Every chromosome carries genes along its length.

What does one gene carry?

1. A. ✓ The instructions for making one protein
2. B. The instructions for making one whole chromosome
   A chromosome carries hundreds of genes.
   Each gene carries the instructions for one protein.

Why: A gene is one stretch of DNA.
That stretch carries the instructions for making one protein.

Here are the four chromosomes of one model cell, shuffled and numbered 1 to 4.

Which chromosome is the homolog of chromosome 1?

1. A. Chromosome 2
   Chromosome 2 is short and chromosome 1 is long.
   Chromosomes of different lengths carry different genes.
2. B. ✓ Chromosome 3
3. C. Chromosome 4
   Chromosome 4 is short and chromosome 1 is long.
   Chromosomes of different lengths carry different genes.

Why: The homolog of a chromosome is the same length and carries the same genes in the same order.
Chromosome 3 is the only other long chromosome.
So chromosome 3 is the homolog of chromosome 1.

A plant’s body cells each hold six chromosomes. Here are the six from one cell, sorted by size.

How many homologous pairs do the six chromosomes form?

1. A. 2
   The six chromosomes come in three lengths, two of each.
   Two chromosomes of the same length are one homologous pair.
2. B. ✓ 3
3. C. 6
   Six chromosomes is six chromosomes, not six pairs.
   A homologous pair is two chromosomes of the same length.

Why: Chromosomes of the same length carry the same genes in the same order, one from each parent.
The six come in three lengths, two of each length.
Each length is one homologous pair, so the cell holds three homologous pairs.

A body cell’s chromosomes are sorted by size.

Which of the following is a homologous pair?

1. A. ✓ Two chromosomes of the same length, with the same genes in the same order
2. B. Two identical sister chromatids joined at one centromere
   Two sister chromatids joined at one centromere are one copied chromosome, not two chromosomes.
3. C. Two chromosomes of different lengths, one from each parent
   Chromosomes of different lengths carry different genes, so they are not a pair.

Why: A homologous pair is two separate chromosomes of the same length.
The two carry the same genes in the same order, one from each parent.

Here is a drawing from one cell of a mouse.

Is this a homologous pair?

1. A. ✓ Yes
2. B. No
   The two rods are the same length, and each has its own centromere: two separate chromosomes carrying the same genes, one from each parent.

Why: The two rods are the same length, so they carry the same genes in the same order.
One is darker, from the mother, and one is lighter, from the father.
Each rod has its own centromere, so they are two separate chromosomes.
So they are a homologous pair.

Here is a drawing from one cell of a pea plant.

Is this a homologous pair?

1. A. Yes
   One rod is long and the other is short.
   Chromosomes of different lengths carry different genes.
2. B. ✓ No

Why: One rod is long and the other is short.
Chromosomes of different lengths carry different genes.
The two members of a homologous pair carry the same genes in the same order, so they are the same length.
So these two are not a homologous pair.

Here is a drawing from one cell of a fruit fly.

Is this a homologous pair?

1. A. Yes
   The lighter rod is clearly shorter than the darker rod.
   Two chromosomes of different lengths carry different genes.
2. B. ✓ No

Why: The lighter rod is clearly shorter than the darker rod.
Chromosomes of different lengths carry different genes.
The two members of a homologous pair are the same length.
So these two are not a homologous pair.

Here is a drawing from one cell of a dog.

Is this a homologous pair?

1. A. ✓ Yes
2. B. No
   A short pair is still a pair.
   The two rods are the same length, and each has its own centromere.

Why: The two rods are the same length, so they carry the same genes in the same order.
One is darker, from the mother, and one is lighter, from the father.
Each rod has its own centromere, so they are two separate chromosomes.
So they are a homologous pair.

Here is a drawing from one cell of a frog.

Is this a homologous pair?

1. A. Yes
   The drawing shows one rod with one centromere: one chromosome.
   A homologous pair is two chromosomes.
2. B. ✓ No

Why: The drawing shows one rod with one centromere, so it shows one chromosome.
A homologous pair is two chromosomes of the same length.
So one chromosome on its own is not a homologous pair.

This drawing shows one X shape. The X is two sister chromatids joined where the arms cross. To count chromosomes, count centromeres.

How many chromosomes does the drawing show?

1. A. ✓ One
2. B. Two
   The X has one centromere, and one centromere is one chromosome.

Why: The X is two identical sister chromatids joined at one centromere, so it is one chromosome.

Here is a drawing from one cell of a grasshopper.

Which of the following does the drawing show?

1. A. A homologous pair
   A homologous pair has two centromeres.
   This X has one centromere.
2. B. ✓ One copied chromosome
3. C. Neither
   One centromere is one chromosome, so the X is one copied chromosome.

Why: The X has one centromere.
One centromere is one chromosome, however many arms it has.
So the X is one copied chromosome: two sister chromatids joined together.

Here is a drawing from one cell of a pea plant.

Which of the following does the drawing show?

1. A. ✓ A homologous pair
2. B. One copied chromosome
   Each X has its own centromere, so the drawing shows two chromosomes, not one.
3. C. Neither
   The two X’s have two centromeres and are the same length, which is a homologous pair.

Why: Each X has its own centromere, so the drawing shows two chromosomes.
Each X is one copied chromosome: two sister chromatids, one chromosome.
The two chromosomes are the same length, one from each parent.
So they are a homologous pair.

Here is a drawing from one cell of a cat.

Which of the following does the drawing show?

1. A. ✓ A homologous pair
2. B. One copied chromosome
   Each rod has its own centromere, so the drawing shows two chromosomes, not one.
3. C. Neither
   The two rods have two centromeres and are the same length, which is a homologous pair.

Why: Each rod has its own centromere, so the drawing shows two chromosomes.
The two rods are the same length, one from each parent.
So they are a homologous pair.

Here is a drawing from one cell of a horse.

Which of the following does the drawing show?

1. A. A homologous pair
   The two rods are different lengths, so they carry different genes.
2. B. One copied chromosome
   The drawing has two centromeres, so it shows two chromosomes, not one copied chromosome.
3. C. ✓ Neither

Why: Each rod has its own centromere, so the drawing shows two chromosomes.
One rod is short and one is long, so the two carry different genes.
So the two are neither a homologous pair nor one copied chromosome.

Here is a drawing from one cell of a salmon.

Which of the following does the drawing show?

1. A. A homologous pair
   The two X’s are different lengths, so they carry different genes.
2. B. One copied chromosome
   The drawing has two centromeres, so it shows two chromosomes, not one copied chromosome.
3. C. ✓ Neither

Why: Each X has its own centromere, so the drawing shows two chromosomes.
One X is long and one is short, so the two carry different genes.
So the two are neither a homologous pair nor one copied chromosome.

A student looks at the drawing below and says: “This X is a homologous pair, and its two arms are the two homologs.”

Is the student correct?

1. A. ✓ No: the X is one copied chromosome, and its two arms are sister chromatids
2. B. Yes: the two arms of the X are the two homologs of a pair
   The X has one centromere, so it is one chromosome.
   Its two arms are two identical copies of that chromosome.

Why: The X has one centromere.
One centromere is one chromosome.
So the X is one copied chromosome, and its two arms are its two sister chromatids.
A homologous pair is two chromosomes with two centromeres.

Two alleles of one gene are two slightly different stretches of DNA.

Where does a DNA molecule carry its information?

1. A. In the number of its strands
   Every DNA molecule has two strands, whatever it codes for.
2. B. ✓ In the order of its bases

Why: The information in DNA is in the order of its bases.

In mice, one gene sets coat color and another gene, further along the same chromosome, sets tail length. One mouse’s homologous pair is drawn below with the version each homolog carries at the two positions.

Which two things are alleles of one gene?

1. A. The coat-color gene and its brown version
   The coat-color gene is the gene, and brown is one version of it.
   Alleles are two or more versions of one gene.
2. B. The two homologs of the pair
   The two homologs are whole chromosomes, and each carries many genes.
   An allele is one version of one gene.
3. C. The black coat version and the long tail version
   Black coat is a version of the coat-color gene; long tail is a version of the tail-length gene.
   Two different genes, so not alleles of one gene.
4. D. ✓ The black version and the brown version

Why: Alleles are versions of one gene at one position.
At the coat-color position this pair carries two different versions, black and brown.
So the black version and the brown version are alleles of the coat-color gene.

The mouse’s homologous pair drawn below carries, on each homolog, a version of the tail-length gene at the tail-length position.

Which of the following does the pair carry at the tail-length position?

1. A. Two different alleles
   Both boxes at the tail-length position read long.
   Two different alleles would read two different words, as black and brown do at the coat-color position.
2. B. ✓ The same allele

Why: Both boxes at the tail-length position read long.
So both homologs carry the same version of the tail-length gene: the same allele.

A pea plant’s flower-color gene comes in a purple version and a white version.

Which of the following is an allele?

1. A. One chromosome of a homologous pair
   A chromosome carries many genes.
   An allele is one version of one of those genes.
2. B. Two different genes at two different positions
   Two different genes are two genes.
   Alleles are versions of one gene at one position.
3. C. ✓ One version of one gene

Why: A gene can come in more than one version.
Each version of that one gene is an allele.

A human body cell’s 46 chromosomes sort by size into 23 pairs.

Which of the following describes the two chromosomes of one pair?

1. A. ✓ Two homologs, one from each parent
2. B. Two sister chromatids of one copied chromosome
   Two sister chromatids are joined at one centromere and are one chromosome.
   The two chromosomes of a pair have two centromeres.

Why: The two chromosomes of a pair are the same length and carry the same genes in the same order.
One came from the mother and one from the father.
So the two are homologs.

A plant’s body cells each hold four chromosomes, two long and two short. Drawings 1 to 3 below each show chromosomes taken from one of the plant’s cells.

Which drawing shows one complete chromosome set?

1. A. Drawing 1
   Drawing 1 shows two long chromosomes and no short one.
   The short chromosome’s genes are missing, so the two are not a complete set.
2. B. ✓ Drawing 2
3. C. Drawing 3
   Drawing 3 shows one long chromosome and two short ones.
   A complete set is one of every pair, and a second short chromosome is one too many.

Why: A complete chromosome set is one chromosome of every pair, every gene once.
The plant has a long pair and a short pair.
Drawing 2 shows one long chromosome and one short one: one of each pair.
So drawing 2 shows one complete set.

A horse’s body cells each hold 64 chromosomes, 32 homologous pairs. A student says: “Take any 32 of the 64 and you have one complete chromosome set.”

Is the student correct?

1. A. ✓ No: any 32 might hold both chromosomes of one pair and neither chromosome of another
2. B. Yes: 32 chromosomes is half of the horse cell’s 64, so any 32 of them make one complete set
   Half the count is not the test.
   A complete set is one chromosome of every one of the 32 pairs.

Why: A complete chromosome set is one chromosome of every pair, every gene once.
Any 32 of the 64 might hold both chromosomes of one pair and neither chromosome of another.
Then that other pair’s genes are missing.
So any 32 is not a complete set.

A body cell’s chromosomes are sorted by size into homologous pairs.

Which of the following is one complete chromosome set?

1. A. Half of the cell’s chromosomes, whichever half
   Half the chromosomes might hold two of one pair and none of another.
   A set is one of every pair.
2. B. ✓ One chromosome of every homologous pair
3. C. Both chromosomes of one homologous pair
   Both chromosomes of one pair carry the same genes twice.
   A set holds every gene once, one chromosome of every pair.

Why: A complete chromosome set is one chromosome of every homologous pair.
One of every pair carries every gene once.

A cell’s chromosomes are counted and sorted by size.

Which of the following describes a diploid cell?

1. A. A cell holding as many chromosomes as a body cell, whichever chromosomes they are
   A count alone is not the test: those chromosomes might hold two of one pair and none of another.
   A diploid cell holds two complete sets, two of every pair.
2. B. A cell holding one complete chromosome set
   A cell holding one complete set is haploid.
3. C. ✓ A cell holding two complete chromosome sets

Why: A diploid cell holds two complete chromosome sets, one from each parent.
So every chromosome in a diploid cell has a homolog.

A cell’s chromosomes are counted and sorted by size.

Which of the following describes a haploid cell?

1. A. ✓ A cell holding one complete chromosome set
2. B. A cell holding two complete chromosome sets, one from each parent
   A cell holding two complete sets is diploid.
3. C. A cell holding half of a chromosome set
   Half of a set leaves genes missing.
   A haploid cell holds one whole set, one chromosome of every pair.

Why: A haploid cell holds one complete chromosome set: one of every chromosome and no homologs.

Here is a cell from a plant whose body cells hold four chromosomes, two long and two short.

Which of the following describes the cell?

1. A. Diploid
   A diploid cell holds two complete sets: two long chromosomes and two short ones.
   This cell has no short chromosome.
2. B. Haploid
   A haploid cell holds one of every pair: one long and one short.
   This cell has two long and no short.
3. C. ✓ Not a complete set

Why: The cell holds two long chromosomes and no short one.
A complete set is one chromosome of every pair.
This cell has two of the long pair and none of the short pair, so the short chromosome’s genes are missing.
So the cell does not hold a complete set.

Here is a cell from a plant whose body cells hold four chromosomes, two long and two short.

Which of the following describes the cell?

1. A. Diploid
   A diploid cell holds two of every chromosome, so every chromosome has a homolog.
   Here the long chromosome has no partner and neither does the short one.
2. B. ✓ Haploid
3. C. Not a complete set
   The cell holds one long and one short chromosome, which is one of every pair, so every gene is present once.
   That is a complete set.

Why: The cell holds one long chromosome and one short one: one of every pair, and no homologs.
One of every pair is one complete set.
A cell with one complete set is haploid.

Here is a cell from a plant whose body cells hold four chromosomes, two long and two short.

Which of the following describes the cell?

1. A. ✓ Diploid
2. B. Haploid
   A haploid cell holds one of every pair.
   This cell holds two long and two short, so it has two of every pair.
3. C. Not a complete set
   The cell has both a long chromosome and a short one, so no gene is missing; it holds every pair twice over.

Why: The cell holds two long chromosomes and two short ones: two of every pair, so every chromosome has a homolog.
Two complete sets make a cell diploid.

A gorilla’s body cells each hold 48 chromosomes. A cell in a gorilla’s testis holds 24 chromosomes, one of each length.

Which of the following describes the cell?

1. A. Diploid
   Two complete sets of a gorilla’s chromosomes are 48, two of every length.
   This cell has 24, one of every length, which is one set.
2. B. ✓ Haploid
3. C. Not a complete set
   24 chromosomes of 24 different lengths means no length is missing and none appears twice.
   That is one chromosome of every pair, which is exactly a complete set.

Why: A gorilla’s 48 chromosomes are 24 homologous pairs, two of every length.
Twenty-four chromosomes of 24 different lengths is one of every pair.
One of every pair is one complete set, so the cell is haploid.

A rabbit’s body cells each hold 44 chromosomes, two of each length.

Which of the following describes one of the rabbit’s body cells?

1. A. ✓ Diploid
2. B. Haploid
   A haploid cell holds one of each length, 22 for a rabbit.
   This cell holds two of each length, 44.
3. C. Not a complete set
   Two of each length means every pair is present, twice over, so no gene is missing.

Why: Two of each length means every chromosome has a homolog.
A rabbit’s 44 are 22 pairs, two complete sets.
A cell holding two complete sets is diploid.

A dog’s body cells each hold 78 chromosomes. A cell in a dog’s ovary holds 39 chromosomes; sorted by size, they show 39 different lengths, one chromosome of each length.

Which of the following describes the cell?

1. A. Diploid
   Two complete sets of a dog’s chromosomes are 78, two of every pair.
   Thirty-nine chromosomes, one of every pair, is one set.
2. B. ✓ Haploid
3. C. Not a complete set
   No length appears twice and none is missing, so the cell holds one of every pair: a complete set.

Why: A dog’s 78 chromosomes are 39 homologous pairs, two of every length.
Thirty-nine chromosomes of 39 different lengths is one of every pair.
One of every pair is one complete set, so the cell is haploid.

A human sperm cell holds 23 chromosomes, one of every pair.

Which of the following describes the sperm cell?

1. A. Diploid
   A diploid cell holds two complete sets, two of every pair: 46 in a human.
2. B. ✓ Haploid

Why: One of every pair is one complete chromosome set.
A cell holding one complete set is haploid.

Cells are taken from a frog and from a tadpole.

Which of the following cells is a gamete?

1. A. A frog’s skin cell
   A skin cell is a body cell with two complete sets.
   It does not join with another cell to start a new frog.
2. B. ✓ A frog’s egg
3. C. A tadpole’s muscle cell
   A muscle cell is a body cell with two complete sets.
   It does not join with another cell to start a new frog.

Why: A gamete is a haploid cell that joins with another to start a new organism.
A frog’s egg holds one complete set, and a sperm joins it to start a new frog.
So the egg is a gamete.

A new animal starts from two cells joining.

Which of the following is a gamete?

1. A. ✓ A haploid cell that joins with another to start a new organism
2. B. The diploid cell made when two gametes fuse into one
   The cell made when two gametes fuse is the start of the new organism, not one of the two cells that joined.
3. C. Any body cell that divides by mitosis to make two
   Body cells divide by mitosis and hold two complete sets; a gamete holds one.

Why: A gamete is a haploid cell, one complete set.
It joins with another gamete to start a new organism.
The sperm and the egg are the two gametes.

A mouse’s sperm holds 20 chromosomes and a mouse’s egg holds 20.

How many chromosomes does the zygote hold?

1. A. 20
   The zygote holds both gametes’ sets: the sperm’s 20 and the egg’s 20.
2. B. ✓ 40
3. C. 80
   Fertilization puts the two gametes’ sets together once, two sets of 20.
   Nothing is copied at fertilization.

Why: The sperm brings one set of 20 chromosomes.
The egg brings one set of 20 chromosomes.
Fertilization puts both sets in one cell.
So the zygote holds 40 chromosomes, two sets of 20.

A zygote with 46 chromosomes divides by mitosis.

How many chromosomes does each of the two new cells hold?

1. A. ✓ 46
2. B. 23
   Before mitosis the cell copies every chromosome once.
   Mitosis then sends one copy of each to each new cell, so the count does not halve.

Why: Mitosis gives two cells with the same chromosomes as the parent cell, so each new cell holds 46.

A frog’s body cells each hold 26 chromosomes. In a pond, frog sperm reach frog eggs, and tadpoles later hatch.

Which cell is the zygote?

1. A. The egg, before the sperm reaches it
   The egg before fertilization is a gamete: haploid, with 13 chromosomes.
   Fertilization has not happened yet, so there is no zygote yet.
2. B. The sperm, once it has entered the egg
   The sperm is a gamete, one of the two cells that fuse.
   The zygote is the one cell the sperm and the egg make together.
3. C. Each cell of the tadpole’s body
   The tadpole’s body cells grew from the zygote by mitosis.
   They are the zygote’s descendants, not the zygote.
4. D. ✓ The single cell made when the sperm and the egg fuse

Why: Fertilization is the fusion of the two gametes.
The single cell it makes is called the zygote.
Here that cell is diploid, with 26 chromosomes, and every body cell of the tadpole grows from it by mitosis.

A sperm and an egg are in the same place.

Which of the following events is fertilization?

1. A. ✓ The two gametes fusing into one cell
2. B. The zygote dividing by mitosis
   The zygote dividing by mitosis is how the body grows, after fertilization has made the zygote.
3. C. A body cell dividing into two
   A body cell dividing by mitosis makes two body cells; no gametes fuse.

Why: Fertilization is the fusion of two gametes, a sperm and an egg, into a single cell.

A sperm has fused with an egg.

Which of the following is the zygote?

1. A. The haploid cell that joined with the egg at fertilization
   The haploid cell that joined with the egg is the sperm, a gamete.
2. B. ✓ The single cell made when the sperm and the egg fused
3. C. Any body cell of the new organism, made by mitosis
   The body cells grew from the zygote by mitosis; they are its descendants.

Why: Fertilization makes one cell from the two gametes.
That single cell, holding both sets, is the zygote.

A new organism is starting.

Which of the following describes sexual reproduction?

1. A. A new organism growing from one cell of one parent by mitosis
   One parent’s cell dividing by mitosis gives a copy of that parent; no gametes fuse.
2. B. A new organism growing all of its body cells by mitosis after it starts
   Every organism grows its body cells by mitosis, after it has started.
   Sexual reproduction is how the organism starts.
3. C. ✓ A new organism starting from two gametes fusing, one from each parent

Why: In sexual reproduction two gametes fuse, one from each parent.
Each parent contributes one chromosome set to the new organism.

Imagine human sperm and eggs each carried 46 chromosomes, like a body cell, and a sperm and an egg still fused at fertilization.

How many chromosomes would the zygote hold?

1. A. 23
   Nothing in this case halves the count; each gamete brings 46.
2. B. 46
   Fertilization adds the two gametes’ chromosomes together, 46 from each.
3. C. ✓ 92

Why: The sperm would bring 46 chromosomes and the egg would bring 46.
Fertilization puts both into one cell.
So the zygote would hold 92 chromosomes, twice 46.

A student says: “Gametes could carry 46 chromosomes each. The zygote would hold 92 and simply throw the extra 46 away.”

Is the student correct?

1. A. Yes: a zygote holding 92 chromosomes would keep 46 of them and throw the other 46 away
   The zygote throws nothing away: it keeps both sets it received.
   Mitosis then gives every body cell the same chromosomes.
2. B. ✓ No: the zygote keeps every chromosome it receives and passes them on by mitosis

Why: The zygote keeps both sets it receives at fertilization.
Mitosis then gives every body cell the same chromosomes as the zygote.
So a zygote holding 92 would give body cells holding 92, and the count would double every generation.

A human egg holds 23 chromosomes, one of every pair.

Which of the following describes the egg?

1. A. ✓ A haploid gamete
2. B. A diploid gamete
   One of every pair is one complete set, so the egg is haploid, not diploid.
3. C. A haploid zygote
   The egg is one of the two cells that fuse; the zygote is the cell their fusion makes.

Why: The egg holds one complete set, so it is haploid.
The egg joins with a sperm to start a new organism, so it is a gamete.

A grasshopper’s body cells each hold 24 chromosomes.

What are the chromosome counts of a grasshopper sperm, a grasshopper body cell and a grasshopper zygote, in that order?

1. A. 24, 24 and 48
   A sperm is haploid, one set of 12, and a zygote holds two sets, 24, not four.
2. B. ✓ 12, 24 and 24
3. C. 12, 12 and 24
   A body cell is diploid, 24, like every cell that grew from the zygote by mitosis.
4. D. 24, 12 and 24
   The sperm, a gamete, is the haploid cell with 12, and body cells are diploid with 24.

Why: A gamete carries one set, 12.
A body cell carries two sets, 24.
The zygote is made from two gametes, so it carries two sets, 24.
Every body cell grows from the zygote.

In a tide pool, sea urchins release sperm and eggs into the water.

Which of the following events is fertilization?

1. A. An egg being released into the water
   An egg in the water is still a gamete on its own; nothing has fused.
2. B. The zygote dividing by mitosis
   The zygote divides by mitosis after fertilization has made it.
3. C. ✓ A sperm fusing with an egg

Why: Fertilization is the fusion of two gametes.
A sperm fusing with an egg is that fusion.

A salmon’s zygote holds 58 chromosomes and divides by mitosis many times as the fish grows.

How many chromosomes does each body cell of the salmon hold?

1. A. 29
   Mitosis never halves the count; each new cell gets one copy of every chromosome.
2. B. ✓ 58
3. C. 116
   Before mitosis the cell copies every chromosome once.
   Mitosis then sends one copy of each to each new cell, so the count does not double.

Why: Mitosis gives each new cell the same chromosomes as the parent cell.
The zygote holds 58, so every body cell holds 58.

A fish’s body cells each hold 50 chromosomes, and so do the body cells of its parents and of its offspring.

Which statement explains why the count stays at 50 from one generation to the next?

1. A. ✓ Halving before fertilization cancels the doubling at fertilization
2. B. Body cells discard one set soon after fertilization
   No set is discarded; the zygote keeps both sets it received and every body cell inherits them by mitosis.
3. C. Cells keep the count at 50 by copying only 25 chromosomes
   Copying, in S phase, makes sister chromatids of every chromosome and never changes the count.
4. D. Fertilization halves the count that two diploid gametes bring
   Fertilization adds the two gametes’ sets together; it never halves anything.

Why: Gametes carry one set, 25 chromosomes, because a halving division comes before fertilization.
Fertilization then joins the two single sets into a double set of 50.
So the halving and the doubling cancel each other, and the count stays at 50.

Imagine a species whose gametes were made by mitosis, with no halving of the chromosome count, and whose eggs and sperm still fused at fertilization.

What would happen to the chromosome count of the body cells over three generations?

1. A. The count would double once and then hold steady
   The doubling would happen again at every fertilization, not once; each generation’s gametes would carry the doubled count into the next zygote.
2. B. The count would halve every generation
   Nothing in this species halves the count; mitosis keeps it and fertilization adds two gametes’ sets together.
3. C. ✓ The count would double every generation
4. D. The count would stay the same
   Fertilization never halves the count; it joins two sets into one cell.

Why: Fertilization adds the two gametes’ chromosome sets together.
With no halving division before it, every generation’s zygote would hold twice its parents’ count.
So the count would double every generation.

Before a cell divides, it copies its DNA once, in S phase.

What does that copying turn each chromosome into?

1. A. ✓ Two identical sister chromatids joined at one centromere
2. B. Two separate chromosomes, each with its own centromere
   Copying does not part the two copies.
   The two copies stay joined at one centromere until a division parts them.

Why: Copying turns every chromosome into two identical sister chromatids joined at one centromere.

A cell goes through meiosis.

What is meiosis?

1. A. One DNA copying, then one division
   One DNA copying and one division is mitosis, which makes two cells with the parent cell’s count.
2. B. ✓ One DNA copying, then two divisions in a row
3. C. Two DNA copyings, then two divisions in a row
   Meiosis copies the DNA once, before meiosis I, and never between the two divisions.

Why: Meiosis is one DNA copying followed by two divisions in a row, meiosis I and then meiosis II.

A cell goes through meiosis.

What is meiosis I?

1. A. ✓ The first division
2. B. The second division
   The second division of meiosis is meiosis II.
3. C. The DNA copying before the divisions
   The DNA copying happens in S phase, before meiosis I begins.

Why: Meiosis I is the first of the two divisions of meiosis.

A cell has finished meiosis I and is about to begin meiosis II.

How many times does the cell copy its DNA between meiosis I and meiosis II?

1. A. ✓ Never
2. B. Once
   The one DNA copying of meiosis happens in S phase, before meiosis I.
   Nothing is copied between the two divisions.
3. C. Twice
   Meiosis has one DNA copying in all, in S phase before meiosis I.

Why: Meiosis copies the DNA once, in S phase, before meiosis I.
Meiosis II follows meiosis I with no copying between them.
So the cell copies its DNA zero times between the two divisions.

Suppose a cell in the anther of a lily, the part of the flower that makes pollen, goes through meiosis.

How many cells does the one cell make?

1. A. Two
   One division makes two cells.
   Meiosis is two divisions in a row, so each of the two cells divides again.
2. B. ✓ Four

Why: Meiosis is two divisions in a row.
The first division makes two cells.
Each of the two divides again.
So the one cell makes four cells.

Suppose a diploid cell in the anther of a lily, the part of the flower that makes pollen, goes through meiosis and makes four cells.

Which of the following describes the four cells?

1. A. ✓ Haploid
2. B. Diploid
   The parent cell was diploid, with two chromosome sets.
   Meiosis turns one diploid cell into four haploid cells, with one set each.

Why: The lily cell began diploid.
Meiosis turns one diploid cell into four haploid cells.
So each of the four cells is haploid.

Two chromosomes of the same length lie side by side in a cell, one from the mother and one from the father.

What are the two chromosomes called?

1. A. Sister chromatids
   Sister chromatids are the two identical copies of one chromosome, joined at one centromere.
   Two chromosomes of the same length from the two parents are a homologous pair.
2. B. ✓ A homologous pair

Why: One chromosome from each parent, the same length and the same genes in the same order, is a homologous pair.

Two cells from a fruit fly are drawn below, numbered 1 and 2, each frozen while its chromosomes move toward the poles. Only some of the chromosomes are drawn.

Which drawing shows meiosis I?

1. A. ✓ Drawing 1
2. B. Drawing 2
   In drawing 2 single V-shaped chromatids move apart, one chromatid of each chromosome to each pole.
   Sister chromatids parting is meiosis II.

Why: In drawing 1 whole X-shaped chromosomes move apart, each still two sister chromatids.
Whole chromosomes moving apart means the two members of each homologous pair are parting.
That is meiosis I.

In a cell from a hamster, the two sister chromatids of every chromosome move to opposite poles.

Which division is this?

1. A. Meiosis I
   Meiosis I never splits a chromosome at its centromere; whole chromosomes move.
   The division that parts sister chromatids is meiosis II.
2. B. ✓ Meiosis II

Why: Sister chromatids are the two copies joined at one centromere.
Meiosis II is the division that parts sister chromatids, as mitosis does.
So this is meiosis II.

In a cell from a hamster, the two members of every homologous pair move to opposite poles.

Which division is this?

1. A. ✓ Meiosis I
2. B. Meiosis II
   Meiosis II parts the two sister chromatids of each chromosome, not the two members of a homologous pair.
   The division that parts homologous pairs is meiosis I.

Why: Meiosis I is the division that parts the two members of each homologous pair.
So this is meiosis I.

In a cell from a moth, whole X-shaped chromosomes, each still two sister chromatids, move toward the poles.

Which division is this?

1. A. ✓ Meiosis I
2. B. Meiosis II
   At meiosis II each centromere splits and single chromatids move, as V shapes.
   Whole X’s moving is meiosis I.

Why: A whole X moving means nothing split at the centromere.
Meiosis I moves whole chromosomes, one member of each homologous pair to each pole.
So this is meiosis I.

In a cell from a grasshopper, each new cell receives one chromatid of every chromosome.

Which division is this?

1. A. Meiosis I
   At meiosis I each new cell receives one whole chromosome of every pair, still two chromatids.
   One chromatid of every chromosome is the outcome of meiosis II.
2. B. ✓ Meiosis II

Why: One chromatid of every chromosome reaches each new cell only when the sister chromatids part.
Sister chromatids part at meiosis II.
So this is meiosis II.

Here is one dividing cell from a fruit fly, drawn with only some of its chromosomes.

Which division is this?

1. A. Meiosis I
   In meiosis I whole X’s move, each still two sister chromatids.
   Here single V-shaped chromatids move, so the centromeres have split: meiosis II.
2. B. ✓ Meiosis II

Why: Single V-shaped chromatids are moving toward the poles.
Single chromatids move only after the two sister chromatids part.
Sister chromatids part at meiosis II.
So this is meiosis II.

Here is one dividing cell from a beetle, drawn with only some of its chromosomes.

Which division is this?

1. A. ✓ Meiosis I
2. B. Meiosis II
   In meiosis II single V-shaped chromatids move.
   Here whole X’s move, each still two sister chromatids: meiosis I.

Why: Whole X-shaped chromosomes are moving toward the poles.
Whole chromosomes move apart when the two members of each homologous pair part.
Homologous pairs part at meiosis I.
So this is meiosis I.

Two cells from a fruit fly are drawn below, numbered 1 and 2, each frozen while its chromosomes move toward the poles. Only some of the chromosomes are drawn. One of the two cells is in meiosis I.

What in that cell’s drawing shows meiosis I rather than meiosis II?

1. A. The chromosomes are moving toward the two poles
   Chromosomes move toward the poles in both divisions.
   What tells them apart is whether whole X’s or single chromatids move.
2. B. Two chromosomes are going to each of the two poles
   Two chromosomes go to each pole in drawing 2 as well, once the sister chromatids of two chromosomes have parted.
   A count per pole cannot tell the two divisions apart.
3. C. Single chromatids are moving apart
   Single chromatids moving apart is meiosis II, which is drawing 2.
   In drawing 1 every moving chromosome is still a whole X.
4. D. ✓ Whole chromosomes of two chromatids are moving apart

Why: In meiosis I the two members of each homologous pair part as whole chromosomes, each still two sister chromatids.
Drawing 1 shows whole X’s moving, so it is meiosis I.
Drawing 2, with single chromatids moving, is meiosis II.

This drawing shows one X shape. The X is two sister chromatids joined where the arms cross. To count chromosomes, count centromeres.

How many chromosomes does the drawing show?

1. A. ✓ One
2. B. Two
   The two sister chromatids share one centromere.
   To count chromosomes, count centromeres: one centromere is one chromosome.

Why: To count chromosomes, count centromeres.
The two sister chromatids are joined at one centromere.
So the drawing shows one chromosome.

A cat’s body cells each hold 38 chromosomes. A student says: “The chromosome count halves at both divisions of meiosis, so each of the cat’s gametes holds 9.5 chromosomes.”

Is the student correct?

1. A. Yes: each gamete holds 9.5 chromosomes
   A cell cannot hold half a chromosome.
   The count halves only at meiosis I, when the two members of each homologous pair part.
2. B. ✓ No: the count halves once, so each gamete holds 19 chromosomes

Why: The count halves at meiosis I, when the two members of each homologous pair go to different cells.
Meiosis II parts sister chromatids, and each chromatid was already counted inside its chromosome.
So the count halves once: each gamete holds 19 chromosomes.

A body cell begins mitosis and enters prophase.

Which of the following happens in prophase?

1. A. The two sister chromatids of each chromosome separate and move apart
   Sister chromatids separate in anaphase.
   In prophase the chromosomes condense into rods and the nuclear envelope breaks down.
2. B. ✓ The chromosomes condense into rods and the nuclear envelope breaks down
3. C. The cell copies its DNA, so each chromosome becomes two chromatids
   The cell copies its DNA in S phase, before mitosis begins.
   In prophase the chromosomes condense into rods and the nuclear envelope breaks down.

Why: Prophase is the first stage of mitosis.
The copied chromosomes condense into rods, the spindle begins to grow between the poles, and the nuclear envelope breaks down.

Suppose a cell in a rat’s testis begins prophase I.

What do the two homologs of each homologous pair do?

1. A. The two homologs move to opposite poles of the cell
   In prophase I nothing moves to the poles yet.
   The two homologs of each pair come together and lie side by side along their whole length.
2. B. The two homologs stay apart from each other, as in mitosis
   Mitosis never pairs the homologs.
   In prophase I the two homologs of each pair come together and lie side by side along their whole length.
3. C. ✓ The two homologs lie side by side along their whole length

Why: In prophase I each homologous pair comes together.
The two homologs lie side by side along their whole length: synapsis.

A cell is in prophase I.

What is synapsis?

1. A. ✓ The two homologs of a homologous pair lying side by side along their whole length
2. B. The two sister chromatids of a chromosome parting at the centromere
   Sister chromatids part in mitosis and in meiosis II, never in synapsis.
   Synapsis is the two homologs of a pair lying side by side along their length.
3. C. The chromosomes standing in a single row along the equator
   A single row along the equator is metaphase of mitosis.
   Synapsis is the two homologs of a pair lying side by side along their length.

Why: Synapsis is the pairing of the two homologs of a homologous pair, side by side along their whole length, in prophase I.

A cell is dividing.

In which of the following does synapsis happen?

1. A. Mitosis
   Mitosis never pairs the homologs.
   Synapsis happens in prophase I, the first stage of meiosis I.
2. B. ✓ Meiosis I
3. C. Meiosis II
   In meiosis II the chromosomes lie single, and nothing pairs.
   Synapsis happens in prophase I, the first stage of meiosis I.

Why: Homologs pair only in prophase I.
Prophase I is the first stage of meiosis I.

A cell is in prophase I.

What is a chiasma?

1. A. Where two sister chromatids are joined to each other
   Two sister chromatids are joined to each other at the centromere.
   A chiasma is a crossing point between the two homologs of a pair.
2. B. The plane halfway between the two poles of the cell
   The plane halfway between the poles is the equator.
   A chiasma is a crossing point between the two homologs of a pair.
3. C. ✓ Where two paired homologs cross and are held to each other

Why: A chiasma is a crossing point at which two paired homologs are held to each other, where an arm of one crosses an arm of the other.

A cell is in prophase I.

Which two things does a chiasma hold together?

1. A. ✓ The two homologs of a homologous pair
2. B. The two sister chromatids of one chromosome
   The two sister chromatids of one chromosome are held together at the centromere.
   A chiasma holds the two homologs of a pair together.
3. C. A chromosome and a spindle fiber
   A spindle fiber attaches to a chromosome at its centromere.
   A chiasma holds the two homologs of a pair together.

Why: A chiasma is where an arm of one homolog crosses an arm of the other homolog.
The two homologs are held to each other there.

Here is one dividing cell from a grasshopper, drawn with only some of its chromosomes.

Which stage is the cell in?

1. A. Prophase of mitosis
   Mitosis never pairs the homologs.
   Here each chromosome lies beside its partner of the same length, so the cell is in prophase I.
2. B. ✓ Prophase I

Why: The chromosomes lie in groups of two, a dark X and a light X of the same length side by side.
Two chromosomes of the same length, one from each parent, are a homologous pair.
Homologs lie side by side only in prophase I.

Here is one dividing cell from a mouse, drawn with only some of its chromosomes.

Which stage is the cell in?

1. A. ✓ Prophase of mitosis
2. B. Prophase I
   In prophase I each chromosome lies beside its homolog.
   Here every chromosome lies apart from the others, so the cell is in prophase of mitosis.

Why: Every chromosome lies on its own, apart from every other.
Homologs lie side by side in prophase I.
Chromosomes lying apart, with no pairing, is prophase of mitosis.

Here is one dividing cell from a fruit fly, drawn with only some of its chromosomes.

Which stage is the cell in?

1. A. Prophase of mitosis
   Mitosis never pairs the homologs.
   Here each chromosome lies beside its partner of the same length, held where their arms cross, so the cell is in prophase I.
2. B. ✓ Prophase I

Why: The chromosomes lie in groups of two, a dark X and a light X of the same length side by side.
Each group is held where two arms cross: a chiasma.
Paired homologs held at chiasmata is prophase I.

Here is one dividing cell from an onion, drawn with only some of its chromosomes.

Which stage is the cell in?

1. A. ✓ Prophase of mitosis
2. B. Prophase I
   In prophase I each chromosome lies beside its homolog.
   Here every chromosome lies apart from the others, so the cell is in prophase of mitosis.

Why: Every chromosome lies on its own, apart from every other.
Homologs lie side by side in prophase I.
Chromosomes lying apart, with no pairing, is prophase of mitosis.

Here is one dividing cell from a lily, drawn with only some of its chromosomes.

Which stage is the cell in?

1. A. Prophase of mitosis
   Mitosis never pairs the homologs.
   Here each chromosome lies beside its partner of the same length, held where their arms cross, so the cell is in prophase I.
2. B. ✓ Prophase I

Why: The chromosomes lie in groups of two, a dark X and a light X of the same length side by side.
Each group is held where two arms cross: a chiasma.
Paired homologs held at chiasmata is prophase I.

A body cell is in metaphase of mitosis.

Where do its chromosomes stand?

1. A. ✓ In a single row along the equator
2. B. Scattered through the cell
   Scattered chromosomes are prophase.
   In metaphase the spindle has pulled every chromosome into a single row along the equator.
3. C. At the two poles of the cell
   Chromosomes reach the poles after anaphase.
   In metaphase every chromosome stands in a single row along the equator.

Why: In metaphase, fibers from both poles have attached to every chromosome and pulled the chromosomes into a single row along the equator, the plane halfway between the poles.

Two cells from an insect whose body cells hold six chromosomes are drawn below, numbered 1 and 2. In both, the chromosomes sit across the middle of the cell with fibers attached.

Which drawing shows metaphase I?

1. A. Drawing 1
   Every chromosome sits on the equator in metaphase of mitosis too.
   Drawing 1 shows six single chromosomes in one row, which is mitosis.
2. B. ✓ Drawing 2
3. C. Both
   Fibers reach every chromosome in both kinds of metaphase.
   What names metaphase I is pairs at the equator, which only drawing 2 shows.

Why: Metaphase I is homologous pairs, not single chromosomes, lined up at the equator, the two homologs of each pair facing opposite poles.
Drawing 2 shows three pairs side by side.
Drawing 1 is a single row, metaphase of mitosis.

A cell is in meiosis I.

What is metaphase I?

1. A. Single chromosomes standing in a single row, each chromosome on its own
   A single row of single chromosomes is metaphase of mitosis.
   Metaphase I is homologous pairs in a double row at the equator.
2. B. ✓ Homologous pairs standing in a double row, the two homologs facing opposite poles
3. C. The two homologs of each pair lying side by side along their whole length, in no row
   Homologs lying side by side in no row is synapsis, in prophase I.
   Metaphase I is homologous pairs in a double row at the equator.

Why: In meiosis I the spindle moves each homologous pair to the equator as a pair.
So the pairs stand at the equator in a double row, the two homologs of each pair facing opposite poles: metaphase I.

In a cell from a dog, homologous pairs stand at the equator in a row two chromosomes wide, the two homologs of each pair facing opposite poles.

Which stage is the cell in?

1. A. Prophase I
   In prophase I the pairs lie where they formed, with no row at the equator.
   Pairs in a double row at the equator is metaphase I.
2. B. Metaphase of mitosis
   In metaphase of mitosis single chromosomes stand in a row one wide.
   Pairs in a double row at the equator is metaphase I.
3. C. ✓ Metaphase I

Why: Homologous pairs at the equator in a double row, the two homologs of each pair facing opposite poles, is metaphase I.

In a cell from a horse, single chromosomes stand in one row along the equator, one chromosome wide.

Is the cell in metaphase I?

1. A. Yes
   In metaphase I homologous pairs stand at the equator, two chromosomes wide.
   A single row, one wide, is metaphase of mitosis.
2. B. ✓ No

Why: Metaphase I is a double row of homologous pairs.
A single row of single chromosomes, one wide, is metaphase of mitosis.
So the cell is in metaphase of mitosis.

In a cell from a dog, every chromosome lies pressed against one other chromosome of the same length, the two touching from end to end. No row has formed.

Which stage is the cell in?

1. A. ✓ Prophase I
2. B. Metaphase I
   At metaphase I the pairs stand in a row at the equator.
   Homologs lying side by side, with no row, is prophase I.
3. C. Prophase of mitosis
   Mitosis never pairs the homologs.
   Homologs lying side by side along their length is synapsis, in prophase I.

Why: Synapsis, the two homologs of each pair lying side by side along their length, happens only in prophase I.

In a cell in prophase I, an arm of one homolog crosses an arm of the other homolog, and the two homologs are held to each other there.

What is the crossing point called?

1. A. A centromere
   The centromere joins the two sister chromatids of one chromosome.
   The crossing point between two paired homologs is a chiasma.
2. B. ✓ A chiasma
3. C. A pole
   A pole is one of the two ends of the cell.
   The crossing point between two paired homologs is a chiasma.

Why: A crossing point at which two paired homologs are held to each other is a chiasma.

A cell is in metaphase I.

Which way do the two homologs of each homologous pair face?

1. A. ✓ Opposite poles
2. B. The same pole
   Fibers from one pole attach to one homolog and fibers from the other pole attach to the other homolog.
   So the two homologs face opposite poles.

Why: Fibers from one pole attach to one homolog’s centromere and fibers from the other pole to the other homolog’s.
So the two homologs of each pair face opposite poles.

Here is one dividing cell from a moth, drawn with only some of its chromosomes.

Which stage is the cell in?

1. A. Prophase I
   In prophase I the chromosomes lie paired, away from any row.
   Here they stand in one row at the equator.
2. B. Metaphase I
   Metaphase I is a double row of homologous pairs, two chromosomes wide.
   Here the row is one chromosome wide: metaphase of mitosis.
3. C. ✓ Metaphase of mitosis

Why: The chromosomes stand in one row along the equator, one chromosome wide.
Each chromosome stands alone, with no homolog beside it.
A single row of single chromosomes is metaphase of mitosis.

In anaphase of mitosis, the spindle pulls partners apart to opposite poles.

Which partners part in anaphase of mitosis?

1. A. ✓ The two sister chromatids of each chromosome
2. B. The two homologs of each pair
   Mitosis never pairs the homologs.
   So it has no pairs to part.

Why: In anaphase of mitosis the connection at each centromere breaks.
Then the spindle pulls the two sister chromatids to opposite poles.

A cell from a lily’s anther, the part of the flower that makes pollen, has reached anaphase I.

Which of the following moves toward each pole?

1. A. Single chromatids, the two sister chromatids of each chromosome parted
   At anaphase I nothing splits at the centromere.
   The two sister chromatids stay joined.
   So whole chromosomes move.
2. B. Whole homologous pairs, both members of each pair together
   The two members of a pair face opposite poles at metaphase I.
   The spindle pulls them apart, one member to each pole.
3. C. ✓ Whole chromosomes, each still two joined sister chromatids

Why: Fibers from one pole are attached to one homolog of each pair, and fibers from the other pole to the other homolog.
Nothing breaks at the centromere.
So the fibers pull whole chromosomes, one member of each pair to each pole.

A student watches a cell at anaphase I and says: “Each centromere has split, so the sister chromatids are moving to opposite poles.”

Is the student correct?

1. A. Yes, each centromere has split
   At anaphase I nothing splits at the centromere.
   Every chromosome reaching a pole is still two sister chromatids joined at one centromere.
2. B. ✓ No, each centromere is still intact

Why: At anaphase I the spindle pulls the two members of each homologous pair apart.
Nothing splits at the centromere.
So whole chromosomes move, each still two sister chromatids.

A cell from a grasshopper’s testis is drawn below. Only some of its chromosomes are drawn.

Which stage is the cell in?

1. A. ✓ Anaphase I
2. B. Anaphase of mitosis
   Every moving chromosome here is still a whole X, two sister chromatids joined at one centromere.
   Whole chromosomes moving apart is anaphase I.

Why: Each moving chromosome is still an X: two sister chromatids joined at one centromere.
Nothing has split at the centromere.
Whole chromosomes moving apart, one member of each pair each way, is anaphase I.

A cell from a mouse is drawn below. Only some of its chromosomes are drawn.

Which stage is the cell in?

1. A. Anaphase I
   Each moving shape here is a single chromatid, a V with its centromere leading.
   Single chromatids moving apart is anaphase of mitosis.
2. B. ✓ Anaphase of mitosis

Why: Each moving shape is a single chromatid: one arm each side of a leading centromere.
The connection at each centromere has broken.
Single chromatids moving apart is anaphase of mitosis.

A cell from a beetle is drawn below. Only one pair of its chromosomes is drawn.

Which stage is the cell in?

1. A. Anaphase I
   Each moving shape here is a single chromatid, a V with its centromere leading.
   Single chromatids moving apart is anaphase of mitosis.
2. B. ✓ Anaphase of mitosis

Why: Each moving shape is a single chromatid: one arm each side of a leading centromere.
The connection at each centromere has broken.
Single chromatids moving apart is anaphase of mitosis.

A cell from a fruit fly’s ovary is drawn below. Only one pair of its chromosomes is drawn.

Which stage is the cell in?

1. A. ✓ Anaphase I
2. B. Anaphase of mitosis
   Both moving chromosomes here are still whole X’s, two sister chromatids joined at one centromere.
   Whole chromosomes moving apart is anaphase I.

Why: Each moving chromosome is still an X: two sister chromatids joined at one centromere.
The two members of the pair are moving to opposite poles.
Whole chromosomes moving apart is anaphase I.

A cell from a rye plant’s anther, the part of the flower that makes pollen, is drawn below. Only some of its chromosomes are drawn.

Which stage is the cell in?

1. A. ✓ Anaphase I
2. B. Anaphase of mitosis
   Every moving chromosome here is still a whole X, two sister chromatids joined at one centromere.
   Whole chromosomes moving apart is anaphase I.

Why: Each moving chromosome is still an X: two sister chromatids joined at one centromere.
One long and one short chromosome move toward each pole.
Whole chromosomes moving apart is anaphase I.

A cell is in meiosis I.

What is anaphase I?

1. A. The stage in which the spindle pulls the two sister chromatids of each chromosome to opposite poles
   At anaphase I nothing splits at the centromere.
   The sister chromatids part later, at meiosis II.
2. B. ✓ The stage in which the spindle pulls the two members of each homologous pair to opposite poles
3. C. The stage in which the homologous pairs line up across the equator
   The homologous pairs line up across the equator at metaphase I.
   Anaphase I is the pull that follows.

Why: At anaphase I the spindle pulls the two members of each homologous pair to opposite poles.
Every chromosome that arrives is still two sister chromatids joined at its centromere.

In mitosis, the separated chromatids have arrived at the two poles.

Which of the following happens next?

1. A. The DNA of every chromosome is copied again
   The DNA was copied in S phase, before mitosis began.
   After anaphase, a nuclear envelope forms around each set.
2. B. The sister chromatids of every chromosome separate
   The sister chromatids separated at the start of anaphase.
   After they arrive at the poles, a nuclear envelope forms around each set.
3. C. ✓ A nuclear envelope forms around each set of chromosomes

Why: After anaphase the chromosomes have arrived at the poles.
The spindle breaks down, and a nuclear envelope forms around each set: telophase.
Then cytokinesis divides the cytoplasm.

A plant’s cell has just finished meiosis I and cytokinesis. Look at one chromosome in one of the two new cells.

How many chromatids does that chromosome have?

1. A. One
   Nothing splits at the centromere in meiosis I.
   Every chromosome in the two cells is still an X: two sister chromatids joined at one centromere.
2. B. ✓ Two

Why: Nothing splits at the centromere in meiosis I.
So every chromosome in the two cells is still two sister chromatids joined at one centromere.
The chromatids part only at meiosis II.

A cell from a fruit fly, whose body cells have 8 chromosomes, has just finished meiosis I and cytokinesis. Each of the two new cells has 4 chromosomes, and each of those chromosomes is still two sister chromatids.

Is each new cell haploid or diploid?

1. A. ✓ Haploid
2. B. Diploid
   A diploid cell has two of every chromosome, a homolog for each.
   Each new cell has one of every pair.
   So it is haploid.

Why: To count chromosomes, count centromeres. A joined pair of sister chromatids is one chromosome, and a separated chromatid is one chromosome.
Each new cell has one chromosome of every pair, 4 of the 8.
One of every chromosome is one complete set.
So each new cell is haploid.

A cell is at the end of meiosis I.

Which of the following happens at telophase I?

1. A. The homologous pairs come together along their length
   The homologous pairs come together in prophase I.
   At telophase I the two sets have already parted and an envelope forms around each.
2. B. ✓ A nuclear envelope forms around each set of chromosomes
3. C. The homologous pairs line up across the equator
   The homologous pairs line up across the equator at metaphase I.
   At telophase I the two sets sit at the poles and an envelope forms around each.

Why: At telophase I the spindle breaks down.
A nuclear envelope forms around each set of chromosomes.
Then cytokinesis divides the cell in two.

In a cell from a mouse’s ovary, the homologous pairs sit side by side across the middle of the cell, one homolog of each pair facing each pole, with a fiber from each pole to each pair.

Which stage of meiosis I is the cell in?

1. A. Prophase I
   In prophase I the paired homologs lie scattered through the cell with no row.
   Here every pair stands across the middle with fibers from both poles.
2. B. ✓ Metaphase I
3. C. Anaphase I
   In anaphase I the two members of each pair have parted and are moving toward the poles.
   Here every pair still sits together at the middle.
4. D. Telophase I
   In telophase I the chromosomes sit at the two ends and the cell is pinching in two.
   Here the cell is whole and the pairs sit at its middle.

Why: The pairs stand side by side across the middle of the cell, one homolog of each pair facing each pole.
A double row of pairs at the equator names metaphase I.

In a cell from a grasshopper’s testis, the two homologs of each pair lie together along their length, an arm of one crossing an arm of the other, and there is no nuclear envelope.

Which stage of meiosis I is the cell in?

1. A. ✓ Prophase I
2. B. Metaphase I
   At metaphase I the pairs stand in a row across the middle of the cell.
   Here the pairs lie together but in no row.
3. C. Anaphase I
   In anaphase I the two members of each pair have parted and whole X’s move toward the poles.
   Here each pair still lies together.
4. D. Telophase I
   In telophase I two cells are forming with the chromosomes gathered at the poles.
   Here there is one cell with paired chromosomes inside it.

Why: The two homologs of each pair lie together along their length, held where an arm of one crosses an arm of the other.
Paired homologs name prophase I.

A cell from a plant whose body cells hold six chromosomes is drawn below.

Which stage of meiosis I is the cell in?

1. A. Prophase I
   In prophase I the homologs lie paired inside one undivided cell.
   Here the cell is pinching in two and the chromosomes are gathered at the poles.
2. B. Metaphase I
   At metaphase I the pairs sit across the middle of one cell.
   Here the chromosomes are at the two ends and a furrow divides the cell.
3. C. Anaphase I
   In anaphase I the chromosomes are still moving through one cell with fibers attached.
   Here the spindle is gone, envelopes are forming and the cell is pinching in two.
4. D. ✓ Telophase I

Why: Two groups of X’s sit at the poles, an envelope forming around each, and a furrow pinches between them.
Two cells forming, each chromosome still doubled, name telophase I.

A cell from a plant whose body cells hold six chromosomes is drawn below.

Which stage of meiosis I is the cell in?

1. A. ✓ Prophase I
2. B. Metaphase I
   At metaphase I the pairs sit in a row across the middle of the cell.
   Here they lie scattered, still paired, with the spindle only reaching in.
3. C. Anaphase I
   In anaphase I the homologs have parted and whole X’s move toward the poles.
   Here each pair still lies together.
4. D. Telophase I
   Telophase I shows two forming cells with the chromosomes gathered at the poles.
   Here there is one cell with paired chromosomes scattered inside it.

Why: The homologs lie paired along their length, arms crossing, with the spindle reaching in and no envelope.
Paired homologs name prophase I.

A cell from a plant whose body cells hold six chromosomes is drawn below.

Which stage of meiosis I is the cell in?

1. A. Prophase I
   In prophase I the homologs lie paired.
   Here they have parted and are moving toward the poles.
2. B. Metaphase I
   At metaphase I the pairs sit still at the equator.
   Here the chromosomes have left the middle and are moving toward the poles.
3. C. ✓ Anaphase I
4. D. Telophase I
   In telophase I the chromosomes have arrived at the poles, envelopes are forming and the cell is pinching in two.
   Here the chromosomes are still moving with fibers attached.

Why: Whole X-shaped chromosomes are moving toward the poles, one of each pair each way.
Whole chromosomes moving name anaphase I.

A cell from a plant whose body cells hold six chromosomes is drawn below.

Which stage of meiosis I is the cell in?

1. A. Prophase I
   In prophase I the paired homologs lie scattered through the cell.
   Here every pair sits at the middle line with fibers attached from both poles.
2. B. ✓ Metaphase I
3. C. Anaphase I
   In anaphase I the homologs have parted and are moving toward the poles.
   Here every pair still sits together at the middle of the cell.
4. D. Telophase I
   In telophase I the chromosomes have reached the poles and the cell is pinching in two.
   Here the cell is whole and the pairs sit at its middle.

Why: Three pairs sit across the middle of the cell, side by side, each pair two chromosomes wide.
A fiber runs from one pole to one homolog and from the other pole to the other.
Pairs at the equator name metaphase I.

A cell from a mouse is drawn below. Only some of the chromosomes are drawn.

Which stage of meiosis I is the cell in?

1. A. Prophase I
   In prophase I the homologs lie paired inside one undivided cell.
   Here the cell is pinching in two and an envelope is forming around each group.
2. B. Metaphase I
   At metaphase I the pairs sit across the middle of one cell.
   Here the chromosomes sit at the two ends and a furrow divides the cell.
3. C. Anaphase I
   In anaphase I the chromosomes are still moving through one cell with fibers attached.
   Here the spindle is gone, envelopes are forming and the cell is pinching in two.
4. D. ✓ Telophase I

Why: Two groups of X’s sit at the poles, an envelope is forming around each group, and a furrow pinches between them.
Two cells forming, each chromosome still an X, name telophase I.

A skin cell is dividing by mitosis. It has reached anaphase, and the connection at each centromere has broken.

What does the spindle pull to opposite poles?

1. A. The two members of each homologous pair
   Mitosis never pairs homologous chromosomes.
   At anaphase of mitosis the connection at each centromere has broken, so the spindle pulls single chromatids.
2. B. Whole chromosomes, each still two sister chromatids
   The connection at each centromere has already broken.
   So the spindle pulls single chromatids, one of each chromosome, to each pole.
3. C. ✓ The two sister chromatids of each chromosome

Why: In mitosis the connection at each centromere breaks at anaphase.
So the two sister chromatids of each chromosome separate.
The spindle then pulls them to opposite poles.

One of the two cells from a grasshopper’s meiosis I has reached anaphase II.

What arrives at each pole?

1. A. One whole chromosome of every homologous pair, still two chromatids
   Homologous pairs parted at anaphase I.
   A cell in meiosis II has no pairs; at anaphase II the two sister chromatids of each chromosome part.
2. B. ✓ One chromatid of every chromosome
3. C. Both chromatids, still joined at their centromere
   At anaphase II the connection at each centromere breaks.
   The two sister chromatids of each chromosome go to opposite poles, so nothing arrives still joined.

Why: At anaphase II the connection at each centromere breaks.
The spindle pulls the two sister chromatids of each chromosome to opposite poles.
So each pole receives one chromatid of every chromosome.

A student says: “Before meiosis II begins, the cell copies its DNA again.”

Is the student correct?

1. A. ✓ No — the cell does not copy its DNA between meiosis I and meiosis II
2. B. Yes — a cell always copies its DNA before it divides
   Every chromosome entering meiosis II is already two sister chromatids, from the copying in S phase.
   No second copying happens.

Why: The cell copied its DNA once, in S phase before meiosis I.
Each chromosome left meiosis I still two sister chromatids.
So the cell enters meiosis II with those chromosomes and copies nothing.

An insect’s body cells hold six chromosomes. One of the two cells its meiosis I produced is drawn below, part way through meiosis II.

Which stage of meiosis II is the cell in?

1. A. Prophase II
   In prophase II the chromosomes lie scattered, only attaching to a forming spindle.
   Here they stand in one row across the middle, with fibers from both poles.
2. B. ✓ Metaphase II
3. C. Anaphase II
   At anaphase II the sister chromatids have parted and single chromatids are moving toward the poles.
   Here every chromosome is still an X at the equator.
4. D. Telophase II
   At telophase II the chromosomes have reached the poles and envelopes are forming.
   Here the chromosomes sit in a row at the middle.

Why: Three X-shaped chromosomes stand in one row at the equator, one deep, with fibers from both poles.
One row of chromosomes on their own, before their chromatids part, is metaphase II.

Here is a cell from a lizard’s ovary, part way through meiosis II. Only some of its chromosomes are drawn.

Which stage of meiosis II is the cell in?

1. A. Prophase II
   In prophase II the chromosomes are still X shapes attaching to a new spindle.
   Here every chromosome is a single rod inside a forming envelope.
2. B. Metaphase II
   In metaphase II the X-shaped chromosomes stand in one row at the equator.
   Here the chromosomes sit at the two ends, in forming envelopes.
3. C. Anaphase II
   In anaphase II the chromatids are still moving toward the poles, as V shapes.
   Here they have arrived, and an envelope is forming around each group.
4. D. ✓ Telophase II

Why: The cell is pinching in two.
A nuclear envelope is forming around each group, and every chromosome is a single rod.
Envelopes re-forming around single chromatids is telophase II.

Here is a cell from a moth’s testis, part way through meiosis II. Only some of its chromosomes are drawn.

Which stage of meiosis II is the cell in?

1. A. Prophase II
   In prophase II each chromosome is still a whole X.
   Here the chromatids have parted and are moving as V shapes.
2. B. Metaphase II
   In metaphase II the whole X shapes stand in one row at the equator.
   Here the chromatids have parted and left the equator.
3. C. ✓ Anaphase II
4. D. Telophase II
   In telophase II the chromatids have reached the poles and envelopes are forming.
   Here the V shapes are still moving, with fibers pulling them.

Why: The chromatids have parted and move toward the two poles as V shapes, the centromere leading.
Sister chromatids moving apart is anaphase II.

Here is a cell from a newt’s ovary, part way through meiosis II. Only some of its chromosomes are drawn.

Which stage of meiosis II is the cell in?

1. A. ✓ Prophase II
2. B. Metaphase II
   In metaphase II the X shapes stand in one row at the equator.
   Here they lie scattered, with the spindle only starting to reach them.
3. C. Anaphase II
   In anaphase II the chromatids have parted into V shapes.
   Here every chromosome is still a whole X.
4. D. Telophase II
   In telophase II the chromosomes sit at the poles in forming envelopes.
   Here they lie scattered, with no envelope.

Why: Whole X-shaped chromosomes lie scattered, and a new spindle is reaching in from the poles.
No chromosome has a partner, so this is a haploid cell.
A new spindle reaching scattered chromosomes is prophase II.

Here is a cell from a grass plant’s anther, part way through meiosis II. Only some of its chromosomes are drawn.

Which stage of meiosis II is the cell in?

1. A. Prophase II
   In prophase II the chromosomes lie scattered.
   Here they stand in one straight row with fibers from both poles.
2. B. ✓ Metaphase II
3. C. Anaphase II
   In anaphase II the chromatids have parted and move as V shapes.
   Here every chromosome is still a whole X standing at the equator.
4. D. Telophase II
   In telophase II the chromosomes sit at the poles in forming envelopes.
   Here they stand in a row at the middle.

Why: Four X-shaped chromosomes stand in one row at the equator, one deep, with a fiber from each pole to every centromere.
One row of whole chromosomes is metaphase II.

Here is a cell from a frog’s testis, part way through meiosis II. Only some of its chromosomes are drawn.

Which stage of meiosis II is the cell in?

1. A. Prophase II
   In prophase II each chromosome is a whole X.
   Here the chromatids have parted into V shapes moving toward the poles.
2. B. Metaphase II
   In metaphase II the X shapes stand in one row at the equator.
   Here the chromatids have parted and are moving away from it.
3. C. ✓ Anaphase II
4. D. Telophase II
   In telophase II an envelope forms around each group at the poles.
   Here the V shapes are still on their way, with fibers pulling them.

Why: Three V shapes move toward each pole, the centromere leading, with fibers still attached.
Sister chromatids moving apart is anaphase II.

A cell in a frog’s ovary completes meiosis.

How many cells does meiosis make from the one cell?

1. A. Two
   Meiosis I makes two cells.
   Then each of the two divides again in meiosis II, so four cells come out.
2. B. ✓ Four
3. C. Eight
   Eight cells would need a third division.
   Meiosis has two divisions: one cell becomes two, then the two become four.

Why: Meiosis I divides the one cell into two.
Meiosis II divides each of the two into two more.
So meiosis makes four cells.

Here is one chromosome from one of the four cells a newt’s meiosis made.

What is the chromosome?

1. A. ✓ A single chromatid
2. B. Two sister chromatids joined at one centromere
   Anaphase II parted the two sister chromatids of every chromosome.
   So a chromosome in a finished cell is a single chromatid.

Why: At anaphase II the spindle pulled the two sister chromatids of every chromosome to opposite poles.
So every chromosome in the four finished cells is a single chromatid.

A fruit fly’s body cells hold eight chromosomes. One of its cells completes meiosis.

What does each of the four cells hold?

1. A. ✓ Four single-chromatid chromosomes, one of every pair
2. B. Four chromosomes, each still two sister chromatids
   Anaphase II parted the sister chromatids.
   So every chromosome in the finished cells is a single chromatid.
3. C. Two single-chromatid chromosomes, half of a set
   The count halves once, at meiosis I, from eight to four.
   Meiosis II parts chromatids and leaves four chromosomes in each cell.
4. D. Eight single-chromatid chromosomes, two of every kind
   Meiosis I sent one member of every pair to each cell.
   So each of the four holds four chromosomes, not eight.

Why: Meiosis I gives each cell one of every pair: four chromosomes from eight.
Meiosis II parts the sister chromatids.
So each of the four cells ends with four single-chromatid chromosomes, a complete haploid set.

A liver cell holds 4 pg of DNA before S phase. In S phase it copies its DNA.

How much DNA does the liver cell hold after S phase?

1. A. 2 pg
   Copying adds DNA; nothing in S phase removes any.
2. B. 4 pg
   Copying makes a second copy of every chromosome, so the amount cannot stay the same.
3. C. ✓ 8 pg

Why: Copying turns every chromosome into two sister chromatids.
So the DNA in the cell doubles, from 4 pg to 8 pg.

A cell in a newt’s testis has finished meiosis I and is about to begin meiosis II.

Does the cell copy its DNA before meiosis II begins?

1. A. Yes
   The DNA is copied once, in S phase before meiosis I.
   The cell enters meiosis II with the chromosomes it left meiosis I with, each still two sister chromatids.
2. B. ✓ No

Why: The DNA is copied once, in S phase before meiosis I.
Between meiosis I and meiosis II nothing is copied.
So the cell begins meiosis II with the DNA it left meiosis I with.

A student says: “After meiosis I each cell has half the chromosomes and half the DNA, so each chromosome must now be a single chromatid.”

Is the student correct?

1. A. Yes — half the DNA means half of every chromosome
   Meiosis I parts homologous pairs, not sister chromatids.
   Each cell gets one whole doubled chromosome of every pair, so it holds half the chromosomes and half the DNA.
2. B. ✓ No — every chromosome after meiosis I is still two sister chromatids

Why: At anaphase I the spindle pulls the two members of each homologous pair apart.
Each cell receives one whole chromosome of every pair, still two sister chromatids.
Half the chromosomes carry half the DNA.
So the DNA halved while every chromosome stayed doubled.

A cell in a moth’s ovary has finished meiosis I. Its chromosome count has halved, yet every chromosome is still two sister chromatids.

Why has the count halved?

1. A. ✓ Each cell received one whole chromosome of every pair, one centromere each
2. B. Meiosis I parted the sister chromatids, and each chromatid now counts as a chromosome
   Sister chromatids part at anaphase II, not at anaphase I.
   At anaphase I whole chromosomes move, each still two sister chromatids.
3. C. Meiosis I removed half of the DNA from every chromosome in the cell
   No DNA is removed from a chromosome.
   The count halved because each cell received one whole chromosome of every pair.

Why: At anaphase I the spindle pulls the two members of each homologous pair to opposite poles.
Each cell receives one whole chromosome of every pair, still two sister chromatids.
Count centromeres: one of every pair is half the count.

A biologist writes the DNA in a cell as 2C.

What is 1C?

1. A. ✓ The DNA in one haploid set of single-chromatid chromosomes
2. B. The DNA in one body cell before it copies its DNA
   A body cell before copying holds two haploid sets.
   So a body cell before copying holds 2C.
3. C. The DNA in one chromosome of a haploid set
   A haploid set is many chromosomes, one of every pair.
   1C is the DNA in the whole set, not in one chromosome.

Why: 1C is the amount of DNA in one haploid set of chromosomes, every chromosome a single chromatid.

A student claims that a cell copies its DNA again between meiosis I and meiosis II.

Which of the following observations shows that the claim is wrong?

1. A. Each cell after meiosis I holds half the DNA of the copied cell
   The halving at meiosis I happens whether or not the cell copies its DNA afterward.
   So it cannot show what happened in the gap between the divisions.
2. B. ✓ Each cell after meiosis II holds half the DNA of a cell after meiosis I
3. C. The chromosome count is the same before and after meiosis II
   The chromosome count depends on centromeres, not on the amount of DNA.
   It would stay the same through meiosis II even after a second copying.
4. D. Every chromosome is two sister chromatids at the moment meiosis I ends
   Doubled chromosomes at the end of meiosis I are what the first copying left.
   They do not show whether a second copying followed.

Why: If the cell copied its DNA between the divisions, each cell would enter meiosis II with twice the DNA it left meiosis I with.
Then the four cells would each hold as much as the two.
Measured DNA halves at meiosis II, so there was no second copying.

Two cells from one plant are measured. Cell 1 holds 2C of DNA and 12 chromosomes, each still two sister chromatids. Cell 2 holds 2C of DNA and 24 chromosomes, each a single chromatid.

Which statement describes the two cells?

1. A. ✓ Cell 1 has just finished meiosis I; cell 2 is a body cell before copying
2. B. Cell 1 is a body cell before copying; cell 2 has just finished meiosis I
   A body cell before copying holds single-chromatid chromosomes, and cell 1’s are doubled.
   A cell after meiosis I holds doubled chromosomes, and cell 2’s are single.
3. C. Both cells have just finished meiosis II
   A cell after meiosis II holds 1C, not 2C.
   Its chromosomes are single chromatids, which does not fit cell 1.
4. D. Both cells have just finished meiosis I
   A cell that has just finished meiosis I holds chromosomes that are still two sister chromatids.
   Cell 2’s are single chromatids.

Why: After meiosis I a cell holds 2C on half the chromosome count, each chromosome two chromatids.
A body cell before copying holds 2C on the full count, each chromosome one chromatid.
So the chromatid count places each cell: cell 1 after meiosis I, cell 2 before copying.

Meiosis has two divisions, meiosis I and meiosis II.

Which partners does meiosis I pull apart?

1. A. ✓ The two members of each homologous pair
2. B. The two sister chromatids of each chromosome
   Meiosis II pulls apart the two sister chromatids of each chromosome.
   Meiosis I pulls apart the two members of each homologous pair.

Why: Meiosis I pulls apart the two members of each homologous pair.
Meiosis II pulls apart the two sister chromatids of each chromosome.

A fruit fly’s body cells hold eight chromosomes. A cell from its testis is drawn below.

Which division is the cell in?

1. A. Meiosis I
   In meiosis I whole X-shaped chromosomes move to the poles.
   Here the moving pieces are V-shaped single chromatids.
2. B. ✓ Meiosis II

Why: V-shaped single chromatids are moving to the poles.
Sister chromatids part only in meiosis II.
So the cell is in meiosis II.

A plant’s body cells hold six chromosomes. A cell from one of its flowers, dividing by meiosis, is drawn below.

Which division is the cell in?

1. A. ✓ Meiosis I
2. B. Meiosis II
   In meiosis II single chromatids move to the poles.
   Here every moving chromosome is still a whole X.

Why: Whole X-shaped chromosomes are moving to the poles, each still two sister chromatids.
Whole chromosomes move to the poles only in meiosis I, when the two chromosomes of each homologous pair part.
So the cell is in meiosis I.

A plant’s body cells hold six chromosomes. A cell from one of its flowers, dividing by meiosis, is drawn below.

Which division is the cell in?

1. A. ✓ Meiosis I
2. B. Meiosis II
   In meiosis II single chromosomes stand at the equator in one row.
   Here the chromosomes stand in pairs, two X’s of the same length side by side.

Why: The chromosomes stand at the equator in homologous pairs, two X’s of the same length side by side.
Homologous pairs stand at the equator only in meiosis I.
So the cell is in meiosis I.

A fruit fly’s body cells hold eight chromosomes. A cell from one of its testes is drawn below.

Which division is the cell in?

1. A. Meiosis I
   In meiosis I the chromosomes stand at the equator in homologous pairs, two wide.
   Here four single chromosomes stand in one row.
2. B. ✓ Meiosis II

Why: Four single chromosomes stand in one row at the equator, with no pairs.
Single chromosomes stand at the equator only in meiosis II.
So the cell is in meiosis II.

A plant’s body cells hold six chromosomes. A dividing cell from one of its flowers, in meiosis, is drawn below.

Which division is the cell in?

1. A. ✓ Meiosis I
2. B. Meiosis II
   At the end of meiosis II every chromosome in the forming cells is a single rod.
   Here every chromosome is still an X.

Why: Two cells are forming.
Every chromosome in them is still an X of two sister chromatids.
Sister chromatids part only in meiosis II, so here they are still joined.
So the cell is in meiosis I.

A fruit fly’s body cells hold eight chromosomes. A dividing cell from its testis is drawn below.

Which division is the cell in?

1. A. Meiosis I
   At the end of meiosis I every chromosome is still an X of two sister chromatids.
   Here every chromosome is a single rod.
2. B. ✓ Meiosis II

Why: Two cells are forming.
Every chromosome in them is a single rod: one chromatid.
Sister chromatids part only in meiosis II.
So the cell is in meiosis II.

A plant’s body cells hold six chromosomes. A cell from one of its flowers, dividing by meiosis, is drawn below.

Which stage is the cell in?

1. A. Metaphase I
   Metaphase I is a double row of homologous pairs at the equator.
   Here three single chromosomes stand in one row, half the plant’s six.
2. B. ✓ Metaphase II
3. C. Prophase II
   In prophase II the chromosomes are still scattered, attaching to a forming spindle.
   Here the chromosomes stand in a row at the equator with fibers from both poles.
4. D. Anaphase II
   At anaphase II single chromatids are moving toward the poles.
   Here every chromosome is still a whole X standing at the equator.

Why: Three single X-shaped chromosomes stand in one row at the equator, half the plant’s six.
Single chromosomes standing at the equator is metaphase II.

A plant’s body cells hold six chromosomes. Cells from one of its flowers are drawn below at the moment one stage of meiosis has just ended.

Which stage has just finished?

1. A. Telophase I and cytokinesis
   Telophase I ends with two cells whose chromosomes are still X’s.
   Here four cells have formed, and each chromosome is a single rod.
2. B. Anaphase II
   At anaphase II the chromatids are still moving inside undivided cells.
   Here the cells have divided, and a nuclear envelope has formed in each.
3. C. Anaphase I
   Anaphase I moves whole X-shaped chromosomes apart inside one cell.
   Only two cells follow anaphase I; here four have formed.
4. D. ✓ Telophase II and cytokinesis

Why: Four cells have formed.
Each cell holds three single-chromatid chromosomes inside a nuclear envelope.
Four finished cells with single chromatids is the end of meiosis: telophase II and cytokinesis have just finished.

A plant’s body cells hold six chromosomes. A cell from one of its flowers, dividing by meiosis, is drawn below.

Which stage is the cell in?

1. A. Anaphase I
   Anaphase I moves whole X-shaped chromosomes.
   Here the moving pieces are V-shaped single chromatids.
2. B. Metaphase II
   At metaphase II the chromosomes stand still in a row at the equator.
   Here the chromosomes are moving toward the poles.
3. C. Telophase I
   At telophase I the cell is dividing and its chromosomes are still X’s.
   Here single chromatids are moving in one undivided cell.
4. D. ✓ Anaphase II

Why: V-shaped single chromatids are moving toward the poles.
Sister chromatids part only in anaphase II.
So the cell is in anaphase II.

A plant’s body cells hold six chromosomes. A cell from one of its flowers, dividing by meiosis, is drawn below.

Which stage is the cell in?

1. A. Prophase I
   In prophase I the homologous pairs lie together in a cell holding all six chromosomes.
   Here three single chromosomes lie apart in a small cell.
2. B. Metaphase I
   Metaphase I is a double row of homologous pairs at the equator.
   Here the chromosomes are scattered and single.
3. C. ✓ Prophase II
4. D. Metaphase II
   At metaphase II the chromosomes stand in one row at the equator.
   Here the chromosomes are scattered, attaching to the forming spindle.

Why: Three single X-shaped chromosomes, half the plant’s six, lie apart and are attaching to a new spindle.
Single chromosomes attaching to a new spindle is prophase II.

A chromosome has been copied. It is now two sister chromatids joined at one centromere.

How many chromosomes is it?

1. A. ✓ One
2. B. Two
   The copied chromosome has one centromere.
   One centromere is one chromosome.

Why: Count centromeres.
The copied chromosome has one centromere, so it is one chromosome, made of two chromatids.

A horse’s body cells hold 64 chromosomes. One of its cells holds 32 chromosomes and 64 chromatids. To count chromosomes, count centromeres. A joined pair of sister chromatids is one chromosome, and a separated chromatid is one chromosome.

Which stage has this cell just finished?

1. A. S phase
   After S phase the cell still holds all 64 chromosomes, as 128 chromatids.
2. B. ✓ Meiosis I
3. C. Meiosis II
   After meiosis II each chromosome is a single chromatid, so the chromatid count equals the chromosome count: 32 and 32.
4. D. Mitosis
   Mitosis gives cells with the parent’s 64 chromosomes, each a single chromatid.

Why: The cell holds 32 chromosomes, half of 64, so the homologous pairs have parted.
Each chromosome is still two chromatids, 64 in all, so the sister chromatids have not parted.
Pairs parted and sister chromatids still joined is the end of meiosis I.

A sunflower’s body cells hold 34 chromosomes. One of its cells holds 17 chromosomes and 17 chromatids. To count chromosomes, count centromeres. A joined pair of sister chromatids is one chromosome, and a separated chromatid is one chromosome.

Which stage has this cell just finished?

1. A. S phase
   After S phase the cell still holds all 34 chromosomes, as 68 chromatids.
2. B. Mitosis
   Mitosis gives cells with the parent’s 34 chromosomes, each a single chromatid.
3. C. Meiosis I
   After meiosis I each chromosome is still two chromatids, so 17 chromosomes would be 34 chromatids.
4. D. ✓ Meiosis II

Why: The cell holds 17 chromosomes, half of 34, so the homologous pairs have parted.
Each chromosome is one chromatid, 17 in all, so the sister chromatids have parted too.
Pairs parted and sister chromatids parted is the end of meiosis II.

A student counts the chromosomes in a cell that has just finished meiosis I and gets 20. The student says the cell must have held 40 chromatids after S phase. To count chromosomes, count centromeres. A joined pair of sister chromatids is one chromosome, and a separated chromatid is one chromosome.

Which of the following did the cell hold after S phase?

1. A. 40 chromosomes and 20 chromatids
   Copying never leaves a chromosome with half a chromatid.
   After S phase each of the 40 chromosomes is two chromatids.
2. B. 40 chromosomes and 40 chromatids
   After S phase every chromosome is two chromatids, so 40 chromosomes carry 80 chromatids.
3. C. ✓ 40 chromosomes and 80 chromatids
4. D. 80 chromosomes and 80 chromatids
   Copying doubles the chromatids, not the chromosomes.
   20 chromosomes after meiosis I means 40 before it, and copying left those 40 as 40.

Why: Twenty chromosomes after meiosis I means the cell held 40 chromosomes before copying.
Copying leaves the chromosome count at 40.
After S phase each of those 40 chromosomes was two chromatids.
So the cell held 40 chromosomes and 80 chromatids.

Both a skin cell and a cell in an ovary spend time in S phase before they divide.

What happens in S phase?

1. A. ✓ The cell copies every one of its DNA molecules
2. B. The cell pulls its sister chromatids apart
   Sister chromatids part later, at anaphase.
   In S phase the cell copies its DNA.
3. C. The cell splits into two cells
   The cell splits at the end of division, after the stages.
   In S phase the cell copies its DNA.

Why: S phase is the copying stage of interphase.
The cell copies every DNA molecule, so each chromosome becomes two sister chromatids.

A cell from a lily is dividing. Its chromosomes are X-shaped, and fibers from two poles are attached to them, so it could be in either mitosis or meiosis.

Which of the following is true of the cell, whichever division it is in?

1. A. The cell will produce four cells by the end
   Only meiosis ends with four cells.
   A cell in mitosis ends with two.
2. B. ✓ The cell copied its DNA once before meiosis or mitosis began
3. C. The cell will produce two cells identical to itself
   Two cells identical to the parent is the outcome of mitosis alone.
   A cell in meiosis makes four haploid cells, none identical to it.
4. D. The cell’s chromosome number will stay the same
   The chromosome number stays the same only in mitosis.
   Meiosis halves it.

Why: Whichever division the cell is in, it copied its DNA once in S phase first.
That copying is why every chromosome is an X.
Four cells and a halved count belong to meiosis alone; two identical cells and an unchanged count belong to mitosis alone.

A plant’s body cells hold six chromosomes. One of its cells is drawn below.

Which division is the cell in?

1. A. Mitosis
   In mitosis the chromosomes stand at the equator singly, in one row.
   Here the chromosomes stand in pairs, two X’s of the same length side by side.
2. B. ✓ Meiosis

Why: The chromosomes stand at the equator in homologous pairs.
Homologous pairs form only in meiosis.
So the cell is in meiosis.

A dog’s body cells hold 78 chromosomes. One of its cells divides and makes two cells with 78 chromosomes each.

Which division was it?

1. A. ✓ Mitosis
2. B. Meiosis
   Meiosis makes four cells, each with half the parent’s chromosomes: 39.
   Two cells with the parent’s 78 is mitosis.

Why: The division made two cells.
Each new cell has 78 chromosomes, the same as the parent.
Two cells with the parent’s count is mitosis.

A plant’s body cells hold six chromosomes. One of its cells is drawn below.

Which division is the cell in?

1. A. ✓ Mitosis
2. B. Meiosis
   In anaphase II of meiosis the cell is haploid: in this plant, three chromatids move to each pole.
   Here six chromatids move to each pole, the plant’s full six.

Why: Single chromatids are moving to the poles, six each way.
Six is the plant’s full count, so this cell is not haploid.
Sister chromatids parting in a cell with the full count is mitosis.

A dog’s body cells hold 78 chromosomes. One of its cells divides and makes four cells with 39 chromosomes each.

Which division was it?

1. A. Mitosis
   Mitosis makes two cells, each with the parent’s 78 chromosomes.
   Four cells with half the count is meiosis.
2. B. ✓ Meiosis

Why: The division made four cells.
Each new cell has 39 chromosomes, half the parent’s 78.
Four cells with half the count is meiosis.

A plant’s body cells hold six chromosomes. One of its cells is drawn below.

Which division is the cell in?

1. A. Mitosis
   In mitosis the chromosomes never pair up.
   Here each X lies against a partner of the same length: three pairs.
2. B. ✓ Meiosis

Why: Each X-shaped chromosome lies against another X of the same length.
Homologous pairs form only in meiosis, in prophase I.
So the cell is in meiosis.

A cell in a mouse’s skin divides and makes two cells with 40 chromosomes each, the same as the skin cell. A cell in the mouse’s testis divides and makes four cells with 20 chromosomes each.

Which of the following happened in the testis cell’s division alone?

1. A. The chromosomes condensed into X shapes before they moved
   Chromosomes condense into X shapes in mitosis as well; the skin cell’s did too.
2. B. The spindle pulled sister chromatids apart
   The spindle pulls sister chromatids apart in mitosis as well.
   Meiosis does it in its second division.
3. C. ✓ Homologous pairs lay together before lining up at the equator
4. D. The cytoplasm divided by cytokinesis at the end
   Both divisions end with cytokinesis; meiosis has it twice.

Why: Four cells with half the count is meiosis.
Only meiosis forms homologous pairs, in prophase I, before they line up.
Condensing, parting sister chromatids and cytokinesis happen in both divisions.