End-of-topic test: Cell Cycle

Unit 4 · Topic 4.5 end-of-topic test

Answer every question. For each multiple-choice question, pick one option and press Check; the feedback tells you what a wrong choice assumed. For the two free-response questions, write your answer in full sentences (and show any calculation), then open the scoring guide and mark your own work against it. Where a question gives the length of the whole cell cycle, use that length; where a graph's caption says its error bars represent ±2SE, the overlap rule applies.

Question 1

A planarian is a flatworm about a centimeter long. Cut one in half across the middle and, within two weeks, each half has grown the parts it lost and is a complete worm.

Where do the cells that rebuild each half come from?

A. Existing cells near the cut divide again and again into daughter cells B. Cells near the cut swell until they are large enough to fill the missing part C. New cells form from the fluid that oozes from the cut surface D. Cells drift in from the water and attach themselves to the cut surface

Question 2

A horse body cell holds 64 chromosomes in its nucleus.

What is one of these chromosomes?

A. A chromosome is a short rod of DNA that exists only while the cell divides B. A chromosome is one of the proteins the cell's DNA is wound around C. A chromosome is the whole set of DNA the cell hands to each daughter cell D. A chromosome is one very long DNA molecule packaged with proteins

Question 3

A stain that binds DNA is added to two cells from a growing bean root. In cell X the nucleus looks evenly grainy. In cell Y there are short, dark rods and no nucleus outline.

Which cell is about to divide, and why does its DNA look as it does?

A. Cell X; its DNA is spread out so it can be moved to the poles B. Cell Y; its chromosomes have coiled into rods that move without tangling C. Cell Y; its DNA has been copied, and copied DNA shows as rods D. Cell X; a grainy nucleus means its chromosomes are already moving toward the poles

Question 4

A cat body cell has 38 chromosomes. During S phase it copies its DNA.

How many chromosomes and how many chromatids does the cell hold at the end of S phase?

A. 19 chromosomes with 38 chromatids B. 38 chromosomes with 38 chromatids C. 38 chromosomes with 76 chromatids D. 76 chromosomes with 76 chromatids

Question 5

The cell cycle of a cultured human cell as a wheel, one turn per cycle, with its five stages numbered 1 to 5 in order from a daughter cell's first stage. Stages 4 and 5 are drawn wider than their true share of the turn so that their numbers fit.

The model below shows the cell cycle of a cultured human cell as a wheel, one turn per cycle, with its five stages numbered 1 to 5 in order from a daughter cell's first stage.

In which numbered stage is the cell's DNA copied?

A. Stage 1 B. Stage 2 C. Stage 3 D. Stage 4

Question 6

Cells of one type were measured every hour as they grew in a dish. Between hour 2 and hour 5 the mean mass of a cell rose from 1.6 to 2.2 ng while the DNA in each cell stayed at 5 pg, the amount a newly formed daughter cell holds.

Which stage were the cells in during these hours, and how can you tell?

A. S phase; the mass rose, which shows the DNA was being copied B. G2; a steady DNA content shows that the copying was finished C. Mitosis; the mass rose as each cell got ready to split in two D. G1; the cells were growing while their DNA was still uncopied

Question 7

DNA per cell through one 22-hour cycle of a trout cell line; a new daughter cell holds 5 pg. Gridlines every 1 hour and every 1 pg.

The graph shows the DNA per cell of a trout cell line through one 22-hour cycle. A new daughter cell holds 5 pg.

During which hours were the cells copying their DNA?

A. Hours 0 to 7 B. Hours 7 to 15 C. Hours 15 to 21 D. Hours 21 to 22

Question 8

In G2 a cell finishes copying one structure it will need for the division that follows.

Which structure is copied, and what will it do?

A. The centrosome, from which the fibers that move the chromosomes will grow B. The nucleus, which will hold one set of chromosomes for each daughter cell C. The centromere, which will hold the two sister chromatids together D. The cell wall, which will grow between the two daughter cells

Question 9

Cells lining the gut divide every few days. The muscle cells of the heart work for a whole lifetime; each one's last division was before birth.

Which statement describes the heart muscle cells?

A. Heart muscle cells are held in G2, ready to divide as soon as the heart is damaged B. Heart muscle cells stay in G1 and grow larger every year instead of dividing C. Heart muscle cells have died, which is why they never divide D. Heart muscle cells have left the cycle from G1 into G0 and work on there

Question 10

In prophase the copied chromosomes coil into rods and the mitotic spindle begins to grow between the two poles. By the end of prophase the nuclear envelope has broken down.

Why does the breakdown of the nuclear envelope matter for what happens next?

A. The spindle fibers can now reach every chromosome and attach at its centromere B. The chromosomes can now uncoil back into chromatin so that their genes can be read C. The DNA can now be copied, because the copying enzymes can reach it D. The cell can now pinch in at a cleavage furrow to divide its cytoplasm

Question 11

Two dividing cells are seen on a slide of a growing pea root. In cell P, rods lie scattered through the cell and fibers from the poles reach only some of them. In cell Q, every rod stands in one line across the middle of the cell with fibers from both ends attached to it.

Which cell is in metaphase, and what marks the stage?

A. Cell P; the spindle fibers have begun to reach some of the scattered chromosomes B. Cell P; the chromosomes have coiled into rods and the envelope has gone C. Cell Q; fibers from both poles hold every chromosome in one row at the equator D. Cell Q; the sister chromatids have separated and started to move apart

Question 12

Dividing cells are treated with a chemical that stops the connection at each centromere from breaking. Their chromosomes line up along the equator, with fibers from both poles attached to every one.

Predict what happens to these chromosomes next.

A. The fibers pull whole chromosomes, two chromatids each, to one pole or the other B. The sister chromatids separate anyway, because the pulling is what breaks the connection C. The nuclear envelope re-forms around the whole row at the equator D. The chromosomes stay put; the sisters must separate before the fibers pull them apart

Question 13

In telophase the chromosomes reach the two poles, a nuclear envelope forms around each set, and the rods uncoil back into chromatin.

Why does the DNA return to the spread-out form at this point?

A. Spread out, the DNA is open to be read, so each new nucleus can start its work B. Spread out, the DNA is easier for the spindle fibers to drag toward the two poles C. Spread out, the DNA takes up less room inside the new nuclear envelope D. Spread out, the sister chromatids can separate from each other

Question 14

At the end of mitosis an animal cell pinches in around its middle until it splits. A plant cell at the same point does something different.

Why does a plant cell divide its cytoplasm by building a plate across its middle rather than pinching in?

A. Its two nuclei sit too far apart for a ring of protein to pinch the cell between them B. Its rigid cell wall cannot be pinched, so it builds a new wall between the nuclei C. Its spindle stays in place after mitosis and hardens into the plate D. Its nuclear envelopes fuse across the middle and seal the two halves apart

Question 15

One cell from a fish embryo, fixed and stained; each body cell of this fish holds four chromosomes.

The drawing below shows one cell from a fish embryo, fixed and stained as it was dividing.

Which stage of mitosis does the drawing show?

A. Metaphase B. Anaphase C. Telophase D. Cytokinesis

Question 16

Four students' drawings of the same cell, which has four chromosomes (two long and two short), each meant to show prophase.

Four students drew the same cell, which has four chromosomes (two long and two short), at prophase. Their drawings are numbered 1 to 4 below.

Which drawing shows prophase correctly?

A. Drawing 1 B. Drawing 2 C. Drawing 3 D. Drawing 4

Question 17

A student counts 300 cells on a slide from the growing root tip of a sunflower seedling and finds 45 of them in one of the four stages of mitosis.

What is the mitotic index of the slide?

A. 0.15% B. 15% C. 18% D. 45%

Question 18

On a slide from a growing root tip, 12% of the cells are in one of the four stages of mitosis. Take the whole cell cycle in this root as 24 hours.

How long does mitosis take in these cells?

A. 0.12 h B. 2.0 h C. 2.9 h D. 12 h

Question 19

Percent of mitotic cells in each stage of mitosis, from 100 mitotic cells counted in untreated chick embryo cells and 100 counted after six hours in the chemical. Gridlines every 20%.

Cells from a chick embryo were grown for six hours in a chemical that stops the nuclear envelope from breaking down. The graph below sorts 100 mitotic cells by stage for untreated cells and for treated cells.

What does the chemical do to the dividing cells?

A. It holds the cells in prophase, so they pile up there B. It speeds the cells through anaphase, so fewer are caught in it C. It stops the DNA being copied, so fewer cells enter mitosis D. It holds the cells at the equator in metaphase, so they pile up there

How to tackle the free-response questions. Read the verb first: describe asks what you see or know; explain asks why or how, so name the mechanism; predict asks what will happen and why; justify asks for the evidence that supports a claim. Each point is earned by one idea, stated in a sentence that names the thing and the mechanism. Extra words earn nothing; a wrong extra can lose the point. If there is a figure or table, use what it shows. When you finish, check the box for each point your answer earns and compare your sentences with the full-credit answer.

Free response 1 · Scientific Investigation · 4 points

A student tests the claim that root cells stop dividing as they get further from the tip of the root. She grows five garlic bulbs until each has roots about 30 mm long. From one root of each bulb she cuts a thin slice 1 mm from the tip and a second slice 10 mm from the tip, stains each slice and, under the microscope, records the stage of 250 cells in it. The table gives her counts for one slice cut 1 mm from the tip. The graph gives the mean percent of cells in mitosis for the five slices at each distance, with error bars that represent ±2SE. Take the whole cell cycle in this root as 24 hours.

Top: the stage of each of the 250 cells counted in one slice cut 1 mm from a garlic root tip. Bottom: the mean percent of cells in mitosis in the five slices at each distance; error bars represent ±2SE. Gridlines every 2%.

(a) Identify the independent variable and the dependent variable in this investigation. (1 point)

A full-credit answer: The independent variable is the distance of the slice from the root tip, 1 mm or 10 mm. The dependent variable is the percent of the 250 cells that were in mitosis (the stage each cell was recorded in).

Check the box for each point your answer earns

both: the independent variable is the distance of the slice from the root tip (1 mm or 10 mm) and the dependent variable is the percent of cells in mitosis (or the stage of each cell counted).

Accept "how far from the tip the cells were" for the independent variable and "the number of cells in mitosis out of 250" for the dependent variable. Do not award the point if the two are reversed, or if a condition kept the same (250 cells per slice, the stain, the 30 mm roots) is named as the independent variable.

Common slip: Naming the stain or the 250 cells per slice as a variable. Those were kept the same for every slice; the student changed the distance from the tip and measured the share of cells in mitosis.

(b) Calculate the time, in hours, that a cell 1 mm from the tip spends in prophase, taking the whole cycle as 24 hours. (1 point)

Write down the values in the question:

cells in prophase = 20

total cells counted = 250

cycle length = 24 h

Write down the equations:

percent in prophase = \frac{cells in prophase}{total cells counted} \times 100

time in prophase = \frac{percent in prophase}{100} \times cycle length

Substitute the values into the equations:

percent in prophase = \frac{20}{250} \times 100 = 8 %

time in prophase = \frac{8}{100} \times 24 h

time in prophase = 0.08 \times 24 h = 1.92 h

1.92 h \times 60 min/h = 115.2 min \approx 115 min

A full-credit answer: A cell 1 mm from the tip spends 1.92 hours in prophase, about 115 minutes.

Accept 1.9 h. Do not award the point for 8 h (the percent read as hours), 0.08 h (the fraction with no cycle length) or 3.84 h (the count divided by 125, the wrong total).

(c) Justify the claim that a smaller share of the cells is dividing 10 mm from the tip than 1 mm from the tip, using the error bars. (1 point)

A full-credit answer: The caption says the bars represent ±2SE, so the overlap rule applies. The 1 mm bar runs from 13.6% to 18.4% of cells in mitosis and the 10 mm bar from 0.4% to 2.0%. The bars do not overlap, so the difference between the means is very unlikely to be chance: far fewer of the cells 10 mm from the tip were caught in mitosis, so fewer of them are dividing.

Check the box for each point your answer earns

the ±2SE bar for 1 mm runs from 13.6% to 18.4% and the bar for 10 mm from 0.4% to 2.0%; the two bars do not overlap, so the lower percent in mitosis at 10 mm is very unlikely to be chance, and far fewer of those cells were caught dividing.

Accept a comparison that quotes both ranges and says they are apart. Do not award the point for comparing the two means alone (16.0% against 1.2%) or for a decision made from the table, which shows one slice only.

Common slip: Comparing 16.0% with 1.2% and stopping. The claim rests on the ±2SE bars, which do not overlap; the means alone could differ by chance.

(d) Explain, in terms of the cell cycle, why so few of the cells 10 mm from the tip are found in any stage of mitosis. (1 point)

A full-credit answer: Near the tip the cells are cycling, so at any instant some are caught in each stage of mitosis. Ten millimeters up the root the cells have left the cycle: they stepped out of G1 into G0, where they carry on their work as root cells without preparing to divide. A cell in G0 never enters mitosis, so almost none of these cells are found in any of its stages.

Check the box for each point your answer earns

the cells 10 mm from the tip have left the cycle, from G1 into G0, where they carry on their work as root cells without preparing to divide, so almost none are ever caught in a stage of mitosis.

Accept "they have stopped cycling and sit in G0". Do not award the point for "they divide faster, so they are caught less often" (a shorter mitosis would lower the count only a little, and these cells show almost none) or for "they are dead" (G0 cells are alive and working).

Common slip: Saying the cells 10 mm from the tip are dividing quickly and so are rarely caught. A quick mitosis would still leave some cells in each stage; almost none here means the cells are out of the cycle, in G0.

Free-response score: 0 of 4

Free response 2 · Conceptual Analysis · 4 points

The drawing shows three cells from the same insect embryo, fixed and stained at one instant and labeled cell 1, cell 2 and cell 3. Each body cell of this insect holds six chromosomes: three long and three short. Every cell in the embryo is cycling.

Three cells from the same insect embryo, fixed and stained at one instant. Each body cell of this insect holds six chromosomes: three long and three short.

(a) Identify the stage of mitosis shown by cell 3, and describe the feature of the drawing that shows it. (1 point)

A full-credit answer: Cell 3 is in prophase. Its chromosomes have coiled into visible rods, each still two sister chromatids joined at a centromere, and they lie scattered rather than in a row; the nuclear envelope around them is breaking into pieces and short spindle fibers are growing in from the centrosome at each pole.

Check the box for each point your answer earns

prophase, shown by the chromosomes condensed into rods (each still two joined sister chromatids, drawn as an X) lying scattered inside a nuclear envelope that is breaking up, with short spindle fibers starting from the two poles.

Accept any one of the three features (scattered rods with no row, the breaking envelope, the spindle starting to grow) with the stage named. Do not award the point for metaphase (the chromosomes are scattered, not in a row) or for interphase (the chromosomes are visible as rods).

Common slip: Naming metaphase because rods with fibers are visible. In metaphase every chromosome stands in one row at the equator; scattered rods with the envelope still breaking up are prophase.

(b) Calculate the number of chromatids in cell 2. (1 point)

Write down the values in the question:

chromosomes = 6

chromatids in each copied chromosome = 2

Write down the equation:

chromatids = 2 \times chromosomes

Substitute the values into the equation:

chromatids = 2 \times chromosomes

chromatids = 2 \times 6 = 12

A full-credit answer: Cell 2 holds 12 chromatids: its six chromosomes were each copied in S phase into two sister chromatids, and at metaphase the sisters are still joined.

Do not award the point for 6 (the chromosome count) or 24 (doubling twice).

(c) Describe the two events, in order, that carry a cell's chromosomes from the arrangement in cell 2 to the arrangement in cell 1. (1 point)

A full-credit answer: First, the connection at each centromere breaks, so the two sister chromatids of every chromosome separate; each one is now counted as a chromosome. Second, the spindle fibers pull the separated chromatids toward opposite poles, so six move toward each pole with their centromeres leading. That is anaphase, and cell 1 is caught in its second event.

Check the box for each point your answer earns

both events in order: first the connection at each centromere breaks so the sister chromatids separate (each now counted as a chromosome), and then the spindle fibers pull the separated chromatids toward opposite poles.

Accept "the sisters split, then the fibers pull one of each pair to each pole". Do not award the point if the pulling is put before the separation, or if only one of the two events is given.

Common slip: Writing that the fibers pull the sisters apart. The pulling comes second: while the sisters are joined, fibers from both poles hold each pair at the equator and nothing moves.

(d) Justify the claim that the two daughter cells formed from cell 1 will each hold six chromosomes carrying the same genome as the parent cell. (1 point)

A full-credit answer: Every chromosome was copied once in S phase into two identical sister chromatids, so cell 1 held twelve chromatids, two copies of each of the six chromosomes. In anaphase the sisters separated and the spindle sent one chromatid of every pair to each pole, so each pole received one copy of every chromosome. Each daughter cell therefore holds six chromosomes, one complete copy of the genome, identical to the parent's.

Check the box for each point your answer earns

every chromosome was copied once in S phase into two identical sister chromatids, and anaphase sent one chromatid of every pair to each pole, so each daughter cell receives one complete copy of the genome: one of each of the six chromosomes, identical to the parent's.

Accept "each daughter gets one copy of every chromosome because each was copied and then the copies were separated". Do not award the point for "the parent shares its six chromosomes out, three to each daughter" or for "the daughters copy their DNA after the division to reach six".

Common slip: Saying the parent divides its six chromosomes into two sets of three. The chromosomes were doubled first, in S phase, so each daughter cell receives all six.

Free-response score: 0 of 4

Multiple choice checked: 0 of 19 correct.

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