Unit 6 · Practice for the Topic 6.4 end-of-topic test
You’ve gone through everything in this topic. The summary video below recaps it all, so you’re ready for the questions.
Watch first: Translation, summed up
Where translation happens; codons, the code chart, AUG and the three stops; redundancy and the near-universal code; initiation, elongation, termination; codon and anticodon; genotype to phenotype through the chain; the retrovirus that runs the flow backward.
A cell lining a hen's oviduct builds ovalbumin, a protein it releases into the egg white.
Where is ovalbumin built?
Suppose a gene switches on in a bacterium and a gene of the same length switches on in a yeast cell. The table shows how long each cell takes to finish copying the gene into mRNA, and how long until the first ribosome starts translating that mRNA.
Which statement explains why translation starts so much sooner in the bacterium?
Read from the first AUG in threes and stop at the first stop codon; the stop codon adds no amino acid. A ribosome reads the mRNA drawn.
Which codon does the ribosome read third?
Three RNA bases make one codon, so there are 64 different codons.
Which statement about the 64 codons is correct?
The genetic code chart is drawn.
How many codons name histidine (His)?
A gene from a moss is put into a yeast cell. The yeast cell's ribosomes read the gene's mRNA and build the moss protein, with every amino acid in its place.
What does this result show?
Read from the first AUG in threes and stop at the first stop codon; the stop codon adds no amino acid. The mRNA drawn above the genetic code chart is translated.
How many amino acids does the polypeptide contain?
A strand is written from its 5′ end to its 3′ end, and both ends are marked. Codons are written as they read on the mRNA, 5′ to 3′; an anticodon is written 3′ to 5′ against its codon. A tRNA's anticodon reads 3′-GAA-5′, as drawn.
Which codon on an mRNA does this tRNA pair with?
A scientist measures how long a ribosome spends at each codon of an mRNA. The bar chart shows the time spent at the four codons that name alanine (Ala), relative to the fastest of them.
At which codon does the ribosome spend the longest time?
The bar chart shows the time a ribosome spends at each of the four codons that name alanine (Ala), relative to the fastest. A cell makes two mRNAs for the same polypeptide: one uses 5′-GCA-3′ at every alanine, the other uses 5′-GCC-3′ at every alanine.
Which mRNA is translated faster, and why?
(a) Identify the amino acid that the codon 5′-UGC-3′ names. (1 point)
A full-credit answer: 5′-UGC-3′ names Cys.
Check the box for each point your answer earns
(b) Describe what the loaded tRNA that pairs with 5′-UGC-3′ does at that codon in a cell where the loading enzyme works. (1 point)
A full-credit answer: In a working cell, the tRNA pairs its anticodon with the codon UGC and brings Cys into the ribosome.
The ribosome joins that Cys to the growing chain.
Check the box for each point your answer earns
(c) Predict how the polypeptide built in the changed yeast cell differs from the polypeptide a working cell builds from this mRNA. (1 point)
A full-credit answer: In the changed cell, the ribosome joins Met–Leu and then stops at the third codon.
The polypeptide is left unfinished, two amino acids long, instead of Met–Leu–Cys–Pro–Gly.
Check the box for each point your answer earns
A prediction that the ribosome skips UGC and builds Met–Leu–Pro–Gly (the chain with Cys left out) earns nothing.
(d) Justify your prediction. (1 point)
A full-credit answer: The ribosome stops there because only a tRNA carrying Cys can bring Cys to the codon UGC.
With the loading enzyme stopped, no tRNA carries Cys.
The ribosome joins the next amino acid and moves on only after a loaded tRNA has paired.
So it waits at UGC, and the chain grows no further.
Check the box for each point your answer earns
(a) Identify the molecule that the host cell's RNA polymerase builds from the DNA copy in an untreated cell. (1 point)
A full-credit answer: The host's RNA polymerase builds RNA from the DNA copy: new copies of the virus's RNA genome and mRNAs for the virus's proteins.
Check the box for each point your answer earns
(b) Describe what the host cell's ribosomes do with the molecule you identified in part (a), in an untreated cell. (1 point)
A full-credit answer: The host's ribosomes read the virus's mRNA codon by codon.
They build the virus's proteins: its coat proteins and its reverse transcriptase.
Check the box for each point your answer earns
(c) Make a claim about whether the treated cell builds new virus particles while the drug is present, and a second claim about whether the DNA copy is passed on when the treated cell divides. (2 points)
A full-credit answer: The treated cell builds no new virus particles while the drug is present.
The DNA copy is passed to both daughter cells when the cell divides.
Check the box for each point your answer earns
Accept for the first point: no new particles once any viral mRNA already present is used up.
Common slip: Claiming that the drug removes the copy. The drug blocks reading of the copy; the copy itself stays in the chromosome.
(d) Support both claims. (2 points)
A full-credit answer: With the promoter blocked, the host's RNA polymerase makes no RNA from the copy.
So the ribosomes get no viral mRNA and build no coat proteins or reverse transcriptase, and no new genome RNA is made.
So no particles can be assembled.
The copy is part of a chromosome, so the cell copies it in S phase with the rest and each daughter cell receives it.
Check the box for each point your answer earns