Cell and Molecular Biology: Concepts and Experiments

Gerald Karp

Chapter 11

Gene Expression: From Transcription to Translation - all with Video Answers

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Chapter Questions

Problem 1

Look at the codon chart of Figure $11.40 ;$ which codons would you expect to have a unique tRNA, that is, one that is used only for that codon? Why is it that many codons do not have their own unique tRNA?

Bryan Valdivia

Bryan Valdivia

Numerade Educator

Problem 2

Proflavin is a compound that inserts itself into DNA and causes frameshift mutations (page 473 ). How would the effect on the amino acid sequence of a proflavin-induced mutation differ between an overlapping and a nonoverlapping code?

Rabeya Zahid

Numerade Educator

Problem 3

You have just isolated a new drug that has only one effect on cell metabolism; it totally inhibits the breakdown of pre-rRNA to ribosomal RNA. After treating a culture of mammalian cells with this drug, you give the cells $\left[^{3} \mathrm{H}\right]$ uridine for 2 minutes, and then grow the cells in the presence of the drug in unlabeled medium for 4 hours before extracting the $\mathrm{RNA}$ and centrifuging it through a sucrose gradient. Draw the curves you would obtain by plotting both absorbance at $260 \mathrm{nm}$ and radioactivity against the fraction number of the gradient. Label the abscissa $(X$ -axis) using S values of RNA.

Hailey Tomashek

Hailey Tomashek

Numerade Educator

Problem 4

Using the same axes as in the previous question, draw the profile of radioactive RNA that you would obtain after a culture of mammalian cells had been incubated for 48 hours in $\left[^{3} \mathrm{H}\right]$ uridine without any inhibiting drugs present.

Hailey Tomashek

Hailey Tomashek

Numerade Educator

Problem 5

Suppose you were to construct a synthetic RNA from a repeating dinucleotide (e.g., AGAGAGAG) and then use this RNA as a messenger to synthesize a polypeptide in an in vitro protein-synthesizing system, such as that used by Nirenberg and Matthaei to produce polyphenylalanine. What type of polypeptide would you make from this particular polynucleotide? Would you expect to have more than one type of polypeptide produced? Why or why not?

Rabeya Zahid

Numerade Educator

Problem 6

Suppose that you found an enzyme that incorporated nucleotides randomly into a polymer without the requirement for a template. How many different codons would you be able to find in synthetic RNAs made using two different nucleotide precursors (e.g., CTP and ATP)? (An enzyme called polynucleotide phosphorylase catalyzes this type of reaction and was used in studies that identified codons.)

Sana Riaz

Numerade Educator

Problem 7

Draw the parts of a $15 \mathrm{S}$ globin pre-mRNA, labeling the noncoding portions.

Marisa A

Numerade Educator

Problem 8

What is the minimum number of GTPs that you would need to synthesize a pentapeptide in a bacterium?

Rabeya Zahid

Numerade Educator

Problem 9

On the same set of axes as in Figure 10.17 , draw two reannealing curves, one for DNA extracted from Xenopus brain tissue and the other for DNA extracted from $X$enopus oocytes.

Hailey Tomashek

Hailey Tomashek

Numerade Educator

Problem 10

Would you agree with the following statement? The discovery that sickle cell anemia resulted from a single amino acid change was proof that the genetic code was nonoverlapping. Why or why not?

Bryan Valdivia

Numerade Educator

Problem 11

Thalassemia is a disease characterized by mutations that convert amino acid codons into stop codons. Suppose you were to compare the polypeptides synthesized in vitro from mRNAs purified from a wide variety of thalassemia patients. How would you expect these polypeptides to compare? Refer to the codon chart of Figure $11.40 ;$ how many amino acid codons can be converted into stop codons by a single base substitution?

Rabeya Zahid

Numerade Educator

Problem 12

Do you think it would be theoretically possible to have a genetic code with only two letters, A and T? If so, what would be the minimum number of nucleotides required to make a codon?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 13

On page 454 , experiments are reported that lead to the synthesis of ribozymes with unique catalytic properties. In 2001 , an artificial ribozyme was isolated that was capable of incorporating up to 14 ribonucleotides onto the end of an existing RNA using an RNA strand as a template. The ribozyme could use any RNA sequence as a template and would incorporate complementary nucleotides into the newly synthesized RNA strand with an accuracy of 98.5 percent. If you were a proponent of an ancient RNA world, how would you use this finding to argue your case? Would this prove the existence of an ancient RNA world? If not, what do you think would provide the strongest evidence for such a world?

Sana Riaz

Numerade Educator

Problem 14

How is it possible that mRNA synthesis occurs at a greater rate in bacterial cells than any other class, yet very little mRNA is present within the cell?

Miwa Wenzel

Numerade Educator

Problem 15

If a codon for serine is $5^{\prime}-\mathrm{AGC}-3^{\prime},$ the anticodon for this triplet would be $5^{\prime}----3^{\prime} .$ How would the wobble phenomenon affect this codon-anticodon interaction?

Fatmah Hani

Numerade Educator

Problem 16

One of the main arguments that proteins evolved before DNA (i.e., that the RNA world evolved into an RNA-protein world rather than an RNA-DNA world) is based on the fact that the translational machinery involves a large variety of RNAs (e.g., tRNAs, rRNAs), whereas the transcriptional machinery shows no evidence of RNA involvement. Can you explain how such an argument about the stages of early evolution might be based on these observations?

Miwa Wenzel

Numerade Educator

Problem 17

Frameshift mutations and nonsense mutations were described on page $474 .$ It was noted that nonsense mutations often lead to the destruction by NMD of an mRNA containing a premature termination codon. Would you expect mRNAs containing frameshift mutations to be subject to NMD?

Bryan Valdivia

Numerade Educator

Problem 18

The arrowheads in Figure 11.16 indicate the direction of transcription of the various tRNA genes. What does this drawing tell you about the template activity of each strand of a DNA molecule within a chromosome?

Rabeya Zahid

Numerade Educator

Problem 19

Genes are usually discovered by finding an abnormal phenotype resulting from a genetic mutation. Alternatively, they can often be identified by examining the DNA sequence of a genome. Why do you suppose that the genes encoding miRNAs were not discovered until very recently?

Miwa Wenzel

Numerade Educator

Problem 20

It was suggested on page 464 that synonymous codon changes do not generally alter an organism's phenotype. Can you think of an occasion where this might not be true? What does this say about the coding requirements of the genetic material?

Miwa Wenzel

Numerade Educator