Essentials of Genetics

William S. Klug, Michael R. Cummings, Charlotte A. Spencer

Chapter 12

The Genetic Code and Transcription - all with Video Answers

Educators

Chapter Questions

Problem 1

In this chapter, we focused on the genetic code and the transcription of genetic information stored in DNA into complementary RNA molecules. Along the way, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions:
(a) How did we determine the compositions of codons encoding specific amino acids?
(b) How were the specific sequences of triplet codes determined experimentally?
(c) How were the experimentally derived triplet codon assignments verified in studies using bacteriophage MS2?
(d) How do we know that mRNA exists and serves as an intermediate between information encoded in DNA and its concomitant gene product?
(e) How do we know that the initial transcript of a eukaryotic gene contains noncoding sequences that must be removed before accurate translation into proteins can occur?

Jennifer Stoner

Jennifer Stoner

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Problem 2

Review the Chapter Concepts list on p. $218 .$ These all center on how genetic information is stored in DNA and transferred to RNA prior to translation into proteins. Write a short essay that summarizes the key properties of the genetic code and the process by which RNA is transcribed on a DNA template.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 3

In studies of frameshift mutations, Crick, Barnett, Brenner, and Watts-Tobin found that either three nucleotide insertions or deletions restored the correct reading frame.
(a) Assuming the code is a triplet, what effect would the addition or loss of six nucleotides have on the reading frame?
(b) If the code were a sextuplet (consisting of six nucleotides), would the reading frame be restored by the addition or loss of three, six, or nine nucleotides?

Jackson Miner

Numerade Educator

Problem 4

The mRNA formed from the repeating tetranucleotide UUAC incorporates only three amino acids, but the use of UAUC incorporates four amino acids. Why?

Jackson Miner

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Problem 5

In studies using repeating copolymers, AC $\ldots$ incorporates threonine and histidine, and CAACAA $\ldots$ incorporates glutamine, asparagine, and threonine. What triplet code can definitely be assigned to threonine?

Jackson Miner

Numerade Educator

Problem 6

In a coding experiment using repeating copolymers (as shown in Table 12.3 ), the following data were obtained. AGG is known to code for arginine. Taking into account the wobble hypothesis, assign each of the four remaining different triplet codes to its correct amino acid.

Khalida Dawar

Numerade Educator

Problem 7

In the triplet binding assay technique, radioactivity remains on the filter when the amino acid corresponding to the experimental triplet is labeled. Explain the basis of this technique.

Sam Limsuwannarot

Sam Limsuwannarot

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Problem 8

When the amino acid sequences of insulin isolated from different organisms were determined, some differences were noted. For example, alanine was substituted for threonine, serine was substituted for glycine, and valine was substituted for isoleucine at corresponding positions in the protein. List the single-base changes that could occur in triplets to produce these amino acid changes.

Jackson Miner

Numerade Educator

Problem 9

In studies of the amino acid sequence of wild-type and mutant forms of tryptophan synthetase in $E .$ coli, the following changes have been observed:
Determine a set of triplet codes in which only a single-nucleotide change produces each amino acid change.

Jackson Miner

Numerade Educator

Problem 10

Why doesn't polynucleotide phosphorylase (Ochoa's enzyme) synthesize RNA in vivo?

Mikayla Stephens

Mikayla Stephens

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Problem 11

Refer to Table $12.1 .$ Can you hypothesize why a mixture of (Poly U) $+$ (Poly A) would not stimulate incorporation of $^{14} \mathrm{C}$ -phenylalanine into protein?

Kathleen Pankow

Kathleen Pankow

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Problem 12

Predict the amino acid sequence produced during translation of the short theoretical mRNA sequences below. (Note that the second sequence was formed from the first by a deletion of only one nucleotide.) What type of mutation gave rise to sequence $2 ?$

Jackson Miner

Numerade Educator

Problem 13

A short RNA molecule was isolated that demonstrated a hyperchromic shift indicating secondary structure (see p. 177 in Chapter 9 ). Its sequence was determined to be
(a) Propose a two-dimensional model for this molecule.
(b) What DNA sequence would give rise to this RNA molecule through transcription?
(c) If the molecule were a tRNA fragment containing a CGA anticodon, what would the corresponding codon be?
(d) If the molecule were an internal part of a message, what amino acid sequence would result from it following translation? (Refer to the code chart in Figure 12.7 .)

Rashmi Sinha

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Problem 14

A glycine residue exists at position 210 of the tryptophan synthetase enzyme of wild-type $E .$ coli. If the codon specifying glycine is GGA, how many single-base substitutions will result in an amino acid substitution at position 210 , and what are they? How many will result if the wild-type codon is GGU?

Jackson Miner

Numerade Educator

Problem 15

Shown here is a theoretical viral mRNA sequence
5'-AUGCAUACCUAUGAGACCCUUGGA-3'
(a) Assuming that it could arise from overlapping genes, how many different polypeptide sequences can be produced? Using the chart in Figure $12.7,$ what are the sequences?
(b) A base-substitution mutation that altered the sequence in part (a) eliminated the synthesis of all but one polypeptide. The altered sequence is shown below. Use Figure 12.7 to determine why it was altered.
$5^{\prime}-$ AUGCAUACCUAUGUGACCCUUGGA-3

Shiksha Dutta

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Problem 16

Most proteins have more leucine than histidine residues but more histidine than tryptophan residues. Correlate the number of codons for these three amino acids with this information.

Jackson Miner

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Problem 17

Define the process of transcription. Where does this process fit into the central dogma of molecular genetics?

Jackson Miner

Numerade Educator

Problem 18

Describe the structure of RNA polymerase in bacteria. What is the core enzyme? What is the role of the $\sigma$ factor?

Alexander Clippinger

Alexander Clippinger

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Problem 19

In a written paragraph, describe the abbreviated chemical reactions that summarize RNA polymerase-directed transcription.

Alexander Clippinger

Alexander Clippinger

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Problem 20

Messenger RNA molecules are very difficult to isolate from bacteria because they are quickly degraded. Can you suggest a reason why this occurs? Eukaryotic mRNAs are more stable and exist longer in the cell than do bacteria mRNAs. Is this an advantage or a disadvantage for a pancreatic cell making large quantities of insulin?

Dennis Howard

Numerade Educator

Problem 21

One form of posttranscriptional modification of most eukaryotic RNA transcripts is the addition of a poly-A tail at the $3^{\prime}$ -end. The absence of a poly-A tail leads to rapid degradation of the transcript. Poly-A tails of various lengths are also added to many bacterial RNA transcripts where, instead of promoting stability, they enhance degradation. In both cases, RNA secondary structures, stabilizing proteins, or degrading enzymes interact with poly-A tails. Considering the activities of RNAs, what might be the general functions of $3^{\prime}$ -polyadenylation??

Shiksha Dutta

Numerade Educator

Problem 22

In a mixed heteropolymer experiment, messages were created with either $4 / 5 \mathrm{C}: 1 / 5 \mathrm{A}$ or $4 / 5 \mathrm{A}: 1 / 5 \mathrm{C}$. These messages yielded proteins with the amino acid compositions shown in the following table. Using these data, predict the most specific coding composition for each amino acid.

Khalida Dawar

Numerade Educator

Problem 23

Shown in this problem are the amino acid sequences of the wild type and three mutant forms of a short protein.
(a) Using Figure $12.7,$ predict the type of mutation that created each altered protein.
(b) Determine the specific ribonucleotide change that led to the synthesis of each mutant protein.
(c) The wild-type RNA consists of nine triplets. What is the role of the ninth triplet?
(d) For the first eight wild-type triplets, which, if any, can you determine specifically from an analysis of the mutant proteins? In each case, explain why or why not.
(e) Another mutation (mutant 4 ) is isolated. Its amino acid sequence is unchanged, but mutant cells produce abnormally low amounts of the wild-type proteins. As specifically as you can, predict where this mutation exists in the gene.

Jennifer Stoner

Jennifer Stoner

Numerade Educator

Problem 24

Alternative splicing is a common mechanism for eukaryotes to expand their repertoire of gene functions. At least one estimate indicates that approximately 50 percent of human genes use alternative splicing, and approximately 15 percent of diseasecausing mutations involve aberrant alternative splicing. Different tissues show remarkably different frequencies of alternative splicing, with the brain accounting for approximately 18 percent of such events.
(a) Define alternative splicing and speculate on the evolutionary strategy alternative splicing offers to organisms.
(b) Why might some tissues engage in more alternative splicing than others?

Shiksha Dutta

Numerade Educator