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Exam 15: The Genetic Code and Translation

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Question 31

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A tRNA anticodon is 5' GCU 3'. What amino acid does it carry?

A) ala
B) arg
C) ser
D) pro
E) thr

F) A) and B)
G) C) and D)

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Question 32

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An auxotrophic E. coli strain requires adenine to grow because of a mutation in a gene for an adenine synthesis enzyme. The following shows part of the wild-type and mutant alleles of the gene, including the start codon. The bottom strand is the template for transcription. $\begin{array} { c c } \text { ade } 1 ^ { + } \text {wild-type allele } & \text { ade1 } ^ { - } \text {mutant allele } \\\text { 5'...TTATGGGCAAGATCCCA...3' } & \text { 5'....TTA TGGGCTAGATCCCA...3' } \\\text { 3'...AATACCCGTTCTAGGGT...5' } & \text { 3'...AATATGGGCTAGATCCCA...3' }\end{array}$ - E. coli strains that have both the original ade1^- mutant allele shown above and a mutant allele for a gene that encodes a lysine tRNA are able to make some normal ade1 enzyme. The mutant lysine tRNA allele makes tRNA that has one base that is different from the wild-type tRNA. Explain how the lysine tRNA mutation allows synthesis of the normal ade1 enzyme.

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The mutant lysine tRNA allele likely all...

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Question 33

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What was Beadle and Tatum's definition of a gene? Give a modern definition and explain how it differs from that of Beadle and Tatum.

An auxotrophic E. coli strain requires adenine to grow because of a mutation in a gene for an adenine synthesis enzyme. The following shows part of the wild-type and mutant alleles of the gene, including the start codon. The bottom strand is the template for transcription. $\begin{array} { c c } \text { ade } 1 ^ { + } \text {wild-type allele } & \text { ade1 } ^ { - } \text {mutant allele } \\\text { 5'...TTATGGGCAAGATCCCA...3' } & \text { 5'....TTA TGGGCTAGATCCCA...3' } \\\text { 3'...AATACCCGTTCTAGGGT...5' } & \text { 3'...AATATGGGCTAGATCCCA...3' }\end{array}$ -Explain why the mutant lysine tRNA allele from the previous question might affect overall cell growth.

Describe the events in prokaryotic translation elongation.

Prokaryotic ribosomes are inhibited by antibiotics like tetracyclines. As eukaryotes, humans are unaffected by these antibiotics. However, ribosomes within human mitochondria more closely resemble prokaryotic ribosomes than eukaryotic ribosomes. Why aren't tetracyclines more toxic to humans if they can, in theory, inhibit mitochondrial ribosomes?

Use the following to answer questions : -After the peptide bond forms, what will happen next?

The antibiotic streptomycin binds to the 30S subunit in bacteria and interferes with translation. Streptomycin resistance (SmR) can occur through mutation in the rpsL gene, which encodes the ribosomal protein S12. The antibiotic can no longer bind to the altered protein, so translation can occur without interference from the antibiotic. However, in the absence of streptomycin, many SmR strains of bacteria grow less well than the non-mutant streptomycin-sensitive (SmS) bacteria. a. Propose a model for why SmR bacteria grow better than SmS bacteria in the presence of streptomycin but not as well in the absence of streptomycin. b. During translation, an incorrect amino acid is occasionally incorporated into the polypeptide chain-that is, the amino acid added does not correspond to the codon at that location in the mRNA. You develop an assay that can assess this rate of incorrect amino acid incorporation and thereby estimate the accuracy of translation in bacteria. You find that SmR bacteria are more accurate than SmS bacteria in this experiment. How might this observation affect your model above?

Codons that specify the same amino acid are said to be:

The function of aminoacyl-tRNA synthetases is to:

List three differences between prokaryotic and eukaryotic translation.

Which of the following mechanisms specifically allows detection and rapid degradation of mRNA containing a premature termination codon?

Which molecule allows the release of mRNA from a stalled ribosome?

During initiation, the _____ subunit is the first part of the ribosome to associate with the mRNA.

Use the following to answer questions Refer to the following sequence: 5' ...GGAGCUCGUUGUAUU... 3' -The sequence 5' ...GGAGCUCGUUGUAUU... 3'is changed to 5'… GGAGACUCGUUGUAUU… 3'. What would be the effect on the amino acid sequence?

A yeast strain was exposed to a chemical mutagen. As expected, exposure to a mutagen resulted in a DNA sequence change in an essential gene you examined. Yet this mutation did not result in any lethal phenotype. How do you explain this apparent discrepancy?

The genetic code is said to be "degenerate" because:

There are _____ different codons, which encode 20 amino acids and 3 stop codons.

Use the following to answer questions Refer to the following sequence: 5' ...GGAGCUCGUUGUAUU... 3' -This sequence is RNA because:

Use the following to answer questions -If the bottom strand of the DNA serves as the template, the amino acid sequence of the protein produced from the RNA would be: