Genetic Code and Translation Questions and Answers

Answer: D

The pair that does not undergo post-translational modification is Ala-Gly.

Post-translational modifications act on reactive side chains, and alanine (a methyl group) and glycine (a hydrogen atom) have none to modify. (An N-terminal glycine can be myristoylated, but that acts on the chain's terminal amino group, not on a side chain.)

Every other pair includes a commonly modified residue. Asparagine is the site of N-linked glycosylation. Serine and tyrosine are phosphorylated by kinases, and serine also carries O-linked sugars. Histidine can be phosphorylated (as in bacterial two-component signalling) and methylated.

Answer: D

The compound that is not a translation inhibitor is rifampicin.

Rifampicin binds the β subunit of bacterial RNA polymerase and blocks transcription, the step before translation, which is why it is used against Mycobacterium tuberculosis.

The other three act on the ribosome. Chloramphenicol binds the 50S subunit and inhibits peptidyl transferase. Cycloheximide blocks translocation on eukaryotic 80S ribosomes. Puromycin mimics the 3′ end of an aminoacyl-tRNA, is joined to the growing chain and causes its premature release.

Answer: B

Arg is encoded by AGA.

RNA₁, (AG)_n, reads AGA GAG AGA ... in every frame, so its only codons are AGA and GAG, giving an alternating Arg-Glu product. RNA₂, (AAG)_n, gives a different codon in each frame: AAG, AGA and GAA, one homopolymer each of Lys, Arg and Glu.

Arg appears in both products, so its codon must be one both templates contain. The only codon common to the two sets is AGA, so AGA codes for Arg. It follows that GAG, the other codon of RNA₁, is Glu; then GAA, the remaining RNA₂ codon besides AGA, also gives Glu, leaving AAG for Lys.

AAG and GAA are absent from RNA₁, and GAG is absent from RNA₂, so none of them could account for Arg in both products.

Answer: D

The incorrect statement is Protein can be reverse-translated to mRNA.

The central dogma allows information to pass from DNA to RNA (transcription), from RNA back to DNA (reverse transcription, by retroviral reverse transcriptase) and from mRNA to protein (translation). It does not allow transfer from protein back to nucleic acid: no enzyme reads an amino acid sequence to build mRNA, and the degeneracy of the code would leave the sequence undetermined anyway.

The other three statements describe processes that do occur.

Answer: D

Codon bias correlates with the relative abundance of tRNA.

Synonymous codons for the same amino acid are not used equally. In highly expressed genes the preferred codons are those read by the most abundant tRNA species in the cell, which speeds and improves the accuracy of translation. Codon usage tables and tRNA gene copy numbers track each other across organisms.

mRNA carries the codons but its abundance does not explain which synonym is preferred. rRNA forms the ribosome and is the same for every codon, and siRNA acts in gene silencing, not in decoding.

Answer: D

The answer is 20.

Each aminoacyl-tRNA synthetase is specific for one amino acid and attaches it to every tRNA that carries that amino acid, including all the isoaccepting tRNAs that read its different synonymous codons. So 20 amino acids need 20 synthetases, not one per codon.

61 is the number of sense codons and 64 the total number of codons including the three stop codons; neither counts synthetases. 40 is not tied to any feature of the code. Fewer than 61 distinct tRNAs are needed in any case, because wobble pairing lets one anticodon read more than one codon.

Answer: B

The sequence is the Shine-Dalgarno sequence.

In bacteria a purine-rich sequence (consensus AGGAGG) lies a few nucleotides upstream of the start codon on the mRNA. It pairs with a complementary sequence at the 3' end of the 16S rRNA in the small (30S) ribosomal subunit, which positions the ribosome so that the initiation codon sits in the P site.

AUG is the start codon itself, read by the initiator tRNA rather than recognised by base-pairing with rRNA. UAG is a stop codon. The Pribnow box (TATAAT) is a promoter element in DNA recognised by RNA polymerase, not a sequence in mRNA.

Answer: C

The protein has 73 amino acids.

Each codon is three nucleotides, so the mRNA stretch described holds:

\[ \begin{aligned} \frac{222}{3} &= 74 \text{ codons} \\ 74 - 1 &= 73 \text{ amino acids} \end{aligned} \]

The initiator codon AUG codes for methionine and is counted, but the termination codon codes for no amino acid; it is read by a release factor that ends translation. So one is subtracted for the stop codon only.

74 is the number of codons, the trap of forgetting that the stop codon adds nothing. 221 and 111 come from counting nucleotides or pairs instead of triplets.