Genetic Code and Translation Questions and Answers

Answer: B

The factor phosphorylated is eIF2.

In reticulocytes, globin synthesis is matched to heme supply. When heme is scarce, the heme-regulated inhibitor (HRI) kinase becomes active and phosphorylates the α subunit of eIF2. Phosphorylated eIF2 binds its recycling factor eIF2B too tightly, so eIF2-GDP is not exchanged to eIF2-GTP, initiator tRNA is not delivered, and translation initiation stops. Heme inhibits HRI, so adequate heme keeps globin synthesis running.

EF1 and EFG are elongation factors, and eIF3 binds the small subunit and helps recruit mRNA; none is the target of this control.

Answer: A

The initiation site is the AUG codon preceded by the Shine-Dalgarno polypurine sequence, the paper's 'Shine-Dalgamo polypurine hexamer'.

In bacteria the start codon, usually AUG, lies a few bases downstream of a purine-rich stretch (consensus AGGAGG) that pairs with the 3' end of the 16S rRNA. That pairing places the start codon in the P site of the small ribosomal subunit, so the right AUG is chosen even in the middle of a polycistronic mRNA.

The 5' cap and its 7-methylguanosine (m7G), which appear in the other options, belong to eukaryotic mRNA, where the small subunit binds the cap and scans for the first AUG. UAG is a stop codon, and AGG codes for arginine; neither is a start codon. GUG can occasionally start bacterial genes, but it is not paired with a cap.

Answer: A

The CCA sequence lies at the 3' termini of tRNAs.

Every tRNA has an acceptor stem in which the 5' and 3' ends pair, and the 3' strand runs on as a single-stranded CCA tail. The amino acid is attached by aminoacyl-tRNA synthetase to the 2' or 3' hydroxyl of the terminal adenosine, so this end carries the amino acid into the ribosome.

The 5' end is the other strand of the acceptor stem and has no CCA. The anticodon arm is at the opposite end of the molecule and carries the anticodon that reads the mRNA. 'Spacer arm' is not a part of tRNA structure; the variable arm lies between the anticodon and T arms.

Answer: B

UGA codes for tryptophan in mitochondrial DNA.

In the standard genetic code UGA is a stop codon. Mitochondria of vertebrates, fungi and many other organisms use a variant code in which UGA is read as tryptophan, one of several mitochondrial departures from the universal code.

Chloroplasts use the standard code, so in chloroplast DNA UGA remains a stop codon; that rules out the chloroplast and 'both' options. In nuclear genes UGA is a stop codon, apart from its special use for selenocysteine in a few genes.