Question

1) When a ribosome encounters a stop codon, what happens to terminate transcription?

a)tRNAs that do not have an amino acid enter the A site to release the polypeptide

b)the small ribosomal subunit immediately dissociates from the large subunit of the ribosome to terminate translation

c)special proteins recognize the stop codon and release the polypeptide from the ribosome

d)the ribosome scans for a polyadenylation site and adds a stop tRNA to the peptide

e) the 5’ methyl-guanine cap loops to the ribosome and pushes off the growing peptide.

2) A tRNA with the anticodon 5’-UAG-3’ would carry the amino acid:

(note the orientation of the anticodon)

a)phenylalanine

b) tyrosine

c) serine

d)threonine

e)tryptophan

f) lysine

g) methionine

h) leucine

i) alanine

Answer 1

1. C. special proteins recognize the stop codon and release the polypeptide from the ribosome

Note: When ribosome meets a termination codon, this codon is recognized by special proteins called Releasing Factors (RFs) which bind to 'A' site of the ribosome.

The RF1 identifies termination codon UAA and UAG, while factor RF2 recognizes UGA.

2. h. Leucine

Note: The anticodon mentioned is in 5' to 3' direction. So 1st changing it into 3' to 5' (which is in a tRNA) it becomes 3'GAU 5'. If GAU is anticodon, then its respective codon would be 5' CUA 3' ,which encodes the amino acid Leucine.