This is a True or False question However I chose False for b

This is a True or False question. However, I chose False for both of the answers and it was incorrect. Can you please explain why both of these answers are TRUE?

25. Charged tRNAs always arrive at the A-site associated with GTP-bound EF-Tu (elongation factor), which will not convert GTP to GDP unless correct codon-anticodon pairing occurs in the A-site. 26. The anticodon of a single charged tRNA can often pair with multiple codons.

Solution

The ribosome has three sites to bind for tRNA- aminoacyl site (A), peptidyl site (P), and exit site (E).

1) The incoming aminoacyl tRNA is carried to the ribosome by GTP- bound EF-Tu and binds to the A-site with complementary base pairing with the mRNA at the A-site. EF-Tu hydrolysis GTP to GDP, when the codon- anticodon pairing is correct. This pairing inturn induces a conformational change in EF-Tu which then dissociates from the tRNA. It delays the GTP hydrolysis if anticodon sequence of tRNA does not match the mRNA codon sequence, thereby providing a chance for the incorrect tRNA to leave the complex.

2) The anticodon of a single charged tRNA can often pair with multiple codons. According to the Wobble hypothesis a single tRNA can recognise and bind to more than one codon, because of less precise base pairing between third base of codon and first base of anticodon.

Because the genetic code is degenerate, i.e., different codons on mRNA code for same amino acid. That is why single tRNA (charged) can pair with with multiple codons.