Question

Notice that the first amino acid in the HBB protein starts with a valine (Val) instead of a methionine (Met). What is the most likely explanation for this based on what we have discussed about gene expression?
(A) Blood cells have a completely different way of starting translation in all their genes.
(B) The messenger RNA for HBB carries a mutation in the start codon.
(C) The start codon for the gene is one of the codons for valine (i.e. GUA, GUC, GUG or GUU).
(D) The protein that is first made gets modified afterwards

Answer 1

Answer:

(D) The protein that is first made gets modified afterwards

Explanation:

The first amino acid in any protein in eukaryotes is always methionine (Met). But, in some proteins any amino acid other than methionine may be found at N terminus of a protein. Like in this example, the HBB protein starts with a valine (Val) instead of methionine. It happened because of post-translational modification which includes the action of aminopeptidases which cleave 1 or 2 amino acids from N terminus of a protein as a result of which any  amino acid other than methionine may exist at amino terminus.