Question

A human hemoglobin variant occurs in certain populations. The variant hemoglobin is slightly defective in its oxygen carrying function, causing a mild anemia. Normal alpha-hemoglobin has 141 amino acids, while the variant α-polypeptide is 150 amino acids long.

Beginning with the final amino acid coding codon (CGU Arg) the 3' region of the normal α-globin mRNA has the sequence:

5’ ...CGU UAA CCU UCG GUA GCA UGU GAU CCU CAC UAG GCC UCC GGG... 3’

a) Based on this information, explain how the variant α-hemoglobin is likely generated.

b) What is the sequence of the four C terminal amino acids of the variant α-hemoglobin.

Answer 1

a.

The second codon from 5' end in the above strand is a stop codon. That is why the translation stops there to make it a 141 amino acid chain.

A point mutation from UAA to UAC or UAU will convert it in to a code for Tyrosine amio acid. Because of this the rest of the triplet codons also code and make it in to a 150 amino acid long. Again UAG is a atop codon. So, when the translation reaches UAC, it does not code for any amino acid, so the translation stops here. This is how 9 triplet codons get added and 9 amino acids are coded more making it 150 amino acid chain. It is a transversion mutation because a purine (A) is changed to pyrimidine (C/U).

b.

In a polypeptide chain, the left end of the polypeptide is called N terminus because it is hear the amio group is free. On the right hand is the carboxyl end because here the carboxyl group is free.

UGU,GAU,CCU,CAC are the sequence of codons towards the C terminal end. They code for following amino acids.

UGU--Cys

GAU--Asp

CCU---Pro

CAC---His

Cys-Asp-Pro-His are the four amino acids towards the carboxyl end or C terminal end of the polypeptide chain of the variant hemoglobin.