Question 3.

Human haemoglobin is a complex protein molecule made up of fourpoly-peptides joined to an iron-containing haem group. In normalhuman adult haemoglobin, haemoglobin A, or HbA, two kinds ofpolypeptides designated as alpha and beta are found. Two identicalalpha and two identical beta chains plus the haem group make upeach molecule of haemoglobin A. Haemoglobin S, or HbS, is ahaemoglobin variant occurring in individuals affected with theheritable disorder sickle cell anaemia. A comparison of the aminoacid sequences of the polypeptides in haemoglobins A and Sindicates that the alpha chains in the two molecules are identical,but that the beta chains differ in a single interior aminoacid.

a.   How many of genes arenecessary for the synthesis of haemoglobin S in an adult who ishomozygous for the S allele?

b.   Assuming that thegene for the beta chain of HbS arose from the gene for the betachain of HbA through a single mutation, what general type ofmutation was most likely responsible?

c.   The differencebetween the beta chains of haemoglobin S and A is restricted to thesixth amino acid from the amino (-NH₂) end of thepolypeptide. In haemoglobin S, the amino acid at the site is valineand in haemoglobin A it is glutamic acid. From your knowledge ofthe genetic code (and with reference to the genetic code) what canyou deduce about the specific (and simplest) base alteration in thebeta polypeptide that would produce the amino acidsubstitution?