Human haemoglobin is a complex protein molecule made up of four poly-peptides joined to an iron-co

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3.   Human haemoglobin is a complex protein molecule made up of four poly-peptides joined to an iron-containing haem group.  In normal human adult haemoglobin, haemoglobin A, (HbA), two different polypeptides designated as alpha (α) and beta (β) are found.  Two identical alpha and two identical beta chains plus the haem group make up each molecule of haemoglobin A.  Haemoglobin S, or HbS, is a haemoglobin variant occurring in individuals affected with the heritable disorder sickle cell anaemia.  A comparison of the amino acid sequences of the polypeptides in haemoglobins A and S indicates that the alpha chains in the two molecules are identical, but that the beta chains differ in a single interior amino acid.
a.   How many genes are necessary for the synthesis of normal functional haemoglobin?



b.   Assuming that the gene for the beta (β) chain of HbS arose from the gene for the beta (β) chain of HbA through a single mutation, what type of mutation was it?



c.   The only difference between the beta (β) chains of haemoglobin S and the beta (β) chain of haemoglobin A is the sixth amino acid from the amino (-NH2) end of the polypeptide.  In haemoglobin S, the amino acid at the site is valine whereas in haemoglobin A it is glutamic acid at the sixth position.  From your knowledge of the genetic code (and with reference to the genetic code) what can you deduce about the specific (and simplest) base alteration in the beta (β) polypeptide that would produce that amino acid substitution?