Explain how mutation can impact genetic code with an example.

Explanation

(i) Comparative studies of mutations (sudden change in a gene) and corresponding alteration in amino acid sequence of specific protein have confirmed the validity of the genetic code.
(ii) The simplest type of mutation at the molecular level is a change in nucleotide that substitutes one base for another and are known as base substitutions which may occur spontaneously or due to the action of mutagens.
(iii) An example is sickle cell anaemia in humans which results from a point mutation of an allele of `beta`-haemoglobin gene (`beta`Hb).
(iv) A haemoglobin molecule consists of four polypeptide chans of two types, two `alpha` chains and two `beta` chains. Each chain has a heme group on its surface. The heme groups are involved in the binding of oxygen.
(v) The human blood disease, sickle cell anaemia is due to abnormal haemoglobin due to a single base substitution at the sixth codon of the beta globin gene from GAG to GTG in `beta`-chain of haemoglobin. It results in a change of amino acid glutamic acid to valine at the `6^(th)` position of `beta`-chain.
(vi) This is point mutation that results in the change of amino acids residue glutamic acid to valine. The mutant haemoglobin undergoes polymerisation under oxygen tension causing the change in the shape of the RBC from biconcave to a sickle shaped structure.