. . Mutation and genetic code 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. The relationship between genes and DNA are best understood by mutation studies.
The simplest type of mutation at the molecular level is a change in nucleotide that substitutes one base for another. Such changes are known as base substitutions which may occur spontaneously or due to the action of mutagens. A well studied example is sickle cell anaemia in humans which results from a point mutation of an allele of β-haemoglobin gene (βHb). A haemoglobin molecule consists of four polypeptide chains of two types, two α chains and two β-chains.
Each chain has a heme group on its surface. The heme groups are involved in the binding of oxygen. The human blood disease, sickle cell anaemia is due to abnormal haemoglobin. This abnormality in haemoglobin is due to a single base substitution at the sixth codon of the beta globin gene from GAG to GTG in β -chain of haemoglobin.
It results in a change of amino acid glutamic acid to valine at the th position of the β -chain. This is the classical G G G G G G G T C T T T C C C C C A A A A Valine DNA sequence of non-template (coding) strand DNA point mutation can lead to a different amino acid sequence Phenotype Sickled red blood cells Normal red blood cells DNA sequence of non-template (coding) strand Amino acid sequence Amino acid sequence Histidine Leucine Threonine Proline Glutamic acid Glutamic acid G G G G G G G T T T T C C C C C C A A A Valine Histidine Leucine Threonine Proline Valine Mutant Glutamic acid T ' ' ' ' Fig. . DNA point mutation Wobble Hypothesis It is a hypothesis proposed by Crick ( ) which states that tRNA anticodon has the ability to wobble at its ’ end by pairing with even non-complementary base