Class 12 Biology Molecular Basis of Inheritance Genetic code

Genetic code

  • The sequence of nucleotides on DNA which determines the sequence of amino acids in a polypeptide chain is termed as Genetic code.
  • The process of translation requires transfer of genetic information from a polymer of nucleotides to a polymer of amino acids but there is no complementarity between nucleotides and amino acids which led to the proposition of a genetic code that could direct the sequence of amino acids during synthesis of proteins.
  • The salient features of genetic code are as follows:
  • The codon is triplet, 61 codons code for amino acids and 3 codons do not code for any amino acids, hence they function as stop codons.
  • One codon codes for only one amino acid thus it is unambiguous and specific.
  • Some amino acids are coded by more than one codon, hence the code is degenerate.
  • The codon is read in mRNA in a contiguous fashion, there are no punctuations.
  • The code is nearly universal. For example, from bacteria to human UUU would code for Phenylalanine (phe).
  • AUG has dual functions, it codes for Methionine (met) , and it also act as initiator codon.



Mutation and genetic code

  • The relationships between genes and DNA are best understood by mutation studies.
  • Point mutation is the insertion or deletion of a single gene in the structural gene.

Example- point mutation is a change of single base pair in the gene for beta globin chain that results in the change of amino acid residue glutamate to valine, which results into a diseased condition called as sickle cell anaemia.


        Sickle cell shaped RBC

  • Frame shift mutation is the insertion and deletion of three or its multiple bases which insert or delete one or multiple codon hence one or multiple amino acids, and reading frame remains unaltered from that point onwards.

Example- cystic fibrosis.

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