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Point mutation (Base substitution mutation)

  • Substitution of a single base by another is known as point mutation. If a purine is replaced by another purine base or if a pyrimidine base is replaced by another pyrimidine base, it is known as transition. Substitution of a purine base by a pyrimidine base or vice versa is known as transversion. A point mutation will affect only one codon. Point mutation can be of following types :-
  1. Silent mutation
  • If the substitution occurs in the third base of a codon, the code word may remain unchanged due to degeneracy of the genetic code.
  • For example, GGC to GGG mutation has no effect as both code for glycine thus, codon containing the changed base codes for the same amino acid (PG1-lJ9), therefore there is no change in amino acid sequence of the protein.
  • The gene is expressed in term of same protein, i.e. there is no change in expression of protein.
  1. Mis-sense mutations
  • If the base substitution changes the code words, i.e. codon containing the changed base codes for different amino acid, it is called mis-sense mutation. The sequence of amino acids in the protein changes. The effect of amino acid substitution is variable :-
  1. Acceptable missense mutation:-
  • The substitution of amino acid has minimal or no effect because either the substituted amino acid is similar to that of original amino acid or the substitution occurs in non critical area.
  • For example, normally hemoglobin (HbA) has valine at position 67 of β-chain in place of valine. This is non-ritical position, i.e. substitution of different amino-acid 'at this position produces normally functional hemoglobin.
  • For example, 'hemoglobin Milwaukee' has glutamic acid at this position (in place of valine), 'hemoglobin bristol' has aspartic acid and 'hemoglobin Sydney' has alanine (in place of valine). 
  1. Partially acceptable missense mutation:-
  • Substitution of one amino acid with other is partially acceptable, i.e. the resultant protein functions normally under certain conditions but not always.
  • For example, substitution of glutamate by valine at position six of β-chain produces HbS. HbS is able to function normally at high oxygen tension but gets precipitated at low oxygen tension.
  1. Unacceptable mis-sense mutation:-
  • Substitution of one amino acid with other produces non-functional protein.
  • For example, in methemoglobin (HbM) histidine is replacement by tyrosine at position 58 of (X- chain. HbM is incapable of combining with oxygen.
  1. Nonsense Mutation.
    Base substitution changes a sense codon into a nonsense (stop or termination) codon. In such cases, protein synthesis will be terminated prematurely and the resulting protein will be usually non-functional.

Frame-shift mutation

Frameshift mutations occur due to insertion or deletion of one or two bases which causes change in the reading frame distal to the mutation.

  1. If 1 or 2 base pair change, whole reading frame is changed distal to the mutation, resulting into entirely different protein molecule.
  2. If 3 base pairs change, single amino acid is incorporated or deleted. The rest of amino acid sequence is same.

Trinucleotide repeat mutation

In this type of mutation a codon (i.e. trinucleotide sequence) undergoes amplification and the same codon is repeated continuously so many times in the chain. Diseases associated with trinucleotide repeat mutation are Huntington's disease (CAG repeat), Spinocerebellar ataxia (CAG repeat), Friedreich ataxia (GAA repeat), fragile-X-syndrome (GGG or GCC repeat), dystrophia myotonica (CTG/CUG repeat), X-linked spinobulbar muscular atrophy (CAG repeat) and dentorubral pallidoluysian atrophy (CAG repeat).


Mutation M

  1. POINT M                     
  2. Frameshift M.              
  3. Trinucleatide Repeat M.
    1. Transition                                       
    2. Transversion                

Effect of Point mutation

Silent M.                                             Missense M                                          Nonsense M.

Acceptable                                       Partially acceptable                                  Unacceptable

Hba,B-67,Valine                                        HbS                                             HbM (Boston)

Hb- Milwaukee-Glutamica                           B-6                                                Alpha -58

Hb-Bristol-Aspartic a                         Valine for Glutamic a.                               Tyrosine for Histidine.

Hb – Hikari B-61

Aspargine for lysine


Transition = Purine replaced by purine & pyrimidine replaced by pyrimidine

Transversion =  Purine replaced by pyrimidine & vice-versa.



Extra Edge
  1. Much of the DNA is associated with histone proteins to form a structure called the nucleosome. Nucleo¬somes are composed of an octamer of histones and 150 bp of DNA
  2. Transcriptionally active DNA (the genes) is often clustered in regions of each chromosome. Within these regions, genes may be separated by inactive DNA in nucleosomal structures. The transcription unit-that portion of a gene that is copied by RNA polymerase-consists of coding regions of DNA (ex¬ons) interrupted by intervening sequences of non coding DNA (introns).
  3. After transcription, during RNA processing, introns are removed and the exons are ligated together to form the mature mRNA that appears in the cytoplasm. This is known as RNA splicing & carried out by Spliceosome.
  4. DNA replication occurs at several sites-called repli¬cation bubbles-in each chromosome. The entire process takes about 9 hours in a typical cell.
  5. Processed genesConsists of DNA sequences identical or nearly identical to those of messenger RNA for appropriate gene product. That is 5’ non transcribed area, the coding region without introns, and 3’ poly A tail all are present contiguously.
  6. Pseudogenes: are processed genes that have been randomly altered through evolution so that they can now not be expressed, hence called pseudo-gene.


Textile DNA

Many sequences in eukaryotic genomes have the potential [0 form triple-stranded DNA structures. Such regions occur with much higher frequency than expected from probability considerations alone. Polypurine tracts over 25 nucleotides long constitute as much as 0.5% of some eukaryotic genomes. These potential triple-helical regions are especially common near sequences involved in gene regulation.


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