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Section B

Types of mutation

Mutation can occur in two way :-

  1. Spontaneous mutation Spontaneous mutations occurs under normal condition of growth. Some of these mutations take place due to mistake during course of DNA duplication. It occurs about one in every million to one in every billion divisions and is probably due to low level natural mutagens normally present in the environment
  1. Substitution of a nucleotide (Point mutations):- A single base, i.e., a change in a single “chemical letter” such as switching an A to a G is replaced by another. Single base substitutions are also called point mutations.These base pair replacements can be of two types:--

Transition cause by tautomeric shift of bases( or ionizationa- process in which the hydrogen atoms of a deoxyribonucleotide base move in a way that changes the properties of its hydrogen bonding. ) if one purine AT is replaced by another purine GC or GC by AT and pyrimidine TA by is replaced by the other pyrimidine CG or GC by TA.

Transversion If a purine is replaced by a pyrimidine or vice-versa, the substitution is called a transversion.In this type AT is replaced by TA or AT to CG and GC to CG or GC to TA.

 

Three different conditions may arise after a point mutation :-

  1. Missense mutation :-(example- Fig 2 Point mutation -missense mutation) This type of mutation is a change in a single DNA base pair that results in the substitution of one wrong amino acid in the protein made by a gene. Due to this missense codons arise which code for another amino acid and usually brings a change of the activity of the protein.
  2. Nonsense mutation :- The specified codon which specified for a particular amino acid is changed to one of the Stop codons (TAA, TAG, or TGA) also known as nonsense codon ,which does not code for an amino acid but act as a stop or comma in the genetic message and the altered DNA sequence prematurely signals the cell to stop building a protein. This type of mutation results in a shortened protein that may function improperly or not at all.

  1. Sense mutation or Silent mutation or samesense mutation:- A change of one of the three nucleotides of the codon usually the last one do not alter the synthesis of specific amino acid or protein and results in a in a new codon still coding for the same amino acid .e. g. the triplets AAA and AAG both code for the production of lysine if AAA is replaced by AAG production is same -lysine.No change in the activity of the protein.

  1. Frame-shift mutation:- In this type of mutation gain or loss (addition or deletion ) of one or more base pairs or substitution(replacement) of one base pair by another causes alternations of the DNA sequence of the gene.The genetic code is a triplet code where three consecutive nucleotides code for a specific amino acid.The order in which the bases are arranged into codons (which is set by the initiator codon AUG) is called reading frame. If a single base pair is added into gene sequence , due to this the reading frame is changed and the most codons after the point of addition is changed. A deletion or an addition (insertion) for any number of nucleotides other than three leads to a frame shift mutation. Such mutation may result in another starting point for transcription of the genetic code or in a reading shift within the code , both of which lead to misreading the genetic code.
  2. Deletion : -Deletions are mutations in which a section of DNA is lost, or deleted.

  1. Insertion :-Insertion mutations are in which extra base pairs are inserted into a new place in the DNA and general result in a nonfunctional protein.

Insertion and deletion mutations cause frameshift mutations, which change the grouping of nucleotide bases into codons. This results in a shift of "reading frame" during protein translation.