Transversion mutation involves the substitution of purine by pyrimidine and pyrimidine by purine. This type of mutation is rare due to steric problems of pairing purines with purines, and pyrimidine with pyrimidines.
This type of mutation takes place due to chemical modification in a base. The chemical modification is caused due to two reactions one is deamination and second is depurination. In deamination Cytosine occasionally loose an amino group and then it is converted to Uracil. This resulted in pairing of Uracil-Guanin instead of Cytosine-Guanin. It is well known that Uracil pairs with Adenin. After Uracil-Adenin pair in first round of replication, thymine takes the place of Uracil in second round of replication. And finally Cytosine - Guanin pair is converted to Adenin - thymine Pair. In depurination loss of purine group (Adenine or Guanin) takes place.
Due to deletion and insertion of one or a few nucleotides in DNA molecule, the shifting of reading frame of nucleotide takes place either in backward or in forward position, this results into a mutation. This is known as frameshift mutation. Generally this mutation occurs where there is a short repeated nucleotide sequence.
Any agent that directly cause damage to the DNA alters the base sequence or interferes with repairing system is known as mutagens. Mutagens may be some chemical or radiation. Here we are discussing how the mutagen can act:
1) Base Analogues: A base analogue is a chemical compound similar to the one or four bases of DNA. During normal replication process this base analogue can be incorporated into growing polynucleotide chain. For Example 5-Bromouracil (5-BU) is an analogue of thymine. It can pair with adenine in polynucleotide chain by replacing thymine. During the replication keto-form of 5-BU Substitutes for thymine and the replication of an initial Adenin:Thymine (A:T) pair becomes an A:BU pair. The rare enol form of 5-BU that pairs with Guanin is the first mutagenic round of replication. In the next round of replication Guanin pairs with cytosine. So that transition is completed from A:T to G:C pair.
2) Chemical Changing the Specificity of Hydrogen Bonding.
There are many chemicals that after incorporation in DNA change the specificity of hydrogen bonding. For example Nitrous Oxide (HNO2) converts the amino group of the bases into keto group through oxidative deamination. Deamination of adenine results in formation of hypoxanthine, the pairing behavior of which is like guanine. Hence it pairs with cytosin