Difference Between Transition and Transversion

Key Difference – Transition vs Transversion
 

It is important to have a general knowledge of base pairing in DNA in order to understand the difference between transition and transversion mutations. There are five different nitrogenous bases in nucleic acids: adenine (A), guanine (G), cytosine (C), thymine (T) and uracil (U). First two bases (A & G) are purines while latter three (C, T and U) are pyrimidines. T is unique to DNA and U is unique to RNA. Purine bases form hydrogen bonds with complementary pyrimidine bases. It is known as a complementary base pairing of nucleic acids. Complement base of A is T in DNA. In RNA instead of T, U is present and A forms hydrogen bonds with U. Complement Base of G is C.  Purine bases are composed of two ring systems and pyrimidine bases are composed of one ring systems. Mutations occur in DNA and RNA base sequences due to interchanges in base pairing. Substitution of an incorrect base during DNA replication repairs by DNA polymerase enzymes. However, transversion and transition are two such mutations that occur in DNA due to substitution errors, and they are not recognized by enzymes for repairing. Transition mutation occurs due to an interchange of purines or pyrimidines. Transversion mutation occurs due to interchanges of pyrimidine for purines or purines for pyrimidines. This is the key difference between transition and transversion.

CONTENTS
1. Overview and Key Difference
2. What is Transition Mutation
3. What is Transversion Mutation
4. Side by Side Comparison – Transition and Transversion
5. Summary

What is Transition Mutation?

Transition is a point mutation that occurs due to an interchange of purines (A ↔ G) or pyrimidines (C ↔ T) in DNA. It is a kind of a substitution mutation. During replication, the correct purine base can substitute another purine. For example, instead of A in the correct sequence, G can be substituted. Once G is substituted, complementary C will substitute to the other strand. In the same way, another pyrimidine base T can be substituted instead of pyrimidine base C and change the complementary base in the other strand. Transition mutations are more frequent than transversions. Single nucleotide polymorphism is a common type of point mutations, and two out of three SNPs are caused by transitional mutations. However, transition mutations are less likely to cause amino acid sequence changes. Hence, they remain neutral and are known as silent mutations.

What is Transversion Mutation?

Transversion is the second type of point mutation that occurs due to the wrong substitution of bases. Transversion happens when a purine base is substituted with a pyrimidine base, or a pyrimidine base is substituted with a purine base as shown in figure 02.

Transversion occurs in two possible ways since two pyrimidines and two purines are present. This type of mutation is less likely to produce incorrect amino acid sequences during translation.

Transversions are caused by ionizing radiation, strong chemicals, etc.

What is the difference between Transition and Transversion?

Transition vs Transversion
Transition is the substitution of a purine from another purine base or pyrimidine from another pyrimidine ( (C ↔T or A↔ G).	Transversion is the substitution of a purine from a pyrimidine or pyrimidine from a purine.
Occurrence
This is the most common type of point mutation.	This is less common than transition.
Possibilities
There is one possible transition.	There are two possible transversions.
Change in Amino Acid Sequence
This is less likely to cause amino acid sequences. So this remains as a silent mutation.	This is more likely to cause amino acid sequence changes. So it has a pronounced effect on the resulting protein.
Interchange of Ring Structure
Interchanges of bases within single ring structures or within double ring structures can be observed.	Interchange occurs in a single ring structure with double ring structure or double ring structure with a single ring structure.

Summary – Transition vs Transversion

Mutation is known as any change that occurs in the base sequence of the DNA. It can be due to an insertion, deletion, duplication, translocation or substitution, etc. Substitution mutations are two types: transition and transversion. In transition, one purine is substituted for another purine or one pyrimidine is substituted for another pyrimidine. In transversion, a purine base is substituted for a pyrimidine base or vice versa. Transition mutations are more common than transversion mutations and are less likely to produce a difference in the amino acid sequence compared to transversions. This is the basic difference between transition and transversion.

Reference:
1. Watanabe, S. M., and M. F. Goodman. “On the molecular basis of transition mutations: frequencies of forming 2-aminopurine.cytosine and adenine.cytosine base mispairs in vitro.” Proceedings of the National Academy of Sciences of the United States of America. U.S. National Library of Medicine, May 1981. Web. 14 Mar. 2017
2. Griffiths, Anthony JF. “The Molecular Basis of Mutation.” Modern Genetic Analysis. U.S. National Library of Medicine, 01 Jan. 1999. Web. 14 Mar. 2017.

Image Courtesy:
1. “Transitions-transversions-v3” By Petulda – Own work (Public Domain) via Commons Wikimedia