What is the Difference Between Transition and Transversion

The main difference between transition and transversion is that transition is the conversion of purine to another purine base or pyrimidine to another pyrimidine base whereas transversion is the conversion of a purine into a pyrimidine or vice versa. Furthermore, in transitions, the ring structure will not be changed while in transversions, the ring structure of the base will be changed.

Transition and transversion are two types of point mutations involved in the substitution of bases in the nucleic acid structure. Generally, the two main classes of nitrogenous bases in nucleic acids are purines, including adenine (A) and guanine (G), and pyrimidines, including cytosine (C) and thymine (T).

Key Areas Covered

1. What is Transition
     – Definition, Type of Base Conversion, Effect
2. What is Transversion
     – Definition, Type of Base Conversion, Effect
3. What are the Similarities Between Transition and Transversion
     – Outline of Common Features
4. What is the Difference Between Transition and Transversion
     – Comparison of Key Differences

Key Terms

Degeneracy of Codon, Point Mutations, Silent Mutations, Substitutions, Transition, Transversion

What is Transition

A transition is a type of base substitution in which a particular nitrogenous base is changed to the other base of the same class. That means; the purines can exchange with each other as A to G and vice versa. On the other hand, pyrimidines can exchange with each other as C to T and vice versa. Therefore, a particular base in the genome has only a single form of transition. However, transitions occur more frequently in the genome since the ring structure is not going to change in transitions.

Figure 1: Transitions and Transversions

Spontaneous deamination results in the transition of 5-methylcytosine more frequently than unmethylated cytosine. It decreases the number of CpG islands in the genome. However, transitions are less likely in amino acid substitutions. Therefore, they persist as single nucleotide polymorphisms (SNPs).

What is Transversion

A transversion is another type of base substitution in which a particular base from class coverts into a base in the other class. That means; the purines convert into pyrimidines, and pyrimidines convert into purines. Here, due to the presence of two types of purines and pyrimidines, the base subjected to the conversion has two possibilities. However, since the ring structure is going to be changed, transversions are less frequent in the genome.

Figure 2: Point Mutations

Furthermore, the effect of a transversion in the genome is more pronounced since it can alter the type of amino acid in a polypeptide chain. For example, transversions in the third base of a codon lead to the degeneracy of the codon, resulting in a different amino acid in the polypeptide chain.

Similarities Between Transition and Transversion

• Transition and transversion are two types of base substitutions which lead to point mutations.
• Both are involved in changing the nitrogenous base of nucleic acid chains.
• Also, both can occur spontaneously or in response to mutagens.

Difference Between Transition and Transversion

Definition

Transition refers to a point mutation in which one base is replaced by another of the same class (purine or pyrimidine) while transversion refers to a point mutation in which a purine is replaced with a pyrimidine or vice versa. Thus, this is the main difference between transition and transversion.

Significance

Another difference between transition and transversion is that in transition, base change occurs within the same class while transversion is responsible for the change of bases from one to the other class.

Ring Structure Change

The ring structure of the nitrogenous base does not change in transitions while the ring structure changes in transversions. Hence, this is another difference between transition and transversion.

Ratio of Transitions to Transversions

Also, there are 8 possible types of transitions while there are 4 possible types of transversions.

Possibility per Base

Moreover, a particular base can undergo a single type of transition while a base can undergo two types of transversions.

Occurrence in the Genome

Besides, another difference between transition and transversion is that transitions are more common in the genome while transversions are comparatively less common.

Occurrence

Furthermore, while transitions occur due to oxidative deamination and tautomerization, transversions occur spontaneously or due to ionizing radiation and alkylating agents.

Effect

Their effect is also a difference between transition and transversion. Transitions are less likely to result in amino acid substitutions, and they occur as silent mutations while transversions have a more pronounced effect since a transversion of the third nucleotide of a codon is responsible for the degeneracy of the codon to a large extent.

Conclusion

A transition is a type of base substitution caused by the conversion of a base to the other base of the same class. Generally, transitions are more frequent in the genome. But, they have a less effect as they remain as silent mutations. In comparison, transversion is a type of base substitution caused by the conversion of a base of one class to another class. Though transversions are less common in the genome, they have an effect on changing amino acids in the protein. Therefore, the main difference between transition and transversion is the type of conversion and the effect.

References:

1. Carr, Steven M. Transitions vs Transversions, 2014, Available Here

Image Courtesy:

1. “All transitions and transversions” By Manudouz.- Own work (CC BY-SA 4.0) via Commons Wikimedia  
2. “Point mutations-en” By Jonsta247 – Own work (CC BY-SA 4.0) via Commons Wikimedia