13. Aspartic acid (Asp) is specified by the codon GAU and GAC. After mutation, Asp is changed to Alanine represented by GCX, where X may be A, U, C and G. The reversion of the mutation could only be done with reactive oxygen species. The nature of mutation is considered to be
(1) transition
(2) transversion
(3) depurination
(4) either transition or transversion

Aspartic acid (Asp) is encoded by the codons GAU and GAC, while Alanine (Ala) is encoded by codons starting with GC (GCU, GCC, GCA, GCG). The mutation from Asp to Ala involves substitution from GAU/GAC to GCX codon. This mutation changes a purine/pyrimidine mix in Asp codon (G is purine, A and U/C are pyrimidines) to a codon starting with GC. Since the first base G remains the same (a purine), but the second base changes from A (purine) to C (pyrimidine), this substitution involves a change from a purine to a pyrimidine base. This type of mutation is called a transversion.

Explanation of Mutation Types and Options

• Transition: A point mutation changing a purine to another purine (A↔G) or a pyrimidine to another pyrimidine (C↔T/U). In the question, A (purine) changing to C (pyrimidine) is not a transition.​

• Transversion: A nucleotide substitution where a purine base (A or G) is replaced by a pyrimidine base (C or T/U), or vice versa. Since the mutation changes A (purine) to C (pyrimidine), this supports transversion.​

• Depurination: Refers to loss of a purine base (A or G) from DNA, leaving an apurinic site, not a base substitution. This is a different kind of mutation damage, not a substitution.​

• Either transition or transversion: This option suggests the mutation could be either, but base analysis clearly shows a purine-to-pyrimidine substitution, so this is not correct.

The mutation reversion only possible by reactive oxygen species indicates oxidative damage relevance, which often causes transversions due to base modifications.

Introduction:
Mutations are changes in the DNA sequence that can alter protein coding. In genetics, point mutations are classified mainly into transitions and transversions, based on nucleotide changes. A classic example is the mutation of aspartic acid (Asp) codons GAU/GAC to alanine (Ala) codons GCX. This article explains the nature of this mutation, focusing on the difference between transition and transversion and why reactive oxygen species play a role in reversion.

This mutation is best classified as a transversion because it involves substituting a purine base (adenine) with a pyrimidine base (cytosine). Transitions, in contrast, maintain base class (purine to purine or pyrimidine to pyrimidine). Depurination involves base loss, not substitution, so it is not applicable here. The oxidative reversion mechanism further supports that the mutation type is a transversion due to the chemical nature of the base change and repair process.