Question

The cells of certain eukaryotic organisms, such as vertebrates and plants, often methylate the cytosines in their DNA, producing 5‑methylcytosine. Organisms that perform cytosine methylation also have evolved a specialized DNA repair system that recognizes G−TG−T base pair mismatches and then repairs the G−TG−T mismatches to form G≡TG≡T base pairs. What is the advantage of a DNA repair system that corrects all G−TG−T mismatches to G≡CG≡C base pairs instead of A=TA=T base pairs when 5‑methylcytosine occurs in the genome?

Answer 1

The G-T mismatches generally originate because of the spontaneous deamination of 5-methylcytosine to thymine. Therefore, correcting G-T to G≡C most probably preserves the original sequence.

In the DNA, the 5-methylcytosine usually goes through the process of deamination to thymine. This transformation may cause a G-T base pair or mismatch of nucleotide pairs. Generally, the mutation like C-T transition is common. The G-T base pair is identified by base excision repair proteins, which withdraws this mismatched base pair.

The repair system encourages methylation of cytosine as the mechanism of the regulation of transcription, which permits the turning off or on the process of transcription on the basis of the requirement of the cells. This also assists in preventing the mutation of DNA.