The cells of certain eukaryotic organisms, such as vertebrates and plants, often

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-T base pair mismatches and then repairs the G-T mismatches to form GC base pairs. What is the advantage of a DNA repair system that corrects all G-T mismatches to G=C base pairs instead of A-T base pairs when 5-methylcytosine occurs in the genome? Intracellular cytidine concentrations are greater than other nucleotides, driving the reaction forward. Repairing G-T mismatches to A-T removes methylation sites, decreasing the fidelity of mismatch repair Because G-T mismatches often arise due to the spontaneous deamination of 5-methylcytosine to thymine, correcting G-T to GC most likely preserves the original sequence. O GC base pairs provide more genomic stability than A-T base pairs due to the additional hydrogen bond. OCorrecting G- Tmismatches to GeC requires less energy than repairing an A-T base pair

Explanation / Answer

In DNA the 5 - methyl cytosine frequently undergoes deamination to thymine. This conversion may result in G- T base pair or mismatch of nucleotide pair. Usually, mutation like C - T (pyrimidine to pyrimidine) transition is common. The G - T base pair is recognized by base excision repair proteins that remove this mismatched base pair. The repair system promotes methylation of cytosine as mechanism of transcription regulation or system that enables turning on or off of transcription according to the requirement of cells. This also helps to restrict the DNA mutation. Therefore, option 3 is correct.

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