Explain what is meant by deamination. How does this mutation affect a cytosine (C) versus a methylcytosine (5meC)? What effect does deamination have on CpG islands, and how does that affect the mutation frequencies in these regions?

Question

What will be an ideal response?

Answer 1

Deamination is the loss of an amino (NH2 ) group from a nucleotide base. Each of the DNA nucleotide bases contains an amino group.
Deamination of cytosine is frequently a source of mutation. When C is deaminated, the amino group is usually replaced by an oxygen atom, forming the nucleotide base uracil. DNA mismatch repair mechanisms recognize uracil as an RNA nucleotide base and remove it from DNA. The excised uracil is replaced by cytosine, so the wild-type sequence is restored.
When deamination occurs on a cytosine that has been methylated, it creates thymine and generates a base-pair mismatch between the newly formed thymine on one strand and the previously complementary guanine on the other strand. If mismatch repair enzymes correct the mismatch before the next DNA replication cycle, two outcomes are possible: Either (1 ) the repair will restore the wild-type G-C base pair, or (2 ) the repair will generate an A-T base-pair transition substitution. If mismatch repair does not occur before replication, the guanine-containing strand will be used to produce a daughter duplex with the wild-type (G-C) sequence, while the thymine-containing strand will be used to synthesize a daughter duplex containing a G-C to A-T base-pair substitution.
CpG dinucleotides are cytosines that are side by side with guanines in a DNA strand and are joined by a phosphodiester bond. Cytosines in CpG dinucleotides are frequent targets for methylation in mammalian promoters where methylation helps regulate transcription. Experimental evidence shows that CpG dinucleotides are hotspots of mutation as a result of deamination of methylated cytosine and the production of thymine. This situation could ultimately result in altered transcriptional activity in genes by mutations in the promoters.