In some organisms, cytosine is methylated at carbon 5 of the pyrimidine ring after it is incorporated into DNA. If a 5-methyl cytosine molecule is then hydrolyzed, what base will be generated?

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Understand the structure of cytosine and the position of methylation: cytosine is a pyrimidine base, and methylation occurs at carbon 5 of the pyrimidine ring, producing 5-methylcytosine.

Recall that hydrolysis of cytosine (or 5-methylcytosine) typically involves deamination, where the amino group (-NH2) is replaced by a keto group (=O).

Recognize that deamination of cytosine converts it into uracil, while deamination of 5-methylcytosine converts it into thymine because the methyl group at carbon 5 remains intact.

Therefore, when 5-methylcytosine undergoes hydrolytic deamination, the base generated is thymine.

Summarize that the key concept is that 5-methylcytosine deaminates to thymine, which is important in understanding mutation processes such as C to T transitions in DNA.

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DNA Methylation

DNA methylation involves the addition of a methyl group to the 5th carbon of cytosine bases, commonly forming 5-methylcytosine. This epigenetic modification affects gene expression without altering the DNA sequence and is important in processes like gene regulation and genomic imprinting.

5-Methylcytosine Deamination

5-Methylcytosine can spontaneously deaminate, losing an amino group and converting into thymine. This chemical change is significant because it can lead to mutations if not repaired, as thymine is a normal DNA base but mismatches with guanine.

Base Hydrolysis and Resulting Products

Hydrolysis of a methylated cytosine base breaks chemical bonds, leading to the release of the base from the DNA backbone. When 5-methylcytosine is hydrolyzed and deaminated, the base generated is thymine, not cytosine, due to the methyl group and deamination altering its structure.