41. In context of DNA methylation, which one of the following statements is FALSE?
(1) Generally, methylation occurs at the 3rd carbon position of cytosine and converts it to 3-methyl cytosine
(2) Maintenance methyl transferase acts constitutively on hemi-methylated sites and converts them to fully methylated sites
(3) During mammalian gametogenesis, the genomic methylation patterns are erased in primordial germ cells
(4) Replication converts a fully methylated site to hemi- methylated site
The false statement regarding DNA methylation is:
(1) Generally, methylation occurs at the 3rd carbon position of cytosine and converts it to 3-methyl cytosine.
Explanation of options:
(1) False. DNA methylation generally occurs at the 5th carbon position of cytosine, not the 3rd carbon, converting it to 5-methyl cytosine (5mC). This methylation mainly occurs at CpG dinucleotide sites in mammals.
(2) True. Maintenance methyltransferase, primarily DNMT1, acts constitutively on hemi-methylated DNA sites (created after DNA replication) and methylates the newly synthesized strand thus converting hemi-methylated sites to fully methylated sites, preserving methylation patterns after replication.
(3) True. During mammalian gametogenesis, genomic DNA methylation patterns are erased in primordial germ cells (PGCs). This epigenetic reprogramming erases previous methylation to allow proper development and establishment of sex-specific methylation patterns later.
(4) True. DNA replication converts a fully methylated site to a hemi-methylated site because the newly synthesized strand initially lacks methylation. The maintenance methyltransferase then restores full methylation.
Understanding DNA Methylation in Detail
DNA methylation is a biochemical process involving the covalent addition of a methyl group (CH3) to the 5th carbon of the cytosine base in DNA, mainly occurring at CpG dinucleotide sites. This modification forms 5-methylcytosine (5mC), an important epigenetic mark that regulates gene expression, genomic imprinting, and genome stability.
Maintenance methyltransferase enzymes such as DNMT1 recognize hemi-methylated DNA formed after DNA replication and methylate the new strand to maintain methylation patterns faithfully through cell divisions. This ensures stable epigenetic inheritance in somatic cells.
During mammalian gametogenesis, primordial germ cells undergo global DNA methylation erasure, which is critical for resetting epigenetic marks for proper development. Methylation patterns are erased actively and passively, allowing the establishment of new methylation profiles specific to sperm or eggs.
DNA replication creates hemi-methylated sites when the parental fully methylated DNA is copied, as the new strand initially lacks methyl groups. Maintenance methyltransferases act quickly to convert these hemi-methylated sites back into fully methylated sites after DNA synthesis.
