In 1975 Robin Holliday and John Pugh propose that a single methyl group on cytosine could serve as a cell's memory.
The Visitor watches a cell divide and sees the memory copied to the daughter strand.
A Copied Memory
- A methyl group on the 5 position of cytosine makes 5-methylcytosine. DNMT1 recognizes hemimethylated CpG sites after replication and methylates the daughter strand. The page gives about 97% fidelity.
- Three enzyme families work as a team. Writers: DNMT3A and DNMT3B place new marks, DNMT1 maintains them. Readers: MBD proteins such as MeCP2 recruit silencing complexes. Erasers: TET enzymes oxidize 5mC in steps toward demethylation.
- A liver cell and a neuron share identical DNA, and their methylation maps give them different identities.
- The page reports that repeat elements make up about 45% of the human genome and that methylation silences them.
DNMT1 follows the fork and copies each methyl mark onto the new strand. The site page carries the writer, reader, and eraser families.
How does the daughter strand receive the parent's methylation pattern?
DNMT1 recognizes the hemimethylated CpG dinucleotides left after replication and methylates the new strand, copying the parental pattern.
A methyl mark copied at every division gives a cell its memory.
The Visitor lights node 04 and the Library act closes with four nodes lit.
See the map