hemimethylated

Hemimethylated refers to a state of a double-stranded DNA molecule in which methyl groups are present on one strand but not on the complementary strand. This situation typically arises transiently after DNA replication, when the parental template strand remains methylated while the newly synthesized daughter strand is initially unmethylated.

In bacteria, hemimethylation is often associated with Dam methylation at the sequence 5'-GATC-3'. After replication, the resulting DNA is hemimethylated and is normally remethylated by Dam to restore fully methylated DNA. Hemimethylated DNA also directs the mismatch repair system, helping to distinguish the original template strand from the newly synthesized strand. This ensures correct repair and maintenance of the genome.

In eukaryotes, cytosines within CpG dinucleotides are methylated. Following DNA replication, the parental strand remains methylated while the new strand does not, creating a hemimethylated state that is a substrate for maintenance methyltransferases. DNA methyltransferase 1 (DNMT1), often aided by the partner protein UHRF1, copies methylation from the parent to the daughter strand to preserve epigenetic patterns. De novo methylation is carried out by DNMT3A and DNMT3B, establishing new methylation marks where appropriate.

Significance and methods: Hemimethylation is central to epigenetic inheritance, imprinting, X-chromosome inactivation, and suppression of transposable elements. It is detected by bisulfite sequencing, methylation-sensitive restriction assays, and related genomic methods.

Overall, hemimethylation represents a transient, replication-coupled intermediate that guides maintenance of DNA methylation patterns across cell generations.