DNA methyltransferase Dnmt1 ensures clonal transmission of lineage-specific DNA 
methylation patterns in a mammalian genome during replication. Dnmt1 is targeted 
to replication foci, interacts with PCNA, and favors methylating the 
hemimethylated form of CpG sites. To understand the underlying mechanism of its 
maintenance function, we purified recombinant forms of full-length Dnmt1, a 
truncated form of Dnmt1-(291-1620) lacking the binding sites for PCNA and DNA 
and examined their processivity using a series of long unmethylated and 
hemimethylated DNA substrates. Direct analysis of methylation patterns using 
bisulfite-sequencing and hairpin-PCR techniques demonstrated that full-length 
Dnmt1 methylates hemimethylated DNA with high processivity and a fidelity of 
over 95%, but unmethylated DNA with much less processivity. The truncated form 
of Dnmt1 showed identical properties to full-length Dnmt1 indicating that the 
N-terminal 290-amino acid residue region of Dnmt1 is not required for 
preferential activity toward hemimethylated sites or for processivity of the 
enzyme. Remarkably, our analyses also revealed that Dnmt1 methylates 
hemimethylated CpG sites on one strand of double-stranded DNA during a single 
processive run. Our findings suggest that these inherent enzymatic properties of 
Dnmt1 play an essential role in the faithful and efficient maintenance of 
methylation patterns in the mammalian genome.