CHI ACGTATCGT methylated cosine Civii i methylation unmethylated cytosine CH3 TGCATAGCA TGCATA ACGTATCGT HIpihI NA mt的n TGCATAGCA replication methylase met ACGTATCGT ACGTATS ..iiI 3 methylation 5 TGCATAGCA TGCATA