describe how the Methyl-directed mismatch repair system in E. coli which DNA strand is the correct strand and determines which DNA strand has the mutation.
describe how DNA methylation is heritable during replication
how epigenetic modifications are involved in genomic imprinting, X-inactivation, and regulation of tissue/cell specific gene expression (including the general roles of TrxG and PcG group proteins). ***Are these heritable during mitosis or meiosis? Are these reversible? Can you support your answer?
The reaction catalyzed by DNA polymerase is error prone, and mismatches occur at a rate of one in thousand bases incorporated. Therefore, to correct the errors during replication, methyl-directed mismatch repair system functions in E.coli.
The DNA in E.coli is methlylated at palindromic sequences of GATC. The enzyme deoxyadenosinemethylase catalyzes this methylation. The enzyme adds methyl group to adenine in both strands of the sequence. The methylation does not alter the base pairing property of adenine, and it pairs normally with thymine base.
When DNA replicates, immediately after replication, the parental strand has methylated bases, but not the new strand. The differences in methylation allows the cell to differentiate between the correct sequence on the parental strand and the incorrect sequence on the new strand.
When mismatch or incorrect base is recognized, the mismatch repair system is activated. A series of proteins called MutS, MutL, and MutH participate in mismatch repair. MutS binds to the mismatched base and then recruits MutL and MutH.
MutL is the protein that recognizes the parental strand. It form a loop in the parental strand, MutH cleaves the unmethylated strand containing the mismatch base. DNA helicase UvrD unwinds the cleaved strand, exposes it to exonucleases. The exonucleases remove the mismatched base, and the gap is filled by DNA polymerase I.
describe how the Methyl-directed mismatch repair system in E. coli which DNA strand is the correct...