Question

For the nucleotide sequence AAC(O6-meG)TGCAC, with a damaged (methylated) G residue, what would be the sequence of each strand of the double-stranded DNA in the following situations?

a) Replication occurs before repair

b) The DNA is acted upon by glycosylase and then repaired, but only after replication has occured

c) Two rounds of replication occur, followed by repair

Answer 1

a) Replication occurs before repair

DNA replication will halt at the damaged site as methylated Guanine residue will make a physical block to replication machinery.

So, the DNA sequence in this case will be

AAC(O6-meG)TGCAC

TTG

b) The DNA is acted upon by glycosylase and then repaired, but only after replication has occured

As the repair happened after replication has occurred, the replication will halt at damaged site. However, due to presence of glycosylase, the base excision repair mechanism will takes place which will result in the replacement of damaged guanine with normal guanine residue.

So, the DNA sequence in this case will be

AACGTGCAC

TTGCACGTG

c) Two rounds of replication occur, followed by repair

After repair, two rounds of replication will results in

1st replication

AACGTGCAC

TTGCACGTG

2nd round of replication

AACGTGCAC

TTGCACGTG

AACGTGCAC

TTGCACGTG