Priming of replication on double-stranded DNA always requires a replicase, SSB, and primase.

TLS Online TPP Program

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#Unit 3. Fundamental Processes

n BER,  An AP endonuclease cleaves the phosphodiester backbone near the AP site

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#Unit 3. Fundamental Processes

In BER,  DNA polymerase I initiates repair synthesis from the free 3’ hydroxyl at the nick, removing (with its 5’-3’ exonuclease activity) and replacing a portion of the damaged strand. 

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#Unit 3. Fundamental Processes

In BER, The nick remaining after DNA polymerase I has dissociated is sealed by DNA ligase.

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#Unit 3. Fundamental Processes

Base excision repair of a T∙G mismatch. 

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#Unit 3. Fundamental Processes

oxoG:A repair, Oxidation of guanine produces oxoG. 

The modified base can be repaired before replication by DNA glycosylase via the base excision pathway. 

If replication occurs before the oxoG is removed, resulting in the misincorporation of an A, then a fail-safe glycosylase can remove the A, allowing it to be replaced by a C. 

This provides a second opportunity for the DNA glycosylase to remove the modified base.

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#Unit 3. Fundamental Processes

Another example of a fail-safe system is a glycosylase that removes a T opposite a G. Such a T:G mismatch can arise, as we have seen, by spontaneous deamination of 5-methylcytosine,which occurs frequently in the DNA of vertebrates.