What is the checkpoint for the completion of DNA synthesis?

Answer 1

I am answering my own question here I'm afraid, but I have since found the answer.

Basically, a checkpoint (otherwise known as the DNA damage checkpoint) response is a stage in DNA replication where the cell cycle stops owing to DNA damage and becomes more susceptible to apoptosis ( programmed cell death). a more detailed answer is shown in the paragraph below.

"In response to DNA damage, eukaryotic cells activate a set of surveillance systems that interrupt cell cycle progression to allow time for repair. These surveillance systems are called checkpoints and have been given an empirical definition. The DNA damage checkpoint acts in three stages in the cell cycle, one at the G1-S phase transition (G1 checkpoint), one at S phase (S-phase checkpoint), and one at the G2-M boundary (G2 checkpoint. With checkpoint failure, the immediate consequence is that the cells increase their sensitivity to being killed, and the long-term consequence is that the cells increase their susceptibility to tumor genesis. S-phase checkpoint monitors progression through S phase, which slows the rate of on-going DNA synthesis."

Ref: http://cancerres.aacrjournals.org/cgi/content/full/62/6/1598

Answer 2

There a variety of different DNA polymerases, the enzymes that synthesise DNA strands, with slightly different functions and uses.

The main polymerase that is used in DNA replication for replication of the leading and lagging strands will usually contain both a 5'-3' polymerase catalytic site and a 3'-5' exonuclease site. These sites are usually next to each other, with the polymerase site in front of the exonuclease site.

If the polymerase catalytic site makes a mistake in DNA replication, the enzyme moves forward as usually, but will be halted by the unusual shape of DNA that is formed by the incorrect non Watson-Crick base pairing. The exonuclease site will recognise that the shape is incorrect and remove the last nucleotide to be added to the strand being synthesised. The enzyme can then "try again" to match the bases correctly.

The incorrect pairing creates an odd shape because base pairs are between one purine and one pyrimidine. Purines have two rings in their structure, so are larger than pyrimidines, which have only a single ring. DNA will not maintain its consistent shape and diameter if two purines are paired, as the bases would be much larger than the normal diameter of DNA. Similarly, DNA cannot maintain its native shape if two pyrimidines are paired, as they will be far too small to fill the gap.