Dna Replication Notes

DNA replication

-when the cell divides by mitosis the DNA has to be replicated so each daughter receives 23 pairs of chromosomes.

-DNA replication occurs during S phase of interphase

-DNA replication occurs through semi conservative replication- DNA double strand is separated each of the strand acts as a template for the synthesis of a complementary strand--- 2 daughter DNA duplexes- one strand comes from the parental DNA whereas the other strand of DNA is newly synthesised from the DNA base strand which acts as a template.

E coli- was grown in media containing 15N incorporated into the DNA- when cells were suddenly switched to 14n containing media- the first generation had DNA which was 50% 15N and 50% 14N

-DNA replication requires a continuous supply of deoxyribonucleotide tri-phosphates (substrate) and once the DNA has replicated it requires histones to package the DNA

Stages involved in replication

• During G1 phase of interphase- pre replication complexes are assembled at origins of replications- unlike circular bacterial chromosomes- single origin of replication, , eukaryotic chromosomes are linear, each have multiple origins of replications- speeds up the rate of replication

• During S phase these complexes are activated, and DNA helicase binds to the origins of replication- break hydrogen bonds and cause the DNA double helix to separate into single strands. The energy to break these bonds comes from the hydrolysis of ATP

• DNA gyrase (topoisomerase enzyme) reduces the torsional strain on the DNA caused by the unwinding stops unwound single DNA from getting tangled by a breaking and rejoining mechanism

• Initiator proteins, REPLICATION PROTEIN A, bind to each of these single strands to prevent the two strands from rehybridisation into a double strand. This results in y shaped replication forks being created with a leading and lagging strand

• RNA primers are synthesised by primase enzymes (DNA polymerase alpha) on each of the single strands of DNA. This is essential as DNA polymerase can only add nucleotides to an existing strand of nucleotides. This is because is requires a base paired 3 hydroxyl group as a substrate

• The PCNA clamp is loaded onto the DNA by RFC and encircles the DNA- the PCNA clamp tether DNA polymerase sigma to the DNA and displaces the primase. Enables the polymerase to be highly processive and synthesis can occur in long segements

• The DNA polymerase binds and elongates the primer in the 5’-3’ to form a complementary strand

• As the replication of DNA is...

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