DNA Replication

Steps of DNA replication

1) Origin of replication – spot on DNA where replication starts. Bacteria have small, circular chromosomes and thus have only ONE origin of replication. Humans have many possible origins of replication per chromosome. Because the base pair A and T have only two hydrogen bonds between them, the are more likely to be an origin of replication.

2) DNA helicase – unwinds/uncoils small portion of supercoiled DNA at origin of replication – there is not enough space in cell for all DNA to be unwound at the same time. One little part of DNA has to be pulled out/unwound from rest of coil. Moves ahead of DNA polymerase with respect to leading strand.
3) Where DNA unwound, double strands separate (“unzip”) (hydrogen bonds between bases are broken). Strands want to join together again, but we can’t have that. So, single-stranded DNA covered by single-strand binding protein (SSB) – prevents strands from reattaching (re-annealing)

4) RNA primase – enzyme that makes a small portion of RNA that’s complementary to small portion of unzipped DNA (ssDNA – single stranded DNA). This small chunk of RNA is called RNA primer. DNA polymerase is enzyme that synthesizes DNA (make new DNA strand). DNA polymerase can’t attachto single stranded DNA. It can only attach to ssDNA that has a RNA primer attached to it. DNA polymerase is the “hand” in the book diagram.

5) DNA polymerase synthesizes new strand of DNA complementary to original strand. Accomplishes this by bringing in nucleotides (really nucleotide triphosphates – ATP, CTP, GTP, TTP) which are complementary to existing strand. Can attach 3 phosphates to A, C, G, and T = all act as energy currency. nTPs – nucleotide triphosphates – have phosphates broken off, providing energy to make new phosphate backbone. A, C, G, and T which were attached to phosphates become the bases – are complementary to existing strand attached to backbone. DNA polymerase can only go in 5 prime to 3 prime direction along a DNA strand. Leading strand can continue to be unwound, unzipped, and then replicated while other strand has to stop and go because DNA helicase is NOT moving ahead of it. Second stand has Okazaki fragments due to stop and go.

6) Okazaki fragments must then be patched together with DNA ligase. Leading strand has no fragments.Telomere: repetitive sequences of DNA on the ends of chromosomes that protect chromosome from degradation (Wikipedia); needed because wherever primer was on end of linear chromosome, DNA can't be replicated - don't code for proteins and so can afford to lost them. When they get too short, cell —> apoptosis. Telomerase in gametes allows the cell to regenerate their telomeres.


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