Lecture notes from Imperial College London, Medical Biosciences BSc, 2nd year, genetics and genomics (GEN) module.
Focus on replication of the genome. Before a cell divides, the entire genome of 3 billion base pairs must first be copied: accurately, quickly and just once per cell cycle. During t...
Replicating the genome
- some believe that increasing telomerase activity may extend lifespan
Basic molecular mechanism of DNA replication
- DNA replicated by a replisome: complex of proteins
=> DNA pol adds nucleotides in the 5’ to 3’ direction to a DNA primer
- DNA helicase unwinds the strands => replication machinery accesses DNA
-
n
1 replication bubble = 2
lagging and 2 leading strands
Control DNA replication in the cell cycle
- high fidelity of DNA pol: 1 error per 10^8 bp --
-
-N -
&
- BUT need control of S-phase (replication) & M-phase
- cells that don’t follow cell cycle:
=> meiotic cells: 2 cell divisions without S-phase (=> halve chr number)
=> megakaryocites: repeated S-phases without M => enlarge + polyploid for platelet production
=> senescent cells: enter Go indefinitely (dormancy) more than 2 homologues
, Replication origins & control of their activation
- prokaryotes: 1 replication origin (= where DNA pol begin DNA synthesis) in circular chr
=> 2 replication forks moving in opposite directions
=> bacterial cells divide every 30min (genome 1000-fold smaller than human genome)
=> bacterial DNA pol synthesises 500 nucleotides/ s (10 times faster than human DNA pol)
- mammalian: larger genomes => multiple replication origins (~50,000: 1 per 70kb)
=> mammalian cells divide every 20h (S-phase in 8h/ up to 30min for specialised cells)
=> simultaneous replication forks
=> replication bubbles coalesce
(3 fuse before S-phase completed)
- eukaryotes do not have a conserved DNA sequence at their replication origin (not same sequence)
=> origin licensing: pre-replicative complex (preRC) assembled
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