BIOC 3021 Lecture Notes - Lecture 22: Okazaki Fragments, Dna Replication, Dna Ligase

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27 Apr 2018
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DNA Replication:
- 2 strands of the double helix have great affinity for one another
- Held together by H-bonds
- Mechanism is required for separating strands in a local region to provide access
to the bases that act as templates
- Specific proteins melt the double helix at specific sites to initiate DNA replication
- Other enzymes (helicases) use the free energy of ATP hydrolysis to move this
melted region along the double helix replication process
- DNA helix is unwound into 2 separate strands
- Local unwinding in one region leads to stressful overwinding in surrounding
regions
- Enzymes (topoisomerases) introduce supercoils that release the strain
caused by overwinding
- DNA replication is highly accurate:
- Free energies associated with base pairing within the double helix suggest that
approximately 1 in 104 bases incorporated will be incorrect
- However, DNA replication has an error rate estimated to be 1 in 1010 nucleotides
- Additional mechanisms allow proofreading of the newly formed double helix
- Replication is very rapid with high accuracy
- One strand is synthesized in a continuous fashion leading strand
- Opposite strand is synthesized in a fragmented, discontinuous fashion → lagging strand
- Enzymes that copy DNA polymerase in
5’ → 3’ direction
- Polymerase strands of DNA run in opposite direction yet appear to grow in the
same direction
-
5’ → 3’ direction is continuous
- 3’ → 5’ direction is fragmented
- Okazaki fragments
- Linked together by DNA ligase
- Specialized structures called telomeres are added by another enzyme to maintain the
information content at the ends of chromosomes
- Replication machinery alone cannot replicate the ends of linear DNA molecules
- Special mechanism is required to prevent the loss of sequence
information with each replication
- Replication Requirements:
- Core Requirements:
- Template DNA strand (copy from)
- Primer Strand (add on to)
- 4 dNTP’s
- DNA polymerase II
- Mg2+ and other protein factors
- Additionally needs:
- Single strand binding proteins (SSB’s)
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Document Summary

2 strands of the double helix have great affinity for one another. Mechanism is required for separating strands in a local region to provide access to the bases that act as templates. Specific proteins melt the double helix at specific sites to initiate dna replication. Other enzymes (helicases) use the free energy of atp hydrolysis to move this melted region along the double helix replication process. Dna helix is unwound into 2 separate strands. Local unwinding in one region leads to stressful overwinding in surrounding regions. Enzymes (topoisomerases) introduce supercoils that release the strain. Dna replication is highly accurate: caused by overwinding. Free energies associated with base pairing within the double helix suggest that approximately 1 in 104 bases incorporated will be incorrect. However, dna replication has an error rate estimated to be 1 in 1010 nucleotides. Additional mechanisms allow proofreading of the newly formed double helix. Replication is very rapid with high accuracy.

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