Replication
Process of which a cell copies its DNA before divison
Replication must be?
Rapid and accurate
What model is used for DNA Replication?
The semiconservative model, each DNA strand serves as a template for the synthesis of new DNA strand
What is the polarity of the template and new strand of DNA?
New strand = 5’-3’
Template strand 3’-5’
What are the requirements for DNA replication?
- Template (single standed DNA)
- Raw materials (DNA triphosphates-dNTPs) to be assembled into a new nucleotide strand
- Enzymes and other proteins (read the template and assemble the substrates into a DNA molecule)
- Primer (short segment of nucleotides(RNA) bound to the template strand. acts as a substrate for DNA polymerase)
What enzymes are present in DNA replication and what is there function?
- Helicase: untwists the DNA
- Primase: constructs RNA primer
- DNA polymerases: polymerization of nucleotide precursors into DNA Chain (5’-3’) and finds the correct nucleotide to pair with nucleotide on template strand
What happens at the site of replication?
Starts at the replication origin
forms a replication bubble that unwinds the helix at the replication fork
How does DNA replication begin?
When the initiator proteins bind to the origin of replication
What happens during the initiation of replication?
- Initiator proteins bind to replication origin
- DNA helicase binds to the initiator proteins
- Helicase loads onto DNA
- Helicase denatures helix and binds DNA primase to form primosome
- Primase synthesizes RNA primer, which is extended as DNA chain by DNA polymerase
What is the polarity of a discontinuous strand of DNA?
5’-3’
What is the polarity of a continuous stand of DNA?
3’-5’
What are the steps of Semidiscontinuous Replication?
- Initiation, RNA primer made by DNA primase and starts replication of the lagging strand (1st okazaki fragment synthesis)
- Further untwisting and elongation of new DNA strands (2nd okazaki fragment elongated)
- Process continues, 2nd okazaki fragment finished and 3rd being synthesized; DNA primase beginning 4th fragment.
- Primer removed by DNA polymerase 1 (Replaces RNA primer with DNA)
- Joining of adjacent DNA fragments by DNA ligase. (glues okasaki fragments together)
Leading Strand
Continuous replication of DNA
5’-3’ —–>
Lagging Strand
Discontinuous replication of DNA
3’-5’ ——>
Okazaki fragments
Template Strand
Stand which new DNA strand is synthesized according to complementary base-pairing rules
What is the process of fixing mistakes made during replication?
1 mistake per billion nucleotides because polymerases are very accurate.
1. Proofreading: if an incorrect base is added the polymerase is stalled and removes the incorrect base and replaces it with the correct one
2. Mismatched repair: if proofreading fails the repair enzymes recognize the deformity in the secondary structure and replace it with the correct one.
What are the functions of DNA polymerase I, II, III?
I: Removes and replaces primers
II: DNA repair (not in exonuclease 5’-3’)
III: Elongates DNA (not in exonuclease 5’-3’)
