Post on 06-Jan-2018
description
DNA Replication
- copying DNA
molecule
Why does DNA need to copy?
• Every time a cell divides (mitosis), an EXACT copy of DNA must go into new cells!
• Growth and repair of body cells
Origins of replication• Prokaryotes –
one origin in two directions along the plasmid
• Eukaryotes – many origins, or forks, along DNA strand
Replication is Semiconservative• One old and
one new
Step One:• Enzyme Helicase breaks hydrogen bonds between base pairs (unwinds at rate of 8,000 rpm)• DNA unzips at Replication Fork
Step Two
RNA primer• Template to begin
replication
Step Three:• Free floating
nucleotides pair up with lone nucleotides in a 5’-3’ direction
Step Four:
Base pairs bonded together
by enzyme called DNA POLYMERASE
Step Five:
• Side of ladder is bonded by enzyme called LIGASE
How do new strands grow?
Leading Strand
• Replicated continuously
• 5’ to 3’ end
Lagging Strand
• Replicated in short fragments called “Okazaki fragments”
(1,000-2,000 bp long)• 5’ to 3’ end
The two new strands are identical to eachother and the
original parent molecule!
Proofreading
• DNA polymerase detects, removes and replaces incorrectly paired nucleotides.
• If not corrected, could lead to mutations, loss of functions etc.
Damage Repair
• Corrects damage caused by X-rays, UV radiation, chemical exposure etc.
• One 4 hour day at the beach – approx.
10 UV induced errors in every skin cell