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