DNA Replication the big event during S phase

The Animation hill.com/sites/ /student_view0/chapter14/animations.html#

1. Helicase breaks hydrogen bonds between bases, unzips and unwinds the helix

Helicase: Is an enzyme (a protein that speeds up chemical reactions) Is made during G1

Begins to unwind the DNA at the ORIGIN OF REPLICATION (a specific nucleotide sequence)

Helicase enzymes move in both directions from the Point of Origin, forming a REPLICATION BUBBLE. At either end of the replication bubble is a REPLICATION FORK, a Y-shaped region where the new strands of DNA are elongating.

In prokaryotic cells there is one origin In eukaryotic cells there are 100’s to 1000’s of origins

2. Single stranded binding proteins hold the DNA strands apart Keeps the separated strands apart and stabilize the unwound DNA

3. RNA nucleotides bind with complementary base sequences under the direction of RNA primase. These RNA nucleotides act as a primer for DNA nucleotides. Primers are short segments of RNA, about 10 nucleotides long Must have a primer because DNA polymerase can only add nucleotides to another nucleotide

4. DNA polymerase III adds DNA nucleoside tri phosphates to the RNA primer sequence in a 5’ to 3’ direction. Two of the phosphates are stripped off in the bonding

New nucleotides can only be added to the 3’ end of a growing DNA chain So we say DNA grows 5’ to 3’

Leading Strand: DNA polymerase III can synthesize a complementary strand on one side of the template in the 5’ to 3’ direction with no problem.

3’ 5’ DNA Polymerase III Can only add to this side … AWAY from the replication fork What about the other strand??

Lagging Strand DNA polymerase III must work away from the replication fork. Makes a short strand of DNA, called an Okazaki fragment. As the bubble widens, it can make another short strand, and so on.

5. RNA primers are removed and replaced with DNA nucleotides by DNA Polymerase I.

6. Along the lagging strand the Okazaki fragments are joined by DNA Ligase to form a single DNA strand.

7. Proofreading by DNA Polymerase III and I occurs, and replication is complete.

DNA replication is... Accurate Only 1:1,000,000,000 nucleotides are incorrectly paired

DNA Replication is semi-conservative: each replicated DNA molecule consists of one “old” and on “new” strand.

DNA replication is... Extremely rapid In prokaryotes, up to 500 nucleotides are added per second 50 per second in eukaryotes

Other Good Animations at/molgenetics/dna-rna2.swf /student/animations/dna_replication/index.html