DNA Replication and Repair

How does DNA replicate?

• Mitosis and meiosis require DNA to replicate itself so that the daughter cells will contain DNA

• But how does DNA replicate itself?• Meselson and Stahl’s experiment

DNA Replication: The Details

• Lots of enzymes and terminology to learn• Enzymes: – DNA gyrase (sometimes called topoisomerase), – DNA helicase– DNA polymerase I– DNA polymerase III– DNA ligase– RNA primase

• Terms: – Replication fork– Okazaki fragment– Parent/Template strand– Daughter strand– RNA primer– Leading strand– Lagging strand– Origins of replication– Replication bubbles

DNA Replication

• See: http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animations.html#

DNA Replication - Steps1. Replication begins at an origin of replication,

causing replication bubbles to open.2. DNA Helicase unwinds the double helix.3. Single stranded binding proteins (ssbp) keeps the

helices apart.4. DNA Gyrase relieves tensional forces.

*At this point, the helix is unwound. Let’s look at the individual strands.

The Leading Strand

• Nucleotides can only be added to the 3’ end, so the leading strand is created from 5’ 3’

1.RNA Primase adds a primer to the beginning of the strand.

2.DNA Polymerase III then adds nucleotides continuously.

The Lagging Strand• The leading strand is made from 5’ 3’, so the

lagging strand is made from 3’ 5’. But, nucleotides can only be added to the 3’ end.

• The lagging strand is therefore made from 5’ 3’ using smaller segments of DNA.

1. RNA Primase adds primers. Nucleotides are added in short Okazaki Fragments by DNA Polymerase III

2. DNA Polymerase I replaces the RNA primers with DNA

The Lagging Strand, Continued

4. DNA Ligase “glues” the Okazaki fragments together.

5. Replication in the lagging strand is non-continuous.

DNA Repair

• DNA Polymerase III and I also have “proofreading” functions

• If there are mistakes, these enzymes can act as an exonuclease, meaning that they can cut out and replace the mistake

Summary

• Please go to the following website:

• http://207.207.4.198/pub/flash/24/menu.swf