DNA Replication and DNA Repair Part 2

DNA replication and DNA repair
Learning Outcomes
1. Explain how the ends of the chromosomes are replicated
2. Outline the importance of DNA repair
3. Describe DNA repair mechanisms

A sliding ring holds a moving DNA polymerase onto the DNA
- Most DNA polymerase molecules will synthesize only a short string of nucleotides before
falling off the DNA template
- The tendency to dissociate from the DNA allows the DNA polymerase that has just
synthesised an Okazaki fragment on the lagging strand to be RECYCLED
- This rapid dissociation however is NOT GOOD for long DNA strands to be synthesised

An accessory protein functions as a regulated sliding clamp
 Keeps the polymerase firmly on the DNA when
it is moving
 Releases the polymerase as soon as it runs
into a Double-stranded DNA
 Forms a large ring around DNA double helix
o One side of the ring binds to the back
of the DNA polymerase
o The whole ring slides freely along the
DNA as the polymerase moves
 The assembly of the clamp around DNA
requires
o ATP hydrolysis by the clamp loader [a
special protein complex]
o This hydrolyses ATP as it loads the
clamp onto a primer-template
junction
o Leading strand  loading needs to
occur once
 DNA polymerase is tightly
bound to the clamp, the 2
remain associated for a long
time
o Lagging strand  polymerase releases itself from the clamp and dissociates from
the template DNA strand
 Polymerase molecule then associates with a new clamp that is assembled to
a RNA primer of the next Okazaki fragment
 The clamp is removed and reattached each time a new Okazaki fragment is
made