DNA Replication, Repair and Recombination 1

Question 1

What happens at the replication fork

Answer 1

Asymmetric

Both strands are simultaneously replicated

DNA poly can only synthesize DNA in 5’ to 3’ direction

Leading strand is continuously

Lagging strand is segments
Okazaki fragments

Question 2

Explain exonucleolytic proofreading.

Answer 2

Immediately after incorrect bases are added

3’ to 5’ exonuclease activity clips off unpaired residues at 3’ primer terminus

DNA poly requires a perfectly paired 3’ terminus

Question 3

What is needed to start DNA replication?

Answer 3

A primer and template strand

Question 4

Why does 3’ to 5’ replication not occur?

Answer 4

There is no high-energy bond to cleave when the incoming correct base is to be added.

5’ to 3’ does have a high energy phosphate bond to cleave.

Allow for error correction…

Question 5

How is 3 the lagging strand synthesised?

Answer 5

In a backstitching process.

RNA primer made by DNA primase

DNA poly adds to new primer to start Okazaki fragment

DNA poly finishes fragment

RNA primer erased by RNAseH

DNA ligand joins Okazaki fragment

Question 6

What are the proteins at the replication fork?

Answer 6

DNA Helicase

Single-stranded DNA binding proteins

Sliding clamp

Clamp loader

Question 7

What are the function of SS DNA?

Answer 7

Stabilize unwound DNA

Prevent formation of hairpins

DNA bases remain exposed

Question 8

Explain mix match repair?

Answer 8

Proofreading proteins

MutS= bind to mismatch

MutL= scans for nick and triggers degradation of nicked strand

Question 9

What does Type I Topoisomerase do?

Answer 9

Relaxes supercoiled DNA, a thermodynamically favorable process

Creates transient single strand

Reveals rapidly does not require energy do to phosphotryrosine

Question 10

What does Type II Topoisomerase do?

Answer 10

Make transient double-stranded break in DNA where double stranded helical cross each other.

Question 11

What are the steps of Type II Topoisomerase?

Answer 11

Break one double stranded helix reversible to create a “gate”

Causes second strand to pass through

Reveals break and dissociates

Can fix 2 locked DNA circles

Question 12

Why do mutation rates need to be low…for obvious reasons?

Answer 12

Germ cells to maintain species

Somatic cells to avoid uncontrolled growth= proliferation/cancer