[lecture 8] DNA replication

Question 1

what is semi conservative replication

Answer 1

one original parental strand, one new daughter strand

Question 2

initiation steps of replication (4)

Answer 2

1. starts at numerous sites on chromosomes called origins of replication (Ori)
2. replication proteins bind to Ori sites and DNA helicase separates 2 strands, breaking hydrogen bonds, creating bubbles
3. this creates a Y-shaped replication fork
4. both parental strands now provide a template for replication

Question 3

elongation steps of replication (6)

Answer 3

1.. 5’ to 3’ original replication direction

2. 1 leading strand (5’ to 3’), 1 lagging strand (3’ to 5’)
3. single RNA primer acts as starting point and elongates leading strand using DNA polymerase III
4. lagging strand synthesized in small okazaki fragments. each fragment has its own RNA primer.
5. DNA polymerse I removes RNA primers
6. DNA ligase joins okazaki

(leading strand- always towards fork, lagging strand- always away from fork)

Question 4

termination steps of replication (3)

Answer 4

1. DNA replication terminates when two replication forks (coming in opposite directions) meet on the same stretch of DNA
2. daughter DNA molecules fuse together
3. two identical double-stranded DNA molecules

Question 5

enzymes in replication (5)

Answer 5

1. DNA helicase: unwinds (breaks H bonds)
2. DNA polymerase III: adds nucleotides
3. primase: synthesizes a RNA primer at the 5’ prime of leading strand and each okazaki fragment on lagging strand
4. DNA polymerase I: removes RNA primers and replaces with DNA nucleotides
5. DNA ligase: joins okazaki fragments of lagging strand

Question 6

how does proofreading of DNA polymerase work

Answer 6

exonuclease activity: when a wrong nucleotide is added, it binds to the exonuclease active site which removes the mispaired base. DNA polymerase then adds the correct base and continues elongation.