Lecture 4

Question 1

DNA Replication

Answer 1

Makes complementary copies of DNA from a DNA template. Semi-conservative process

Question 2

DNA replication steps

Answer 2

1. Topisomerase cuts strands. Helicase unwinds and separates complementary DNA by breaking H bonds at replication fork
2. RNA primase binds to the origins (AT rich and less stable), signalling the starting point of replication. DNA polymerase binds to separated DNA strands at primer sites and adds complementary base units. DNA lygase helps sugar-phosphate backbone together. DNA is replicated 5’-3’ and because DNA is antiparallel, replication will be continuous for leading strand but not lagging strand

3.DNA Polymerase reaches end of DNA molecule and 2 identical daughter strands have been produced which recoils to double helix. Has 5’-3’ exonuclease enzyme to removed mismatched nucleotide.

4. DNA religates

Question 3

nucleotides

Answer 3

Uses nucleotide triphosphates as substrate. adds monophosphate to the 3’OH end of the growing chain and for each one added.

Question 4

Bonds

Answer 4

Forms a high energy phosphodiester bond and release pyrophosphate (PPi)

Question 5

Lagging strand replication

Answer 5

DNA can only replicate 5’-3’ and so, primer heads away from replication fork (leading stand heads towards) meaning that only a small section is replicable. Hence, multiple primers are placed and replicates until it runs into the next primer. Small fragments are made (Okazaki fragments) which are unjoined DNA sections. DNA Polymerase I removes the RNA and replaces with DNA. DNA ligase joins it together.

Question 6

Leading vs laggin strand

Answer 6

Leading: Primase makes an RNA primer to begin then DNA polymerase III makes a DNA copy of the strand in the 5’-3’ direction. Continuous copying

Lagging: Primase makes multiple RNA pri,ers. DNA Polymerase III synthesises in 5’-3’ direction until it runs into the next primer making Okazaki fragments. When the replication forks come together, DNA Polymerase I replaces the RNA with DNA which is joined by DNA ligase.