lecture 2: DNA replication, the cell cycle and mitosis

Question 1

what is the nature of DNA replication? what are the two main stages?

Answer 1

DNA replication is semi conservative opening of the DNA helix DNA synthesis

Question 2

how does the opening of the DNA happen?

Answer 2

• DNA must be unwound and unzipped - DNA helicase separates the two strands of the DNA by using ATP to break the hydrogen bonds between the base pairs - topoisomerase then unravels the DNA - the new strand is made by DNA polymerase

Question 3

which end does DNA polymerase add nucleotides to?

Answer 3

can only add to the 3’ end

Question 4

what does the DNA synthesis require?

Answer 4

• DNA polymerase - template strand - oligonucleotide primer - DNA primase - dNTPs

Question 5

how does DNA synthesis happen?

Answer 5

• DNA polymerase adds dNTPs to the 3’ end of the DNA - the hydrolysis of the triphosphate releases energy which drives the reaction a free 3’ end is needed

Question 6

in what direction does DNA synthesis take place?

Answer 6

happens from the 5’ to 3’ direction

Question 7

what is the origin of replication? what is the replication fork?

Answer 7

this is the specific place o the DNA molecule where replication begins the replication fork is the site of DNA synthesis

Question 8

* what does the replication fork look like?

Answer 8

insert the pic

Question 9

where does DNA polymerase attach to the replication fork?

Answer 9

the DNA polymerase can only attach nucleosides at the 3’ end

Question 10

what is the nature of replication of the leading strand?

Answer 10

the replication is continuous because its 3’ end is in the same direction as the movement of the replication fork

Question 11

what is the nature of replication of the lagging strand?

Answer 11

the DNA synthesis is discontinuous instead the DNA is synthesised in fragments called okazaki fragments

Question 12

* how are the okazaki fragments of the lagging strand synthesised?

Answer 12

• a type of RNA polymerase known as DNA primate synthesises a short RNA fragment - DNA polymerase adds nucleotides to the 3’ end of the RNA primer - this continues to synthesise the okazaki fragment until reaches the end of the previous okazaki fragment - the RNA primer of the previous okazaki fragment is removed and replaced wit DNA - two okazaki fragments are joined using DNA primase ( insert the pic)

Question 13

how are the okazaki fragments joined?

Answer 13

• the ribonuclease removes the RNA primer using 5’ to 3’ exonuclease activity - the repair DNA polymerase replaces the RNA with DNA - DNA ligase joins the two okazaki fragments together -

Question 14

what is the function of the single strand DNA binding protein?

Answer 14

• prevents the single stranded DNA from locally folding

Question 15

what is the function of the sliding clamp?

Answer 15

• this makes sure that the DNA polymerase is in the right place

Question 16

what does exonuclease activity mean?

Answer 16

• this is the ability of an enzyme to remove a nucleotide at any time from the end of a polynucleotide chain

Question 17

what is the proof reading mechanism?

Answer 17

• DNA polymerase checks to make sure the previous nucleotide is correct

Question 18

why is the proofreading mechanism important?

Answer 18

• it is important because mutations can be dangerous

Question 19

eukaryotic organisms: origins of replication? direction?

Answer 19

multiple origins of replication bidirectional

Question 20

what is the cell cycle?

Answer 20

G1- DNA present as single linear double helix of DNA S phase - DNA is replicated G2 - each chromosome has 2 identical sister chromatids mitosis - two chromatids separate to the daughter cells G0 - cells have stopped dividing