Lecture 5 - DNA Synthesis

Question 1

Semi-conservative replication

Answer 1

DNA replication results in one old strand and one new strand present in each daughter DNA molecule.

Question 2

DNA Helicase

Answer 2

Helps uncoil the DNA. Breaks Hydrogen bonds.

Enzyme that catalyses the unwinding and separating of strands in DNA replication.

Question 3

DNA Polymerase

Answer 3

Acts in 5’ to 3’ direction.
Enzyme that catalyses the formation of phosphodiester bonds between adjacent nucleotides in DNA replication.
It requires a DNA template, a DNA or RNA primer (because DNA polymerase requires an –OH on the 3’ carbon to start adding nucleotides) the four deoxyribonucleoside triphosphate (dNTP) building blocks and Mg2+ ions.

Question 4

DNA REPLICATION (4)

Answer 4

Initiates at specific sites on DNA - replication origins.
RO unwinds, forming a replication bubble allowing access to the replication machinery.
Replication bubbles will grow larger until they coalesce and form two daughter chromosomes.

Question 5

DNA Synthesis (2) B (2) M (2)

Answer 5

Occurs in S phase.
Involves complete unwinding of the parental DNA.

Bacterial cycle (M and S) - 20/30 mins
Has a single replication origin.

Mammalian cell cycle (M, G1, S and G2) - takes longer cause we have more chromosomes.

Question 6

Eukaryotic DNA polymerase

Answer 6

β - Repair
α / Δ (delta) / Ε (epsilon) - Replication
ϒ (gamma) - Mitochondrion

Question 7

Bacteria DNA Polymerase

Answer 7

I /II / Others - Repair

III - Replication

Question 8

Replication fork (5)

Answer 8

5’ - 3’
Leading strand
Continuous strand

3’ - 5’
Lagging strand
Discontinuous strand
Okazaki fragments - replication has to be originated again and again.

Question 9

Enzyme that cuts DNA strand (3)

Answer 9

Topoisomerase
Topoisomerase I - Cuts only one strand.
Topoisomerase II - Cuts both strands. Used infront of the helicase and gets rid of the coils in DNA (which becomes coiled due to helicase).

Question 10

Primase

Answer 10

Lays down some RNA (produces 3’ –OH), so that DNA polymerase knows where to start replicating.

Question 11

DNA binding proteins

Answer 11

Stabilise the single stranded DNA, stop it being re-annealed with the other parent strand.

Question 12

DNA Ligase

Answer 12

Splices the fragments together.

Question 13

Defects in DNA replication

Answer 13

Inherited defects in mismatch repair genes are involved in (colon) cancer, failure to correct DNA synthesis may lead to cancer. Error prone cell.

Question 14

DNA replication inhibitors

Answer 14

Antibacterial: Ciprofloxacin, Levofloxacin, Novabiocin (Gyrase inhibitors).
Antitumour: Etoposide, Doxorubicin, Mitoxantrone (Topo II inhibitors). Used in chemotherapy against cancers.
Antiviral: AZT (Reverse transcriptase).