DNA replications, the cell cycle and mitosis

Question 1

What two types of nucleotides are needed for DNA synthesis?

Answer 1

-Oligonucleotide primer (RNA primer made by DNA primase)
The primer is synthesised by an RNA polymerase called primase.
-dNTPs (deoxynucleoside triphosphates)

Question 2

what do DNA polymerases do and what do they need?

Answer 2

Add deoxynucleotide tri-phosphates to the 3’ end of a DNA molecule.
-synthesis is driven by the release of energy from the hydrolysis of the tri-phosphate
NEED:
-template
-dNTPS
-oligonucleotide primer
they cannot start from scratch.

Question 3

what is the place at which replication start called?

Answer 3

Origin of Replication.

Question 4

Explain how the lagging strand is synthesised.

Answer 4

-DNA is synthesised in short fragments called okazaki fragments
The oligonucleotide primers attach and DNA polymerase moves in a 5’ to 3’direction adding the dNTPs to the growing chain. When the newly synthesised strand reaches the oligonucleotide primer of the previous okazaki fragment, the DNA polymerase is removed. Ribonuclease removes the RNA primer using 5’ to 3’ exonuclease activity. and Repair DNA polymerase replaces the RNA with DNA. DNA ligase then joins the adjacent okazaki fragments.

Question 5

Which three enzymes are involved in the joining of Okazaki fragments?

Answer 5

Ribonuclease using 5’ to 3’ exonuclease activity,
Repair DNA Polymerase,
DNA ligase

Question 6

What stuff are there that ensures DNA binding

Answer 6

Single strand DNA binding protein: prevents the single stranded DNA from locally folding.
Sliding Clamp: makes sure DNA polymerase is in the right place
Lagging strand forms a loop so both strands can be synthesised in a coordinated manner.

Question 7

What is the proofreading mechanism

Answer 7

• Before adding a new nucleotide, DNA polymerase checks to make sure previous nucleotide is correct, Any incorrect bases are removed by 3’ to 5’ exonuclease activity of DNA polymerase. A new, correct nucleotide is then added.

Question 8

What is the difference between eukaryote and prokaryote/E.coli DNA replication?

Answer 8

Replication with E.coli starts at OriC.
2 replication forks proceed in opposite direction.
-single replication origin
-bi-directional

Eukaryote:

• multiple replication origins
• Bi-directional

Question 9

What are nucleoside analogs and what do they do and the 4 drugs?

Answer 9

Replace dNTPs and terminate the chain.
-do not have hydroxyl group at 3’ position, so stops DNA polymerase from working.

Dideoxycytidine (ddC) / Zalcitabine: Has 3’ (-H) group, used to treat HIV
Azidothymidine (AZT) / Zidovudine: Has 3’ (-N3) group, used to treat HIV
Acyclovir: Does not have a ring structure to its sugar group (so not a sugar at all), used to treat Herpes
Cytosine Arabinose: Has a tetrose sugar instead of pentose, used in Chemotherapy

Question 10

What happens during early and late prophase?

Answer 10

During Early Prophase: Chromosomes pair up + Crossing Over occurs
During Late Prophase: Chromosomes condense and become visible + Spindle fibres move to opposite ends ofcell

Question 11

Describe the mammalian cell cycle?

Answer 11

A mammalian cell cycle takes about 24 hours in tissue culture.

M phase: Mitosis 1 hr; cell division, metaphase. 2 chromatid seperate to the daughter cells

G1; gap phase 1; 10 hrs
DNA of each chromosome present as a single linear double helix of DNA

S phase; DNA replication, 9 hrs
Synthesis, DNA is replicated

G2; gap phase 2; 4 hrs
Each chromosomes has 2 identical sister chromatids

G0; cells which have stopped dividing.

G1, G0, S and G2 are interphase.