Medical Cell Biology and Genetics - DNA and Replication

Question 1

Name the purines and pyrimidines

Answer 1

Purines - Two ring structure (G and A)

Pyrimidines - One ring structure (C U and T)

Question 2

Are RNA and DNA chains polar?

Answer 2

Yes - they each have a 5’ phosphate end and a 3’ hydroxyl end.

Question 3

How is eukaryotic DNA condensed into chromosomes?

Answer 3

During the S phase (synthesis) DNA is replicated.

The DNA is wrapped round a histone core to form nucleosomes (beads on a string)
The Nucleosomes then coil to form solenoid structures which make up chromosomes.
Genome is unexpressed when condensed.

Question 4

Describe the 3 stages of DNA replication?

Answer 4

1. Initiation - helicase unravels the double helix. Primase is a special enzyme needed to initiate each strand because DNA polymerase can only extend from 3’ to 5’.
2. Elongation. The leading strand is replicated from 5’ to 3’ like normal but the lagging strand is replicated discontinuously in okazaki fragments (which are then joined by DNA ligase)
3. Termination

Question 5

Describe the cell cycle

Answer 5

Mitosis
G1 - all the cell organelles are replicated
(there is a check point to ensure this is all done correctly so no incorrect cells are duplicated)

S - DNA replication
G2 - the cell checks the duplicated chromosomes are correct then grows ready for division