Genes in Action

Question 1

6 Key stages in discovering how DNA replicates

Answer 1

1. Chargoff finds that the percentage of G = C and A = T
2. Watson and Crick describe the double helix
3. Meselson and Stahl prove that DNA replication is semi conservative
4. Kornberg characterised DNA polymerase
5. Cairns identified the replication fork
6. Proof of bidirectionality

Question 2

Describe the hierarchical nature of DNA packing

Answer 2

*DNA double helix
*Extended chromatin
Condensed chromatin
Scaffold-associated chromatin
Condensed scaffold-associated chromatin
Metaphase chromosone (most compact)

*stage where DNA is able to be read

Question 3

How is chromatin formed

Answer 3

Chromatin = DNA + histones

Histone octomers (2 copies of each of the main histones) interact with DNA
H1 is used as a clamp
One of these is a nucleosome

Question 4

How did meselson and stahl proove semi conservative DNA replication?

Answer 4

Density gradient seperation - centrifuge DNA, which means the DNA strands find an equilibrium in the tube

1. DNA grown on N15 medium to form ‘heavy’ DNA
2. Moved to N14 medium

Question 5

How were the other two proposals for DNA replication ruled out?

Answer 5

Conservative: ruled out after generation 1 (not two bands)
Dispersive: ruled out after generation two when there were two bands (as opposed to one band getting lighter)

Question 6

How was replication prooved to be bidirectional?

Answer 6

Cells grown in low specific activity H thymidine to allow uniform DNA labelling
Pulse of high specific activity used to identify newly synthesied DNA - and both replication forks were shown to be active

Question 7

How were replication forks visulised?

Answer 7

Grown on H labelled thymidine (to label them)

Question 8

What structure does a replicating chromosone have?

Answer 8

theta struture θ

Question 9

How is DNA so tightly packed into a eukaryotic chromosone?

Answer 9

Problem: electrostatic repulsion due to negative charge
Histones are very positivly charged due to high concentration of argine and lysine amino acids

Question 10

Explain Base Excision Repair

Answer 10

Repairs damage to specific bases
- Eg. C can spontaneously deaminate to a U

Glycosylases removes the base
Another enzyme removed the nucleotide
DNA pol fixes the nick

Question 11

Describe mismatch repair in e.coli

Answer 11

MutS recognises problem and initates mutL

MutL: Identifies which strand is the template (through which one is methylated) strand and activates MutH

MutH: Has exonuclease activity and removes a stretch of DNA.

DNA pol III and DNA ligase used to fill gap

Question 12

Describe nucleotide excision repair in prokaryotes

Answer 12

3 proteins

UvrA recognises problem

Forms complex with UvrB, which unwinds the DNA

UvrC is recrutited, cuts the fragement

DNA polI I and ligase fix the nicks

Question 13

What is photoreactivation

Answer 13

UV light damage to DNA

Often results in dimerisation of the prymidines (T-T) and (C-C)