Control of Gene Expression 2

Question 1

What is epigenetics?

Answer 1

Heritable changes in gene expression which occur without changes in the DNA sequence

Question 2

What is the length of all the DNA contained within the nucleus of a human cell

Answer 2

2m

Question 3

What is chromatin made up of?

Answer 3

DNA

Histone proteins

Question 4

What is a nucleosome?

Answer 4

146bp of DNA wrapped 1.7 times around an octomer of histone proteins found in eukaryotes

Question 5

What is the length of a nucleosome?

Answer 5

11nm

Question 6

What histone proteins are present in a nucleosome?

Answer 6

2 x H2A

2 x H2B

2 x H3

2 x H4

Question 7

How is DNA compacted?

Answer 7

Wrapped around histones to form nucleosomes

Nucleosomes packed into 30nm fibres

Looped and folded into chromosomes

Question 8

What is the basic structure of a histone protein?

Answer 8

Globular head

Charged N-terminal tail

Question 9

By what processes can the histone tails be modified?

Answer 9

Acetylation

Methylation - lysine residues

Phosphorylation - turn genes on

Question 10

What is DNA methylation?

Answer 10

Methylation of cytosine at position 5 by DNA methyltransferase

Question 11

What is an example of a DNA methyltransferase?

Answer 11

S-adenosylmethionine

Question 12

What type of cytosine can be methylated?

Answer 12

Cytosines which are 5’ of guanines

Question 13

Explain how methylation of DNA affects gene expression.

Answer 13

Methyl group protrudes from major groove producing a novel epitope

Methylated DNA can be more tightly compacted

Turns genes ‘off’

Question 14

What is the evidence that only CpG cytosines can be methylated?

Answer 14

Mspl and Hpall = isoschizomers which both cut GCGC but only Mspl will cut if first C is methylated

Compare Southern Blot patterns to find methylated sites

Many genes found to be inactivated by methylation of promoters

Question 15

What is the evidence that methylation of DNA inactivates genes?

Answer 15

5-aza-cytidine is a potent demethylating agent

Can reactivate many inactive genes

Question 16

What are CpG islands?

Answer 16

Areas of genome rich in CpGs (up to 60%)

Tend to be upstream of genes/coding regions

Question 17

What percentage of CpGs are methylated?

Answer 17

50-60%

Question 18

What is lyonisation?

Answer 18

Inactivation of X chromosome by DNA methylation to ensure normal gene dosage

Question 19

Explain why the calico cat pattern cannot be cloned.

Answer 19

Colour gene is X-linked

Lyonisation pattern in blastula is not genetic/is random

Question 20

How could SAHA be used in future epigenetic therapy?

Answer 20

Histone deacetylase inhibitor used in treatment of T-cell lymphoma

Prevents deacetylation of histone tails to allow gene activation

Question 21

How could 5-aza-cytidine be used in future epigenetic therapy?

Answer 21

Cytosine analogue used in myelodysplastic syndrome

Inhibits DNA methylation as it is incorporated into DNA

Helps reactivate genes