Topic 3: Epigenetics

Question 1

What is the epigenome? (1 point)

Answer 1

1. Chemical markers that are attached to DNA or histones

Question 2

What types of chemical markers do we know? (2)

Answer 2

(1) Methyl or (2) acetyl groups

Question 3

What are the types of epigenetic changes?

2 types

Answer 3

(1) DNA methylation and (2) histone modification

Question 4

What is the role of the epigenome? (1 point)

Answer 4

1. Regulation of gene expression – controls which genes are switched on and transcribed to produce proteins, and which are switched off

Question 5

What are the steps of how the environment influences the epigenome to affect the phenotype?
(4 steps)

Answer 5

1. Environmental factors (Medicines/drugs, chemicals, diet, smoking, infectious disease)
2. Can trigger changes in the epigenome
3. affects gene expression (proteins made)
4. Affects phenotype

Question 6

(The effect of diet on the epigenome in mice)

What would you say in an exam Q to state that two mice have the same genotype but different phenotypes? (1 point)

Answer 6

1. These mice have the same agouti gene /genotype but are epigenetically different, so have different phenotypes.

Question 7

What does the agouti gene produce? (1 thing)

What does the product do? (2 things)

Answer 7

1.Produces the agouti protein

which binds to and blocks:

1. MSH receptors in skin
2. Feeding behaviour receptors in brain

Question 8

How does the agouti protein impact the phenotype? (the result) (2 point)

What will the phenotype be? (1 point)

Answer 8

1. MSH cannot bind to MSH receptors, melanin (dark pigment) is not made, so mouse is lighter in colour.
2. Mouse overeats and becomes obese
3. Phenotype = yellow/obese

Question 9

How does a mouse become brown/healthy/normal weight?

Suggest what epigenetic change might switch off the gene
name epigenetic change + 3 steps

Answer 9

DNA methylation:

1. methyl group can attach to cytosine of DNA
2. prevents RNA polymerase binding to DNA
3. agouti gene is switched off - no mRNA made - no agouti protein made

Question 10

What about the agouti gene means that a mouse is brown? (1 point)

Answer 10

1. Agouti gene is methylated, so agouti gene is switched off and not expressed.

Question 11

What about the agouti gene means that the mouse is yellow? (1 point)

Answer 11

1. Agouti gene is not methylated so Agouti gene is switched on, so expressed, so (transcribed) and translated producing the agouti protein.

Question 12

(in mice) Methylation means the gene is switched ___

Answer 12

OFF

Question 13

(in mice) No methylation means the gene is switched ___

Answer 13

ON

Question 14

(in mice) If the agouti gene is switched on, the mouse will be ____ and ____.

Answer 14

yellow

obese

Question 15

(in mice) If the agouti gene is switched off, the mouse will be ____ and ____.

Answer 15

brown

healthy

Question 16

In an experiment,
Some pregnant yellow mice are fed a methyl-rich diet.
Other pregnant yellow mice are fed regular mouse food.

What are going to be the results?
(3 points)

Answer 16

1. Mother fed a methyl-rich diet, had predominantly brown offspring
2. Mother fed regular mouse food, had predominantly yellow offspring (like the mother, the mother is yellow)
3. dietary supplement (methyl) is the environmental factor
   which affects gene expression in the offspring

Question 17

In an experiment,
Some pregnant yellow mice are fed a methyl-rich diet.
Other pregnant yellow mice are fed regular mouse food.

Mother fed a methyl-rich diet, had predominantly brown offspring

Mother fed regular mouse food, had predominantly yellow offspring (like the mother, the mother is yellow)

Dietary supplement (methyl) is the environmental factor which affects gene expression in the offspring.

What is the conclusion of the experiment? (3 points)

Answer 17

1. If pregnant yellow mice are fed a methyl-rich diet, this methylates DNA,
2. so agouti gene switched off, agouti protein not made, MSH and brain feeding receptors not blocked, {melanin made/normal eating} and they produce healthy, slim, brown offspring
3. The epigenetic change must happen early in embryo development

Question 18

Key Summary: How does a METHYL RICH DIET (environment) affect the epigenome and therefore phenotype of a mouse?
(7 steps)

Answer 18

1. Methyl rich diet causes methylation of DNA of agouti gene
   (epigenetic change)
2. RNA polymerase cannot bind so agouti gene is switched off – gene not expressed
3. No agouti protein made
4. No blocking of MSH and brain feeding receptors
5. MSH can bind to MSH receptors, melanin is made - mouse is brown
6. No blocking of brain feeding behaviour receptors so mouse does not overeat and does not become obese
7. Phenotype – healthy/slim brown mouse

Question 19

Key Summary: How does a METHYL POOR DIET (environment) affect the epigenome and therefore phenotype of a mouse?
(7 steps)

Answer 19

1. Diet lacks methyl, so no methylation of DNA of agouti gene
2. Agouti gene is switched on and expressed
3. Agouti protein made
4. MSH receptors on melanocytes and brain feeding receptors blocked
5. MSH cannot bind to MSH receptors so no melanin is made, mouse is yellow
6. Brain feeding behaviour receptors are blocked, so mouse overeats and becomes obese
7. Phenotype – Unhealthy/obese yellow mouse

Question 20

Experiment:
A good mother (calm, licks and grooms offspring frequently)
A bad mother (nervous/easily frightened, licks and grooms offspring rarely)
Their offspring are then observed over many generations.

What are the observations?
(3 points)

Answer 20

1. Over many generations – rat offspring were similar to their mothers
2. Bad mother – offspring were bad mothers
3. Good mother – offspring were good mothers

Question 21

Experiment:
A good mother (calm, licks and grooms offspring frequently)
A bad mother (nervous/easily frightened, licks and grooms offspring rarely)
Their offspring then observed over many generations.

Over many generations – rat offspring were similar to their mothers
Bad mother – offspring were bad mothers
Good mother – offspring were good mothers

What is the conclusion of the experiment? (1 point)

Answer 21

1. Researchers concluded that there is a genetic basis for ‘mothering’ – it is affected by genotype.

Question 22

What were the results when the researchers swapped the offspring of each group at birth?
(Good and bad mother rat experiment)
(2 points)

Answer 22

1. Bad mother + offspring from good mothers:
   Offspring became nervous adults/grooms rarely
2. Good mother + offspring from bad mothers:
   Offspring became calm adults/grooms frequently

Question 23

(Good and bad mother experiment)

What was the conclusion when the researchers swapped the offspring of each group at birth? (2 point)

Answer 23

1. Levels of anxiety and licking/grooming are determined by the mother who brings up the pup (so the environment/behaviour) rather than the birth mother.
2. This means the two types of offspring are epigenetically different.

Question 24

Key Summary:
Explain how a BAD mother means its offspring will have the same characteristics (of a BAD mother)
(good and bad mother experiment)
(9 steps)

Answer 24

1. Two types of rat offspring are epigenetically different
2. Bad mother – no licking and grooming in first week of life
3. Offspring’s GR gene is methylated
4. Methylation prevents GR gene being expressed
5. No hormone receptor for glucocorticoid/stress hormone
   made
6. No receptor to bind hormone
7. When exposed to stress not enough hormone binds to
   receptors (calming signals not sent out)
8. No relaxation of offspring after stress/stressed for longer
9. Methylation remains – so offspring and adult rats are more anxious/don’t groom

Question 25

Key Summary:
Explain how a GOOD mother means its offspring will have the same characteristics (of a GOOD mother)
(good and bad mother experiment)
(8 steps)

Answer 25

1. Two types of rat offspring are epigenetically different
2. Good mother – licking and grooming in first week of life
3. Offspring’s GR gene is NOT methylated
4. GR gene expressed
5. Hormone receptor for glucocorticoid/stress hormone
   made
6. When exposed to stress hormone binds to receptors
   (calming signals sent out)
7. Relaxation of offspring after stress
8. No methylation remains – so offspring and adult rats are less anxious/groom

Question 26

The environmental factor results in _________ _________ which are ____ __________ and _________ __________

Answer 26

epigenetic changes

DNA methylation and histone modification

Question 27

The epigenetic changes regulate _____ __________

Answer 27

gene expression

Question 28

When a cell divides, epigenetic changes to genes may be copied and passed on to ________ _______

Answer 28

daughter cells

Question 29

When a cell divides, epigenetic changes to genes may be copied and passed on to daughter cells.

If epigenetic changes are passed on, the genes that were switched on/off in the parent cell are also… (1 point)

If the environment changes the epigenome (to help us deal with a changed environment), it is useful for the same epigenetic markers to be passed to daughter cells to help them too.

Changes may then be passed to offspring in _______.

Answer 29

1. …. switched on/off in the new cells

gametes