Epigenetic Control Of Gene Expression

Question 1

What is meant by epigenetics

Answer 1

It’s the process by which environmental factors can cause heritable changes in gene function without changing the base sequence of DNA

Question 2

What is the epigenome

Answer 2

Chemical tags on DNA and histones form a second layer known as epigenome

Question 3

What does the epigenome determine

Answer 3

The shape of the DNA histone complex

E.g. keeps genes that are inactive in a tightly packed arrangement and therefore ensures that they cannot be read (keeps them switched off) - this is known as epigenetic silencing

or

It unwraps active genes so that the dna is exposed and can easily be transcribed (switches them on)

Question 4

What is epigenetic silencing

Answer 4

keeps genes that are inactive In a tightly packed arrangement and therefore ensures that they cannot be read (keeps them switched off)

Question 5

Why is the epigenome flexible

Answer 5

Because it’s chemical tags respond to environmental changes.

Factors like diet and stress can cause the chemical tags to adjust the wrapping and unwrapping of dna and so switch genes on and off

Question 6

What is important in shaping the epigenome in early development

Answer 6

The signals come from within the cells of the fetus and the nutrition provided by the mother

Question 7

The environmental signal stimulates proteins to carry its message inside the cell from where it is passed by a series of other proteins into the nucleus. Here the message passes to a specific protein which can be attached to a specific sequence of bases in the dna. Once attached the protein has two possible effects, what can it change ?

Answer 7

Acetylation of histones leading to the activation or inhibition a gene

Methylation of DNA by attracting enzymes that can add or remove methyl groups

Question 8

Where the association of histones with DNA is weak, what’s the dna-histone complex like

Answer 8

It is less condensed (loosely packed)

Question 9

What can the dna histone complex do when it’s less condensed

Answer 9

It is accessible by transcription factors, which can initiate production of mRNA, that is, can switch the gene on

Question 10

What’s the condition like when the association is stronger between the dna histone complex

Answer 10

It’s more condensed, the DNA is not accessible by transcription factors, which therefore cannot initiate production of mRNA, that is, the gene is switched off

Question 11

What does condensation of the DNA histone complex do?

Answer 11

Inhibits transcription

It can be brought about by decrease acetylation of these histones or by methylation of DNA

Question 12

What is acetylation

Answer 12

An acetyl group is transferred to a molecule

Question 13

What’s deacetylation

Answer 13

Is the reverse reaction where an acetyl group is removed from a molecule

Question 14

What does a decreased acetylation do

Answer 14

Increase the positive charges on histones and therefore increases their attraction to the phosphate groups of DNA

The association between dna and histones is stronger and the dna is not accessible to transcription factors.

These transcription factors cannot initiate mRNA production from DNA. In other words, the gene is switched off

Question 15

What is methylation

Answer 15

Is the addition of a methyl group (CH3) to a molecule

Question 16

How does methylation inhibit transcription of genes in 2 ways?

Answer 16

Preventing the binding of transcriptional factors to the DNA

Affecting proteins that condense the DNA- histone complex (by inducing deacetylation of the histones) making the DNA inaccessible to transcription factors

Question 17

What’s the effect on
DNA
DNA-histone complex
Chromatin type
Transcription factors
Gene 

If there is a decreased acetylation in the histones

Answer 17

DNA- increased methylation 
DNA-histone complex - more condensed 
Chromatin type-heterochromatin
Transcription factors-no access 
Gene - inactive

Question 18

What’s the effect on
DNA
DNA-histone complex
Chromatin type
Transcription factors
Gene 

If there is a increased acetylation in the histones

Answer 18

DNA- decreased methylation 
DNA-histone complex- less condensed 
Chromatin type- euchromatin 
Transcription factors- access
Gene - active

Question 19

What can altering the epigenetic processes do

Answer 19

Can cause abnormal activation or silencing of genes

Question 20

What can increased methylation do to lead to cancer

Answer 20

Has led to protective genes been switched off. As a result, damaged base sequences in DNA are not repaired and can lead to cancer

Question 21

How can we treat diseases with epigenetic therapy

Answer 21

drugs that inhibit enzymes that cause DNA methylation can reactivate genes that have been silenced.

Can do tests that can identify the level of DNA methylation and histone acetylation at an early stage of disease - allowing a better chance to seek early treatment

Question 22

Describe the mechanism involving small double stranded sections of siRNA

Answer 22

1. An enzyme cuts large double stranded molecules of RNA into smaller sections called small interfering RNA (siRNA)
2. One of the two siRNA strands combines with an enzyme
3. The siRNA molecule guides the enzyme to a mRNA molecule by pairing up its bases with the complimentary ones on a section of the mRNA molecule
4. The mRNA is no longer capable of being translated into a polypeptide
5. This means that the gene has not been expressed, that is, it has been blocked

Question 23

One of the 2 strands of siRNA combines with an enzyme and guides it to an mRNA molecule which it then cuts. Explain why the mRNA is unlikely to be cut if the other siRNA strand combines with the enzyme

Answer 23

The other strand would have complimentary bases. It is unlikely that these opposite base pairings would complement a sequence of the mRNA. The siRNA, with enzyme attached, would therefore not bind to the mRNA and so would be unaffected