Topic 8—A: Mutations and Gene expression- 8. Epigenetic control of gene expression

Question 1

In eukaryotes what can epigenetic control determine?

Answer 1

• whether a gene is switched on or off- i.e. whether the gene is expressed (transcribed and translated) or not

Question 2

How does epigenetic control work?

Answer 2

• it works through the attachment or removal of chemical groups (epigenetic marks) to or from DNA or histone proteins
• these epigenetic marks don’t alter the base sequence of DNA
• Instead, they alter how easy it is for the enzymes and other proteins needed for transcription to interact with and transcribe the DNA

Question 3

Where do organisms inherit their DNA base sequence?

Answer 3

From their parents
- most epigenetic marks on the DNA are removed between generations but some escape the removal process and are passed on to offspring
- this means that the expression of some genes in the offspring can be affected by environmental changes that affected their parents or grandparents

Question 4

What are the two epigenetic mechanisms used to control gene expression?

Answer 4

1. Increased methylation of DNA
2. Decreased acetylation of histones

Question 5

Increased methylation of DNA

Answer 5

• Methylation is when a methyl group (an example of an epigenetic mark) is attached to the DNA coding for a gene
• The group always attaches to a CpG site which is where a cytosine and guanine base are next to eachother in DNA ( linked by a phosphodiester bond)
• Increased methylation changes the DNA structure so that the transcriptional machinery (e.g. enzymes) can’t interact with the gene- so the gene is not expressed- it’s switched off

Question 6

Decreased acetylation of histones

Answer 6

• histones are proteins that DNA wraps around to form chromatin, which makes up chromosomes
• Chromatin can be highly condensed or less condensed
• how condensed it is affects the accessibility of DNA and whether or not it can be transcribed
• histones can be epigenetically modified by the addition or removal of acetyl groups (another example of an epigenetic mark)
• when histones are acetylated the chromatin is less condensed
• this means the transcriptional machinery can access the DNA allowing genes to be transcribed
• When acetyl groups are removed from the histones, the chromatin become highly condensed and genes in the DNA can’t be transcribed because the transcriptional machinery can’t physically access them
• histone deacetylase (HDAC) enzymes ae responsible for removing the acetyl groups

Question 7

What is good about epigenetic changes being reversible?

Answer 7

• It makes them good targets for new drugs to combat diseases they cause
• these drugs are designed to counteract the epigenetic changes that cause the diseases

Question 8

What are the problems for developing drugs to counteract epigenetic changes?

Answer 8

• these changes take place normally in a lot of cells, so its important to make sure the drugs are as specific as possible
  E.g. drugs used in cancer therapies can be designed to only target dividing cells to avoid damaging normal body cells