Epigenetics

Biology: AQA A Level, topic 8.2.6

Epigenetic Control

Epigenetic regulation interacts with DNA to control the access to DNA. Controlling access to the DNA alters gene expression without actually changing the DNA code. These changes can be inherited.

Chromatin

DNA in the nucleus combines with proteins called histones.
The combination of DNA and histones is called chromatin.
A chemical layer surrounds the chromatin. This is called the epigenome.

Epigenome

The epigenome interacts with the chromatin and changes its structure.
The epigenome can cause the chromatin to become either:
More condensed. This prevents transcription factors from binding to DNA so transcription is inhibited.
Less condensed. This allows easier access to transcription factors, promoting transcription.

Epigenetic markers

Chromatin becomes more or less condensed when epigenetic markers are attached or removed to the DNA or histone proteins.
Epigenetic markers are groups (e.g. methyl groups) that do not alter the base sequence but influence chromatin structure.
E.g. Methylation of DNA makes chromatin more condensed.

Epigenetic Markers

Epigenetic markers bind to DNA or histone proteins to influence transcription of DNA. Examples of epigenetic markers are methyl groups and acetyl groups.

Increased methylation

Methyl groups bind to a CpG site on DNA. CpG sites are areas in DNA where cytosine and guanine are together in the base sequence.
Methyl groups cause the chromatin to be more condensed.
When chromatin is more condensed transcription factors can’t reach the DNA.
Methylation inhibits transcription.

Decreased acetylation

Acetyl groups (CH₃CO) are removed from histone proteins.
Removal of acetyl groups increases the positive charge on histone proteins. This increases the attraction to phosphate groups on DNA.
Decreased acetylation causes the chromatin to condense.
When chromatin is more condensed transcription factors can’t reach the DNA.

Inheritance

The action of epigenetic markers results in changes in the chromatin structure.
Epigenetic markers can be inherited by offspring.
Inheritance of epigenetic control means that environmental factors (e.g. methylation) experienced by an individual can influence the gene expression of their offspring.
E.g. Starvation of human adults can influence the gene expression in their offspring.

Test yourself on Epigenetics

Which of the following statements about epigenetic control are true?
What is the name of the structure that is made from DNA and histone proteins?
What is the effect of increased methylation on chromatin?
Which of these statements about inheritance of epigenetic markers is true?

Practise this topic for free