What does epigenetics control?
Gene expression (protein synthesis)
What is epigenetics?
The heritable change in gene function, without changing the base sequence of DNA. These changes are caused by the environment and can inhibit transcription.
What factors can add chemical tags to the DNA?
Diet
Stress
Toxins
What is the epigenome?
A single layer of chemical tags on the DNA and this impacts the shape of the DNA-histone complex and whether the DNA is tightly wound so won’t be expressed or unwound so it will be expressed.
What happens if the DNA is tightly wound?
Transcription factors can’t bind so the epigenome can inhibit transcription.
Describe heterochromatin.
Tightly coiled DNA.
Transcription won’t occur as DNA is tightly coiled so transcription factors can’t bind. This is caused by increased methylation and decreased acetylation.
Describe chromatin.
Loosely coiled DNA.
Can result in transcription.
Caused by decreased methylation and increased acetylation.
Describe increased methylation.
Increased methylation of DNA inhibits transcription.
When methyl groups are added to DNA, they attach to the cytosine base.
This prevents transcriptional factors from binding and attracts proteins that condense the DNA-histone complex.
In this way methylation prevents a section of DNA from being transcribed.
Methyl group is positively charged, DNA is negatively charged so the DNA and methyl groups attract and tightly coil together.
Describe the acetylation of histone proteins.
Acetyl groups bind directly to the histones.
Acetyl groups are negative and so is the DNA so they repel each other so DNA doesn’t tightly coil and is far more loosely packed so DNA is more accessible for the transcriptional factors to bind so transcription of that gene can occur.
What do tumour suppressor genes cause?
They produce proteins to slow down cell division and to cause cell death if DNA copying errors are detected. Thus it regulates mitosis.
Describe what could happen if a mutation occurs in the tumour suppressor gene.
The tumour suppressor gene won’t produce the proteins to carry out its function so cell division continues and mutated cells will not be identified and destroyed.
What are two examples of known mutated tumour suppressor genes?
BRCA1 and BRCA2 - breast cancer.
What does methylation cause?
A gene to turn on or off.
What does it mean if tumour suppressor genes become hypermethylated?
An increased number of methyl groups attach to it (Tightly coiled). This results in the gene being inactivated and becomes turned off.
Where does hypomethylation occur? Describe hypomethylation.
Oncogenes.
This reduces the number of methyl groups attached. This results in the gene being permanently switched on
