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The key points covered in this video include:
2. Chromatin Remodelling
3. DNA Methylation
Gene expression can be regulated by chemical modifications to chromosomes. These are known as epigenetic changes. Epigenetic changes are not like mutations – they do not change the base sequence of DNA. Instead, chemical tags are added onto DNA or onto associated proteins called histones. Epigenetic changes occur during development of an organism but importantly, they can also be caused by environmental factors. When a cell divides to make daughter cells, many of these chemical tags are also passed on to the next cell. This means that epigenetic changes may be heritable. Epigenetics refers to changes in DNA that alter the expression of genes without changing the base sequence of DNA itself. These changes can be caused by environmental factors and are heritable. All of the chemical tags attached to the DNA and histones make up the epigenome of a cell. The epigenome is therefore a result of the lifetime accumulation of signals that the cell has received from the environment.
Epigenetic changes are able to regulate transcription by remodelling chromatin – this is the complex formed by DNA and histones. Signals from the environment cause chemical tags to be added to the histones or the DNA, changing how tightly packed the chromatin in. When the chromatin is tightly packed, the DNA wrapped around it may not be accessible to RNA polymerase and transcription factors. If RNA polymerase cannot bind to the DNA, transcription cannot occur and the gene will not be expressed. Tightly packed chromatin is known as heterochromatin. When the chromatin is loosely packed, the DNA is exposed and is accessible to RNA polymerase and transcription factors. RNA polymerase can therefore bind to the DNA and the gene will be transcribed. Loosely packed chromatin is known as euchromatin. These chemical tags therefore control which genes are switched on or off by regulating the formation of euchromatin or heterochromatin.
One method that eukaryotes use to regulate gene expression by epigenetic changes is by methylation of DNA. Methyl groups are added to DNA at specific locations called CpG sites – this is where cytosine is found next to guanine in the DNA chain. Methyl groups are added to the cytosine base by an enzyme called DNA methyltransferase. DNA methylation always inhibits transcription and this takes place in one of two ways: Transcription factors may no longer be able to bind to the DNA, therefore also preventing RNA polymerase from binding. To methyl groups may attract proteins that condense the chromatin, making the genes inaccessible for transcription. Methyl groups can be removed from DNA in a process called demethylation. Demethylation has the reverse effect of methylation – the chromatin is more loosely packed and the genes are accessible for transcription.
Gene expression can be regulated by epigenetic changes – these are chemical modifications to chromosomes
These chemical tags are attached to DNA or histones as a result of environmental signals
All the chemical tags on all the chromosomes in a cell make up the epigenome
One type of epigenetic change is methylation of DNA
Methylation of DNA inhibits transcription as the methyl groups prevent the RNA polymerase from binding
The chromatin also becomes more densely packed so the DNA is not accessible for transcription
Densely packed chromatin is called heterochromatin
Demethylation has the reverse effect of methylation, and therefore loosens the chromatin and activates genes
Loosely packed chromatin is called euchromatin