Epigenetics involves heritable changes in gene function, without changes to the base sequence of DNA. These changes are caused by changes in the environment that inhibit or activate transcription.DNA in the nucleus is wrapped around proteins called histones, forming chromatin. Chemical groups (tags) can be attached to the DNA and to the histones. Together, these tags make up the epigenome of the cell. Unlike the DNA base sequence, which is fixed, the epigenome is flexible and can respond to the environment.The epigenome controls how tightly the chromatin is packed:
• If chromatin is highly condensed, transcription factors and RNA polymerase cannot reach the DNA, so transcription is inhibited.
• If chromatin is less condensed, the DNA is accessible and genes can be transcribed.The two mechanisms you need to know are increased methylation of DNA and decreased acetylation of histones, both of which inhibit transcription (see the next two lessons).Environmental factors that can change the epigenome include diet, stress, smoking and toxic chemicals. Signals from the environment activate or inhibit the proteins that add or remove the tags.Epigenetic changes are heritable. When a cell divides by mitosis, the tags are copied to the daughter cells, so specialised cells keep their pattern of gene expression. Some tags are not removed when gametes form, so they can be passed from parents to offspring. Epigenetics also explains why identical twins, who have the same DNA base sequence, develop differences in gene expression as different environments change their epigenomes.
• If chromatin is highly condensed, transcription factors and RNA polymerase cannot reach the DNA, so transcription is inhibited.
• If chromatin is less condensed, the DNA is accessible and genes can be transcribed.The two mechanisms you need to know are increased methylation of DNA and decreased acetylation of histones, both of which inhibit transcription (see the next two lessons).Environmental factors that can change the epigenome include diet, stress, smoking and toxic chemicals. Signals from the environment activate or inhibit the proteins that add or remove the tags.Epigenetic changes are heritable. When a cell divides by mitosis, the tags are copied to the daughter cells, so specialised cells keep their pattern of gene expression. Some tags are not removed when gametes form, so they can be passed from parents to offspring. Epigenetics also explains why identical twins, who have the same DNA base sequence, develop differences in gene expression as different environments change their epigenomes.