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Human Biology (Year 11) - DNA

Ben Whitten

What is epigenetics?

Consider this; all of our cells have the same DNA, yet our bodies contain many different types of cells. How can this be?

Cells, tissues, and organs differ because they have certain sets of genes that are ‘turned on’ or expressed and genes that are ‘turned off’ or inhibited. Epigenetic silencing is one way of turning genes off and can contribute to differential expression. This may explain why genetic twins may not be phenotypically identical!

Key Definitions

  • Epigenetics: The study of how DNA interacts with chemical compounds in cells, which results in the activation or deactivation of genes; mitotically heritable changes in gene activity or expression that occur without altering the genome.

  • Gene expression: The process of transcribing DNA into RNA, which is then translated into proteins in the ribosomes. The resulting proteins determine a cell’s characteristics and function.

  • Genome: Hereditary information encoded in their DNA.

  • Epigenome: Sum of all the factors determining when, where and which genes are switched on or expressed. If the epigenome is abnormal, cell function and structure may be abnormal, resulting in disease.

The Epigenetic Code

  • DNA methylation:

    • Represses transcription (switches a gene off)

    • Methyl groups attach to the DNA to a cytosine directly before a guanine

    • CpG are common in the promotor region (binding site for RNA polymerase)

  • Histone modification:

    • Acetyl groups attach to the histone tails which removes the positive charge of the histone

    • Reduces attraction between DNA and histones allowing DNA to uncoil

      • Histone acetylation = Enhances transcription

      • Histone acetylation = Represses transcription

Epigenetics and the Environment

  • External influences (i.e. environment and diet) can affect DNA methylation patterns, permanently changing your epigenetic imprint.

  • Factors that affect DNA methylation - Age, diet, stress, heavy metals, pesticides, diesel exhaust, tobacco smoke.

Abnormal Methylation Patterns

  • Abnormal DNA methylation can lead to abnormal gene activity or inactivity e.g. modifying the wrong gene, failing to add a compound to a gene.

  • Factors associated with abnormal DNA methylation – Pesticides, heavy metals, tobacco smoke, stress.

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