Epigenetics, Prader-Willi Sydrome, Angelman Syndrome

While genetics is the study of DNA sequence, Epigenetics is the study of heritable changes that do not affect the DNA sequence. Genetics determines what is transcribed, while epigenetics determines how much is transcribed. Remember that negatively charged DNA is wrapped around positively charged histone proteins like beads on a string in the nucleus. Epigenetics primarily changes how tightly the DNA is condensed. DNA that is wrapped very tightly around the histones is not actively transcribed. It is clumped up so tight that the enzymes needed for transcription don’t have enough room to work. Alternatively, DNA that is loosely associated with histones is more easily accessible.

  • Euchromatin – loosely packed chromatin that is easily accessible by transcription enzymes and actively transcribed. Think People from the EU (European Union) tend to be relaxed and less uptight
  • Heterochromatin – tightly packed chromatin that is not actively transcribed (silent)

Euchromatin Heterochromatin USMLE

The most common type of epigenetic changes are DNA methylation, histone acetylation, and histone methylation. These changes affect the hydrophobic and/or electrical properties that control the attraction between histones and DNA.

  • Histone Acetylation removes the histones positive charged decreasing its level of attraction to DNA causing uncoiling and more active transcription
  • Histone or DNA Methylation – makes histone/DNA more hydrophobic which causes it to clump up in the hydrophilic environment. Causes heterochromatin and less active transcription

Acetylation –> Active

Methylation –> genes are Missing

 Imprinting is when one allele is naturally inactivated by epigenetic changes and only 1 allele is expressed. This type of inheritance is only present in a small number of human genes. For some of these genes the allele from the mother is expressed while the allele from the father is silenced. Other genes are the opposite. When only one allele is being expressed, a single mutation can change the phenotype of a homozygous individual.

  • Prader-Willi Syndrome – involves inheriting a mutated allele from the father while the allele inherited from the mother is naturally silenced. Causes mental retardation and Hyperphagia (excessive eating).
  • Angelman Syndrome – involves inheriting a mutated allele from the mother while the allele inherited from the father is naturally silenced. Excessive laughter, happy demeanor, seizures, and mental retardation.

Angelman Syndrome Mnemonic Prader Willi Syndrome


Now that you have finished this video you should check out the next video in the Genetics section which covers different types of inheritance like Autosomal Dominant, Autosomal Recessive & X-linked recessive.


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