October 16


Epigenetics, Prader-Willi Sydrome, Angelman Syndrome, Methylation, Imprinting, Heterochromatin

By heheals

October 16, 2020


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)

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
Angels are Mother-like and Happy
Angelman’s = Maternal Mutation with Excessive Laughter

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 (http://www.stomponstep1.com/genetic-inheritance-autosomal-dominant-x-linked-recessive-mitochondrial-disease/)

Picture Used: “Schutzengel” by Bernhard Plockhorst available at http://en.wikipedia.org/wiki/File:Bernhard_Plockhorst_-_Schutzengel.jpg under Public Domain
• “The basic unit of chromatin organization is the nucleosome” available at http://commons.wikimedia.org/wiki/File:The_basic_unit_of_chromatin_organization_is_the_nucleosome,_which_comprises_147_bp_of_DNA_wrapped_ar.jpg by Creative Common 3.0 Attribution Share Alike



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  1. Basically, you need the Dad's PWS gene (because moms is always silenced) or you will get Prader Willi.
    You need Mom's AS gene (because dad's is silenced) or you'll get angelman.
    The disomy issue seems confusing but think about it: "Uniparental Disomy" means you get both copies of that chromosome from only one parent! meaning you don't ever get the other parents non-imprinted (gene expressing) allele, thus you'll be missing a functional PWS or AS gene.

  2. Hi Mark C, I hope you see this message. I can't respond directly to you comment (I think that may be based on your account seetings). But to answer your question, I’m pretty sure the way it is in the video is correct. I always remember it that Prader Willi sounds like a more masculine name while Angelman sounds more feminine

  3. I was wondering if you could clarify Angelman and Prader-Willi or if i am picking up your video wrong.  In the video for Prader-Willi you say that the mutation is inherited from the father with silencing of the mothers allele (on chr 15) and vice versa in Angelman.  However, i thought uniparental disomy of chromosome 15 from mother and silencing of the fathers alleles resulted in Prader-Willi and the opposite occured in Angelman.  I had always remember Paternal was silence in Prader-Willi and Maternal in angelMann (i.e. P with P and M with M as a memory aid).  Can you please clarify? sorry if i have misinterpreted the video

  4. Thank you for these videos! And thank you for making them available for free! Please do keep on making more of these – we need them! I don't know if this is asking too much, but if you are looking to further improve on your already great work, you might incorporate animations of what you are describing (didactically, this would definitely make a difference).

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