Epigenetic Factors – Chromosome Theory and Sex Linkage

by Georgina Cornwall, PhD

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    00:00 One example, we will see here is small RNA sequences that interact with DNA as it packs.

    00:01 Really cool stuff in epigenetics because this is where we are seeing nature of nurture.

    00:08 It is not really a question anymore of nature or nurture. It is an impact of where nature is sort of being impacted by the nurture, what we experienced during life. Let us look at some of the DNA packing questions that are epigenetic factors. You recall that DNA is all unwound in order to have access to the genes, the machinery of the cells needs it unwound so that it can translate and transcribe and make the appropriate proteins. And we know that DNA packs into chromosomes using a sequence of coiling and folding and supercoiling and eventually we pack into a chromosome and that chromosome packing can impact. They don't always uncoil all the way, that is what I am saying. You can have euchromatin, which is available to be transcribed and translated that would be the very unwound or loosely bound form of DNA and heterochromatin, which is more tightly packed. Now clearly if the chromosome is very tightly packed and everything is round up, then we don't have access to the DNA and if we were not going to transcribe genes in that region of the DNA, why unravel it all during G1 and S and G2. We don't need to unravel everything all the time, just the sections that need to be expressed. Whether there is heterochromatin or euchromatin will determine whether genes can be expressed.

    01:39 That is the very basic level of gene packing. If you do unwind the DNA most of the way, you will recall that the DNA is wound about 2 and 1/4 times around the histone core and there are tails on these histone proteins that sort of hold on to the DNA around the waist like it is a belt, so that is a sort of belt loops I guess.

    02:01 acetylation increases the expression of genes as it neutralizes the positive charge of the histone tails that hold the DNA in place.

    02:11 This makes the DNA more accessible to transcriptional mechanisms de acetylation, on the other hand, or the removal of the acetyl group causes the histone tails to grip more tightly, which results in less accessible DNA and thus reduced gene expression.

    02:30 Even if DNA is veryunwounded in the euchromatin form and theoretically there would be access to that DNA so that we could transcribe and translate and make the necessary proteins. DNA methylation is another mechanism that can prevent those transcription factors from binding to the DNA. Methylation involves adding methyl groups to cytosine residues along the DNA strand and it binds essentially into the major group and prevents the transcription factors from binding.

    03:07 In another lecture, you will learn that the genome contains all sorts of noncoding stuff that ends up in the production of RNA. Some of these RNAs are involved as epigenetic factors and also preventing access to the DNA. These RNAs will be transcribed into RNAs, but never translated into proteins and they interact with the DNA in a way to kind of keep it coiled, keep it all locked up. Finally, I introduce the idea that environment impacts these epigenetic factors and can change the DNA not only that but we are learning that these changes to DNA could be heritable. In this lecture, we have explored a number of different ideas about chromosomes and chromosome theory, sex linkage and epigenetic factors that are not in the chromosomes themselves. By now, you should be able to explain the chromosomal theory of inheritance and describe the nature of sex linkage as well as discuss how chromosomes are involved in determining sex. Also, you should be able to evaluate the impact of X inactivation and explain exceptions to chromosomal theory. Thank you so much for listening.

    About the Lecture

    The lecture Epigenetic Factors – Chromosome Theory and Sex Linkage by Georgina Cornwall, PhD is from the course Understanding Genetics.

    Included Quiz Questions

    1. Heterochromatin is not available to be transcribed, whereas euchromatin is available to be transcribed.
    2. Euchromatin is not available to be transcribed, whereas heterochromatin is available to be transcribed.
    3. Euchromatin a is tightly wound around histones while heterochromatin is not.
    4. Heterochromatin is a term used for alternate copies of an allele, whereas euchromatin is two of the same copies of an allele.
    5. Both heterochromatin and euchromatin can not be epigenetically modified.
    1. It involves the addition of methyl groups to the DNA strands.
    2. It changes the DNA activity without any changes in the DNA sequence.
    3. It represses gene transcription.
    4. It helps the transcription factors to recognize the gene to be expressed.
    5. It participates in the DNA replication process.

    Author of lecture Epigenetic Factors – Chromosome Theory and Sex Linkage

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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