Heterochromatin, epigenetics and gene expression

Published on February 4, 2014   51 min

Other Talks in the Series: Epigenetics, Chromatin, Transcription and Cancer

Other Talks in the Series: Molecular Genetics of Human Disease

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The title of this talk is Heterochromatin, Epigenetics and Gene Expression. I'm Joel Eissenberg. I'm a professor in the Department of Biochemistry and Molecular Biology at Saint Louis University School of Medicine. My goal in this talk is to survey our understanding of heterochromatin and its connection to the idea of epigenetic control of gene expression. Terms like heterochromatin and epigenetics are frequently used to disguise, rather than explicate, our understanding of how cells organize and use genetic information. In this presentation, I'll define these terms and discuss epigenetics in the context of heterochromatin and the evidence that it can influence gene expression.
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The term heterochromatin was coined by Emil Heitz to refer to the material in the eukaryotic nucleus that fails to decondense after telophase in the cell cycle. On the left in this slide is an image showing the chromosomes of a cell at or near telophase stained with a fluorescent dye that labels DNA. The bright fluorescent staining, is coextensive with the chromosomes. In contrast, the interphase nucleus on the right is filled with DNA, but only certain regions stain brightly, the heterochromatin. The condensed state of heterochromatin concentrates the DNA, making these regions stand out on the background of the rest of the DNA fluorescents in the nucleus. This property distinguishes it from the remaining so-called euchromatin, or true chromatin, that undergoes cyclic condensation and decondensation. In contrast, heterochromatin exhibits heterocyclic behavior, hence the term heterochromatin. Thus, the term heterochromatin was originally coined to describe a cytological phenomenon, not a genetic or biochemical phenomenon. Heitz ultimately showed that most or all eukaryotic chromosomes are differentiated along their lengths by zones of euchromatin, which he recognized as relatively gene rich, and heterochromatin, which he recognized as relatively gene poor. More recently, the term heterochromatin has been used more promiscuously to describe any form of chromatin associated with transcriptional silencing and/or chromatin enriched for certain biochemical markers, such as cytosine methylation, or certain histone modifications. In this presentation, I'll stick to examples that are consistent with the original definition of the word.
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Heterochromatin, epigenetics and gene expression

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