3.6.2007 - 3.6.2007 Histone H3 on nucleosomes let go of the...

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3.6.2007 Histone H3 on nucleosomes let go of the DNA a little bit to make it available to transcription factors and RNA polymerase with help of HAT histone acetyl transferase Chromatin can go the other was as well, for further packaging necessary for cell division chromatin with radial loop domains are folded back and forth (further looping and coiling) vague description Histone methylase (takes euchromatin) and can methylate the histone (H3) on a radial loop domain and a methylated histone H3 creates a binding site for another protein, the other protein is a non-histone protein that is specific to heterochromatin o So the methylated chromatin is a binding site for a heterochromain protein and this other protein binds up the chromatin very densely to make it heterochromatic o One reason for this is during development a certain tissue like skin will never again need to transcribe genes in a certain area along a chromosome, not needed for skin cells, one way this is handled is the histones H3 in this area might be methylated to turn it into heterochromatin which makes those genes far far less likely to be transcribed – one way trip – no enzyme to take methyl groups off of histones – once it is converted to heterochromatin it is heterochromatin – this is okay because these cells will never need these genes again This is facultative heterochromatin because it is available in some other tissue where it is necessary and it was euchromatic before and hasn’t always been heterochromatic (if something becomes heterochromatic it is facultative) Need to know what holds the DNA in each level of packaging and the cases of how the level of packaging can be changed by one or another enzyme and why it would be changed and to what effect Can describe a sequence of DNA as o Unique: unique sequences are found only in one copy in the genome, found in one location and no other Most protein encoding genes are unique sequences, there is one copy of the gene per genome You think of yourself as having two copies of one from each parent that’s okay we have two genomes and one per genome Some proteins are needed in a large quantity but if the one unique gene that encodes this protein is encoding the RNA and many copies of this can be made and a single mRNA can be translated many times o Moderately repetitive sequences In 10-1000 copies per genome Genes like ribosomal RNA genes and transfer RNA genes are in this category We need a lot of rRNA and tRNA to function so this makes sense, and there isn’t the same opportunity for amplification for rRNA and tRNA like for mRNA o Highly repetitive sequences Many thousands of copies Categorized according to how the copies are arranged Tandem: some are arranged tandemly which means you find the copies head to tail, same thing over and over again (telomeric sequence) – a lot of copies are in one location o centromere: tandemly repetitive as well, same sequence over and over again within
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This note was uploaded on 04/07/2008 for the course BIO 325 taught by Professor Saxena during the Spring '08 term at University of Texas.

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3.6.2007 - 3.6.2007 Histone H3 on nucleosomes let go of the...

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