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Cells - Bacteria, Eukarya and prokarya- ch 12- reading.docx

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York University
BIOL 1000
Julie Clark

12.5-Dna Organization in Eukaryotic Versus Prokaryotic cells  Chromatin – associated proteins that bind with DNA to form Chromosomes  Chromosome – a collection of DNA associated with chromatin  In Eukaryotes, the proteins histone and nonhistone are associated with DNA structure and regulation in nucleus. These proteins are known as chromosomal proteins of eukaryotes  DNA and it’s proteins, termed Chromatin, is structural building block of chromosome  Prokaryotic cells have fewer associated proteins, yet it has 2 classes of proteins with similar functions to those of histones and non-histones: one class organizes DNA structurally and one that regulates gene activity. 12.5a-Histones pack eukaryotic DNA at successive levels of organization  Histones are positivesly charged proteins made of DNA in the chromosomes of eukaryotes  Histones link to DNA because DNA have negatively charged phosphate groups  5 types of histones: H1, H2A, H2B, H3, and H4-they all have relatively the same function: one function is to pack DNA molecules into the nucleus. Histones and DNA packing  Histones pack DNA at several levels of chromatin structure – most fundamental structure called nucleosome. 2 molecules of (H2A, H2B, H3 and H4 = 8 molecules ) combined to form nucleosome core particle around which DNA winds. A Linker (short portion of DNA) helps connect one nucleosome to the next – the nucelosome is also known as the 10nm chromatin fibre Histones and Chromatin Fibres  H1 histone connects nucleosome to linkers, causing it to package into colied structure 30nm indiameter with 6 nucleosomes per turn (this provides the DNA structure with more protection from chemical and mechanical damage) Packing at Still Higher Levels: Euchromatin and Heterochromatin:  During interphase, some chromatin fibres are held loosely together (euchromatin) whereas others are densely packed (heterochromatin) – experiements show that heterochromatin represents large block of genes that are turned off and put into storage form (this show
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