2) Chromatin the Dynamic Structure

This is the famous picture of beads on a string you

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This is the famous picture of  “beads on a string”.  You see this only if H1 is removed, why?? BIO 1140 Unit 1-2:Chromatin a Dynamic  Structure 11
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The Solenoid or 30nm Fibre 6-8 nucleosomes per turn structure less well defined  Sequences that are more distant  can now be closer together!!! 6-8 nucleosomes per turn structure less well defined  Sequences that are more distant  can now be closer together!!! BIO 1140 Unit 1-2:Chromatin a Dynamic  Structure Fig. 12-21, p. 279 12
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Higher order structures: the Looped  Structure** and the “chromosome” While the previous structures are well defined at several levels, there are  higher ordered structures which are less well defined  (WHY!).  What are they and what is the evidence for them?  Data Models Seq are BIO 1140 Unit 1-2:Chromatin a Dynamic  Structure 13
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HETEROCHROMATIN   More highly condensed  Genes are generally inactive  DNA is more highly methylated.  15% or higher of genome **  Centromeric and telomeric  regions  Highly enriched in DNA repeats General Properties of Chromatin: In the interphase cell, chromatin can be experimentally divided into: Heterochromatin can be constitutive or  facultative; heterochromatin and  euchromatin can change by  a process called chromatin remodelling. EUCHROMATIN  Less condensed, more dispersed  Genes are generally active  DNA is less highly methylated. BIO 1140 Unit 1-2:Chromatin a Dynamic  Structure 14
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Eukaryotic DNA wraps around  histones , to form  nucleosomes, etc.  Promoters are less accessible! Chromatin remodeling   makes gene promoters more  accessible Activators recruit remodeling complexes that displace nucleosomes Activators recruit enzymes modify histones and loosed their   assocation with DNA. The best studied system is  actelyation of  Lysine . Other modifications can include methylation of Lys, methylation  of Arg and His, phosphorylation of Ser and His. These modifications affect the “tails” (amino termini) on the surface of the nucleosome. The influence of these modifications on transcription  is known as the    HISTONE CODE” Chromatin Remodeling BIO 1140 Unit 1-2:Chromatin a Dynamic  Structure 15
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Histone code: importance Post-translational modifications of histones introduce meaningful variations into  chromatin and provide a regulatory platform for controlling  ..... transcription,...  repair & ...replication. 
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  • Winter '07
  • FENWICK
  • DNA, Berlin U-Bahn

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