First level - Each nucleosome core(octameric protein disc...

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First level DNA is bound to basic proteins known as histones [contain basic amino acids]. DNA with bound histone is termed chromatin . Five classes of histones: H1 H2A H2B H3 H4 Histones are extremely rich in the positively charged basic amino acids lysine and arginine . The positive charge of the histones is one of the major features of the molecules, enabling them to bind to the negatively charged phosphates of the DNA. The histones are highly evolutionarily conserved. Especially H3 and H4 Less so with H1. H4 from cows differs by only 2 amino acids from H4 of peas. What you can conclude from this is that the structure/sequence of the histones (and therefore function) has not diverged in the ~ 10^9 years since plants and animals diverged. The appearance of DNA complexed to histones : beads on a string The beadlike structures are called nucleosomes
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Unformatted text preview: . Each nucleosome core (octameric protein disc) is found to consist of: Two molecules each of histones • H2A • H2B • H3 • H4 Around this core of histones a 146 base pair segment of DNA which is wrapped like a ribbon (1.75 times). DNA ( linker DNA links nucleosomes one to another) 146 base pairs in association with nucleosome pretty consistent from organism to organism. The linker DNA does vary quite a bit in length. H1 is then bound to the histone octamer and to linker DNA between nucleosome cores. When H1 binds, there are two full turns of the DNA (166 base pairs) around the nucleosome core particle. In this conformation the structure is referred to as a chromatosome . The resulting chromatin fiber is approximatley 10nm in diameter. Overall length of DNA has been shortened 6X at this point. See Fig. 4....
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