essential biology图资料-核小体-细胞周期.ppt

* D2. Chromatin Structure (eukaryotic cells) Chromatin: The highly ordered DNA-protein complex which makes up the eukayotic chromosomes. 50% of the mass is protein Solving the packing problem: chromatin The length of a chromosome is cm level, but the the diameter of nucleus is at mm level. The DNA concentration in nucleus is about 200 mg/ml Chromatin structure also enables the chromosomes to alter their compactness as the cell progress the cell cycle. meta phase中期- highly condensed interphase间期- more diffuse Thus, chromatin is the basic organization unit of eukaryotic chromosomal structure Histones: positively charged, rich in lysine and arginie core histones (H2A, H2B, H3 and H4): 10-20 kDa located in the nucleosome core [octamer core (H2A)2(H2B)2 (H3)2 (H4)2 highly conserved in their sequences H1 histone: 23 kDa Located outside of nucleosome, bind to DNA more loosely Less conserved in its sequence D2. Chromatin Structure Histones : The major protein components of chromatin are the histones; small , basic (positively charged) proteins which bind tightly to DNA. There are four families of core histone, H2A, H2B, H3, H4, and a further family, H1, which has some different properties, and a distinct role. Individual species have a number of variants of the different histone proteins. D2. Chromatin Structure Nucleosomes and micrococcal nuclease Micrococcal nuclease: an endonuclease that cleaves naked/non-supercoiled double-stranded DNA. Treatment of chromatin with micrococcal nuclease led to the isolation of DNA fragments with discrete sizes (~200 bp), the primary structure unit of chromatin termed nucleosome. Nucleosomes : The nucleosome core is the basic unit of chromosome structure, consisting of a protein octamer containing two each of the core histones, with 146 bp of DNA wrapped 1.8 times in a left-handed fashion around it. The wrapping of DNA into nucleosomes accounts for virtually all of the negative supercoiling in eukaryotic DNA. Nucleosome core