Chromosomal structures
Chromosomal Structures
Eukaryotic Chromosomes are composed of DNA, Histone and nonhistone proteins Histones:H1 H2A. H2B.H3.H4. The later 4 form nucleosomes(100 A) and thus called core histones. H1 plays a role in forming 300 A superhelix Histones are rich in basic amino acids Nonhistone proteins play a variety of functions
Eukaryotic Chromosomes are composed of DNA, Histone and Nonhistone Proteins • Histones: H1, H2A, H2B, H3, H4. The later 4 form nucleosomes (100 A) and thus called core histones. H1 plays a role in forming 300 A superhelix • Histones are rich in basic amino acids • Nonhistone proteins play a variety of functions
TABLE 1 1. 1 Some Molecular Characteristics of the Histone Proteins Found in Calf Thymus* Histone Basic/Amino Acids Molecular Weight Total Amino acid residues H1 54 23,000 224 H2A 14 13,960 129 H2B 1.7 13,774 125 1.8 15,273 135 2.5 11,236 102 .Note the small size of the histones; in addition, their predominantly basic amino aaid constitution ensures that they will be positively charged in physiological conditions
Nucleosomes H2B H4 H2A H3 Linker dnA Linker DNA
Scaffold: the structural backbone, can be revealed after mild treatment of the chromosome
Scaffold: the structural backbone, can be revealed after mild treatment of the chromosome
Nucleosomes: beads on a string
Nucleosomes: beads on a string
Higher level packaging models N。 scaffold protein 20 A in diameter Loop 100A chromatin Nonhistone scaffold 300A proteins Additional nonhistone c scaffold components Rosettes compressed into a compact bundle Interphase Metaphase chromosome chromatin
EM picture supports the radial loop- scaffold model SARs: scaffold-associated reglons Agure 11.5 Experimental support for the radial loop- Fi affold model. a close-up of the image in Fig 11.2, this electron
EM picture supports the radial loopscaffold model. SARs: scaffold-associated regions
TABLE 11.2 The Different Levels of Chromosome Compaction Mechanism Status what it Accomplishes Nucleosome Confirmed by crystal structure Condenses naked dna 7-fold to a 100-A fiber Supercoiling Hypothetical model (the 300-A fiber predicted Causes additional 6-fold compaction, achieving a by the model has been seen in the electron 40-to 50-fold condensation relative to naked dna microscope) Radial loops--scaffold Hypothetical model (preliminary experimental Through progressive compaction of 300-A fiber, support exists for this model) condenses dna to rodlike mitotic chromosome that is 10,000 times more compact than naked DNA
Specialized chromosomal elements Replication Telomere Centromere
Specialized chromosomal elements • Replication • Telomere • Centromere