《遗传学》课程教学资源：英文版 Chromosomal structures

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 loop￾scaffold 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