13 and 15 September, 2006 Chapter 7 RNA and Chromosome Structure

Overview In the cell, DNA is always associated with proteins. In eukaryotes, histones package DNA into chromatin. Proteins serve a structural role, and compact DNA, with regulated access. Chromosomes may be maintained as haploid or diploid. Gene density is highest in prokaryotes. Chromosomes replicate in S phase, segregate in M phase. Nucleosomes are the building blocks of chromosomes.. Histone H1 stabilizes the 30 nm fiber. Chromatin structure is highly regulated, often by covalent modification of histones. Nucleosome positioning is sequence dependent and allows access to transcription factors. Histone inheritance and assembly is complex, and maintains the modification state of the parent genome. Nucleosome assembly requires accessory proteins.

Prokaryotic and Eukaryotic Chromosomes

Gene Density

Eukaryotic Genome Structure

Chromosome Structure

Cell Cycle

S PhaseM Phase

Mitosis Review

Meiosis Review

Cohesins and Condensins

Nucleosomes

Histones are small positively-charged proteins.

Nucleosome Assembly

Nucleosome Symmetry

H3 and H4 bind the middle and the ends of the DNA, H2A and H2B bind a smaller region.

Histones make nonspecific contacts with the phosphodiester backbone near the minor groove.

Histone H1 binds to linker DNA and stabilizes the 30 nm fiber, which also required N-terminal tails.

Chromosome Structure

Nucleosome structure is dynamic.

Some Nucleosomes are positioned.

Histone modification alters nucleosome function.

Ordered Modification of Chromatin Structure

Nucleosome Inheritance

Histone Chaperones