Nucleosome composition

Chromatin is composed of nucleosomes, where each comprise 147 base pairs of DNA wrapped around an octamer oftwo copies of each histone H2A, H2B, H3, and H4. Nucleosomes are folded into higher-order structures that are stabilized by linker histones. Chromatin structure can be altered by enzymes that posttranslationally modify histones (e.g., through phosphorylation, acetylation, methylation, or ubiquitination) or by ATP-driven chromatin-remodeling complexes that alter nucleosome position and/or composition. 

Of important note, native nucleosomes are intrinsically heterogeneous both in terms of their DNA sequence and histone compositions and both of which can contribute to the stability of the complex. 

Nucleosome structure - The bead particles are nucleosomes (or more precisely, nucleosomal core particles). They have a simple, definite composition that is practically invariant over the whole eukaryotic kingdom. Both nucleosomes and nucleosome histone cores have been crystallized. 

Nucleosomes always contain 146 bp of DNA, wrapped about an octamer of histone molecules--two each of H2A, H2B, H3, and H4. This composition explains the equal amounts of each of the four histones in chromatin. 

Describe the composition and structure of the nudeosome, and relate the nucleosome to the proposed structure of the chromatin fiber. 

Describe the composition of chromatin. List the types of histones and describe their general characteristics. Note the evolutionary stability of the sequences of the H3 and H4 histones and assign the types of histones to their locations within the nucleosome. 

In general, the remodeling of sperm chromatin that accompanies stage I decondensation involves the replacement of sperm-specific basic proteins with histones from the egg and results in the formation of nucleosomes. It now seems clear, at least in amphibian egg and Drosophila embryo extracts, that this remodeling is mediated by specific factors that participate in both assembly and disassembly processes. These changes in chromatin composition and structure can be analyzed by one- or two-dimensional polyacrylamide gel electrophoresis and micrococcal nuclease digestion. 

---