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many levels of packaging of the DNA within the nucleus involving a variety of DNA

binding proteins.

First-order packaging involves the winding of the DNA around a core complex of

four small proteins repeated twice, termedhistones(H2A, H2B, H3 and H4). These are

rich in the basic amino acids lysine and arginine and form a barrel-shaped core

octomer structure. Approximately 180 bp of DNA is wound twice around the structure

which is termed anucleosome. A further histone protein, H1, is found to associate with

the outer surface of the nucleosome. The compacting effect of the nucleosome reduces

the length of the DNA by a factor of six.

Nucleosomes also associate to form a second order of packaging termed the 30 nm

chromatin fibrethus further reducing the length of the DNA by a factor of seven

(Fig. 5.11). These structures may be further folded and looped through the interaction

with other non-histone proteins and ultimately form chromosome structures.

DNA is found closely associated with the nuclear lamina matrix, which forms a

protein scaffold within the nucleus. The DNA is attached at certain positions within

Chromatin

H2A

H3

H2B

H4

A nucleosome

Histone proteins H2A H2B H3 H4

each repeated twice and

approximately 180 bp DNA

Fig. 5.11Structure and composition of the nucleosome and chromatin.

151 5.4 Location and packaging of nucleic acids