The nucleosome is the basic repeat element of chromatin, and consists of 147 base pairs (bp) of DNA wrapped 1.7 times around an octamer of histone proteins (two copies each of the core histones H2A, H2B, H3, and H4).
Nucleosomes are connected by about 20 to 60 bp of linker DNA to form the 10-nm "beads-on-a-string" array. This can be further compacted into a "30-nm" chromatin fiber.
Two classes of model for chromatin have been proposed: (a) the "one-start helix" in which nucleosomes, connected by bent linker DNA, are arranged linearly in a higher order helix; or (b) the "two-start helix" in which nucleosomes, connected by straight linker DNA, zigzag back and forth between two adjacent helical stacks.
To distinguish between these two competing models of higher order chromatin folding, Dorigo and co-workers employed a fully defined in vitro system to generate regular nucleosomal arrays. Analysis of the length of the nucleosome stacks, now connected only by internucleosomal cross-links, revealed a two-start rather than a one-start organization. This interpretation was corroborated by electron microscopy. Thus, local interactions between nucleosomes can drive self-organization into a higher order chromatin fiber. Adapted from: Adone Mohd-Sarip and C. Peter Verrijzer (Science 2004 306:1484)1. B. Dorigo et al., Science 306, 1571 (2004)
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