DNA Packaging: Nucleosomes and Chromatin

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At the top right portion of the diagram, a vertical double-ended arrow indicates that the DNA double helix strands are 2 nm apart. The strands are represented as gray ribbons connected by vertical colored bars that are either half red/half green or half yellow/half cyan.

As the DNA strand reaches the left side of the illustration, all colors are replaced by gray. Box 1 has the text “At the simplest level, chromatin is a double-stranded helical structure of DNA. The DNA strand turns down and goes back toward the right, still compacting along the way.

Below this is Box 2, with the text “DNA is complexed with histones to form nucleosomes.” Toward the center of the schematic are three sets of two brown discs, each disc quartered, and the cylinders are wrapped 1.65 times by the DNA, which has now compacted into a thick gray thread shape. Each nucleosome consists of eight histone molecules.

To the right of the first nucleosome complex is Box 3, with the text “Each nucleosome consists of eight histone proteins around which the DNA wraps 1.65 times.” The second nucleosome has a vertical red bar, about as long as the nucleosome is high, attached to the side of the nucleosome. This bar is labeled H1 histone. A horizontal, double-ended, black arrow indicates the nucleosome with DNA has a diameter of 11 nm. A third nucleosome to the right of the second is labeled “Chromatosome.” Above and to the right of the chromatosome is Box 4, with the text “A chromatosome consists of a nucleosome plus the H1 histone.”

Below this, the nucleosomes are folded in on each other to form a hollow, tube-like fiber, where many nucleosomes are arranged in parallel rings to form the tube’s outer layer. To the right of this is a vertical, double-ended, black arrow labeled 30 nm. To the right of this arrow is Box 5, with the text “The nucleosomes fold up to produce a 30-nm fiber…” The nucleosome tube continues to compact to form a gray spiral and gray squiggles as it continues leftward. Above this is Box 6 with the text “… that forms loops averaging 300 nm in length.” A black, vertical, double-ended arrow is labeled 300 nm. The squiggles compact further, going down and back toward the right, coiling like a telephone cord. Below this is Box 7 with the text “The 300-nm fibers are compressed and folded to produce a 250-nm-wide fiber.” A black, vertical, double-ended arrow is labeled 700 nm. Two, inward-pointing, black arrows indicate a gap labeled “250-nm-wide fiber.”

These coils continue to the right and compress further, forming a horizontal, X-shaped, chromosome. A black, vertical, double-ended arrow is labeled 1400 nm. Below this is Box 8 with the text “Tight coiling of the 250-nm fiber produces the chromatid of a chromosome.”

这里的欠缺在于,从图框6到图框8之间的折叠细节没有交代清楚。而且染色质\染色体的构象是随着时间在发生动态变化的,并非一成不变的。所以上述描述可以说是一个剪影。

http://www.nature.com/scitable/topicpage/DNA-Packaging-Nucleosomes-and-Chromatin-310

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