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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