Scripps Research scientists shed light on how DNA is unwound so that its code c...(f cells within a human body it has been ...)

So DNA must be packaged into tidy little chromosomes. The DNA in each gene first assembles into what looks like a string of beads: the string is the DNA and to compact its length, it is wrapped two times around a spool-like bead of histone protein, to form a nucleosome. But there is so much DNA in a single gene that each gene is packed into a necklace of nucleosomes on a DNA string. These beads then become further compressed into twisted ropes that eventually form chromatin, in which DNA is compacted about 10,000 times from its extended length.

What the Scripps Research scientists set out to do is to understand how the RSC complex unwinds DNA from the many histone beads within a gene so that other molecular machines can read the genetic code.

RSC is a huge complex of 13 different proteins and the scientists first found that it holds an individual nucleosome in what looks like a vise grip. They then found that RSC creates a little bulge in the DNA that can be propagated around the nucleosome and make possible translocation of the DNA with respect to the histones, exposing the DNA so that it can be read.

"Imagine a rubber band wrapped twice around a water glass. The easiest way to move the band is to pull a little of it away from the glass and then slide it" Asturias says. "By using energy from an external source (ATP hydrolysis) RSC can repeatedly pull DNA away from the histones and eventually expose all of the DNA."

The researchers believe that by translocating a nucleosome along the DNA, RSC eventually slides into the next adjoining nucleosome, causing the histones to be ejected and exposing the DNA. "Interestingly, although its DNA is gradually exposed, the nucleosome to which RSC is bound remains intact," Asturias says.

The structure RSC interacting with a
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