Live cell microscope images show histones (green) at heat shock genes (indicated by the dotted lines), which are highly condensed under normal conditions (top). When the temperature of cells is raised, heat shock genes undergo extensive disruption (below) as seen by the formation of large puffs with RNA Polymerase II (red), as the histones unpack to make the gene available for expression.
The human genome contains some 3 billion base pairs that are tightly compacted into the nucleus of each cell. If a DNA strand were the thickness of a human hair, the entire human genome would be crammed into a space the size of a softball, but if it were unraveled and all the strands lined up, they would stretch from Ithaca, N.Y., to Boston. A Cornell study, published in the Jan. 13 issue of Molecular Cell, teases out how cells undergo transcription, where compacted DNA unravels, and a complex enzyme called RNA polymerase II reads the desired gene's DNA base pairs and transcribes them into RNA. The RNA then instructs the cell to make specific proteins based on a gene's blueprint. In particular, the researchers expand on their previous work that showed that the unraveling of compacted DNA occurs independently of transcription by RNA polymerase II. Many scientists previously believed that RNA polymerase II played a major role in decompacting the DNA.
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