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Researchers have discovered a cellular mechanism that helps nocturnal mammals see in the dark. Mice, cats, deer, lemurs, and other mammals that are active at night remodel the DNA within their eyes to turn photoreceptor cells into light-collecting lenses, according to a study published today (Apr. 16) in Cell.
In nearly all eukaryotic nuclei, chromatin -- the structural building block of chromosomes -- is spatially separated into distinct compartments. Condensed, non-coding heterochromatin is usually localized to the periphery of the nucleus, while extended, active euchromatin typically resides in the nuclear interior. This "conventional" pattern is nearly universal, and probably helps cells regulate essential nuclear functions such as how and when genes are expressed.
But some nuclei are special. In 2006, a team led by Didier Devys, a molecular biologist at the Institute of Genetics and Molecular and Cellular Biology in Illkirch, France, showed that mouse rod photoreceptor cells have a different arrangement in which the chromatin is "inverted." In these cells, but not in other mouse cell types, heterochromatin is shunted to the interior, where it is enveloped by a thin ring of euchromatin. With this layout, all transcription takes place at the nuclear margins rather than at the core of nucleus as per usual.
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