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The eyes of nocturnal mammals contain particularly large numbers of the highly light-sensitive rods, the photoreceptor type used for night vision. This allows the detection of light levels millions of times lower than daylight. Researchers at the Ludwig Maximilians University Munich, the Max Planck Institute for Brain Research Frankfurt and the Cavendish Laboratory Cambridge have now shown that the nocturnal lifestyle and its visual challenges had a unique impact on rod nuclear organisation: The distributions of the densely packed inactive and the less densely packed active regions of DNA differ remarkably from those in other somatic cells of nearly all organisms from protozoans to multicellular animals, including the rods of diurnal mammals. With this unique arrangement, the rod nuclei of nocturnal mammals act as micro-lenses that focus the incoming light. Computer simulations show that stacks of such nuclei effectively guide the light to the light-sensitive outer segments of the rods. Hence, this change in rod nuclear organisation improves vision at nocturnal low light levels. It also provides new insights into the evolution of the mammalian retina and furthers our understanding of the nuclear architecture in general. (Cell, April 17th, 2009)
The DNA of a mammalian cell is about two metres long if all base pairs were aligned in one string. To fit this genetic material in a nucleus of only a few microns diameter, the DNA is wrapped around millions of so-called histon proteins that are arranged like pearls on a string, leading to a 10,000-fold compaction of the DNA. This DNA-protein complex is termed chromatin. Chromatin regions with genetic information that needs to be read out and transcribed by enzymes in a given cell at a given time are less tightly packed and more easily accessible. These chromatin pa
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