The perfect lens, or in praise of mammals

Oct 09, 2009 20:28

...nuclear architecture of rod photoreceptor cells differs fundamentally in nocturnal and diurnal mammals. The rods of diurnal retinas possess the conventional architecture found in nearly all eukaryotic cells, with most heterochromatin situated at the nuclear periphery and euchromatin residing toward the nuclear interior. The rods of nocturnal retinas have a unique inverted pattern, where heterochromatin localizes in the nuclear center, whereas euchromatin, as well as nascent transcripts and splicing machinery, line the nuclear border. The inverted pattern forms by remodeling of the conventional one during terminal differentiation of rods. The inverted rod nuclei act as collecting lenses, and computer simulations indicate that columns of such nuclei channel light efficiently toward the lightsensing rod outer segments. http://www.cell.com/abstract/S0092-8674(09)00137-8

Here it is explained better:

...the C-bands of all the chromosomes including the centromere coalesce in the center of the nucleus to produce a dense chromocenter. Second, a shell of LINE-rich G-band sequences surrounds the C-bands. Finally, the R-bands including all examined protein-coding genes are placed next to the nuclear envelope. The nucleus of this cell type is also smaller so as to make the pattern more compact. This ordered movement of billions of basepairs according to their “barcode status” begins in the rod photoreceptor cells at birth and continues for weeks and months. Why the elaborate repositioning of so much “junk” DNA in the rod cells of nocturnal mammals? The answer is optics. A central cluster of chromocenters surrounded by a layer of LINE-dense heterochromatin enables the nucleus to be a converging lens for photons, so that the latter can pass without hindrance to the rod outer segments that sense light. In other words, the genome regions with the highest refractive index - undoubtedly enhanced by the proteins bound to the repetitive DNA - are concentrated in the interior, followed by the sequences with the next highest level of refractivity, to prevent against the scattering of light. The nuclear genome is thus transformed into an optical device that is designed to assist in the capturing of photons. This chromatin-based convex (focusing) lens is so well constructed that it still works when lattices of rod cells are made to be disordered. Normal cell nuclei actually scatter light. http://www.evolutionnews.org/2009/04/shoddy_engineering_or_intellig.html

That's absolutely amazing piece of engineering. Bravo, mammals!

evolution

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