Backwards retina: cells guide light (Introduction)

by David Turell , Saturday, June 08, 2019, 23:49 (1995 days ago) @ David Turell

More on how light is guided to retina to overcome low light vision:

https://phys.org/news/2019-06-genome-nucleus.html

"Research carried out by a team led by Irina Solovei at LMU's Biocenter, in cooperation with Job Dekker (University of Massachusetts Medical School) and physicists from the group of Leonid Mirny at MIT (Institute for Medical Engineering and Science) now suggests that the driving force in chromatin segregation is the inactive heterochromatin and that in the 'default' chromatin distribution of euchromatin and heterochromatin are reversed. The new findings appear in the leading journal Nature.

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"Many mechanisms have been proposed to explain how chromatin is segregated within the nucleus, however none of them were conclusive, largely, because it is difficult to analyze the interactions of the two chromatin types in the context of conventional nuclei with heterochromatin tethered to the nuclear membrane. "For our study, we therefore chose so called inverted cell nuclei," says Solovei. She and her Munich colleagues discovered these nuclei about 10 years ago in the retina of nocturnally active mammals, where they are restricted to the type of photoreceptor cells known as rods. In rods, the tightly condensed heterochromatin is packed in the interior of the nuclei, while the active euchromatin is localized directly under the nuclear membrane—a unique exception to the general rule. It turned out that the heterochromatin core of rod nuclei serves as a microlens condensing light and thus improving optical properties in the nocturnal retinas. Subsequent study from the same group disclosed the mechanism of inversion by revealing that these atypical nuclei lack two protein complexes that normally link the heterochromatin to the inner surface of the nuclear membrane, the nuclear lamina.

Comment: What this means is junk DNA is used to focus light in the rods. Confirmed by this commentary:

https://evolutionnews.org/2009/04/shoddy_engineering_or_intellig/

Reporting in the journal Cell, Irina Solovei and coworkers have just discovered that, in contrast to the nucleus organization seen in ganglion and bipolar cells of the retina, a remarkable inversion of chromosome band localities occurs in the rod photoreceptors of mammals with night vision (Solovei I, Kreysing M, Lanctôt C, Kösem S, Peichl L, Cremer T, Guck J, Joffe B. 2009. “Nuclear Architecture of Rod Photoreceptor Cells Adapts to Vision in Mammalian Evolution.” Cell 137(2): 356-368). First, the C-bands of all the chromosomes including the centromere coalesce in the center of the nucleus to produce a dense chromocenter. Keep in mind that the DNA backbone of this chromocenter in different mammals is repetitive and highly species-specific. 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, at least in the mouse, 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. So the next time someone tells you that it “strains credulity” to think that more than a few pieces of “junk DNA” could be functional in the cell — that the data only point to the lack of design and suboptimality — remind them of the rod cell nuclei of the humble mouse.

Comment: Never question God's designs as wrong!