A group of geneticists has peered into the eyes of nocturnal animals, and say they may have found the secret to these animals’ keen night vision: light-sensing cells with unusually structured DNA that turns each cell’s nucleus into a tiny lens.
The researchers were examining mice’s rod cells, the cells in the retina of the eye that operate under low light. Usually, they say, active genes are clustered in the center of each cell’s nucleus for convenient access to cellular machinery. But rod cells in the mouse retina shove active genes to the outside of the nucleus, the researchers found. The center of the nucleus is instead occupied by densely-packed inactive DNA called heterochromatin. Mice put this type of DNA front and center in their rod cells. “Everything that must be inside is outside, and everything that should be outside is inside,” [lead researcher Boris] Joffe says. “It was an absolutely heretic finding” [Science News].
In the study, published in the journal Cell, researchers note that placing densely packed inactive DNA raises the refractivity index — the degree to which the material decreases the speed of light traveling through it. The photons travel faster through the loosely packed DNA containing active genes, called euchromatin, and slower through the dense heterochromatin. Slowing down the photons creates a lens to focus light in the center of the cell. Rod cells form columns in the retina of nocturnal animals, so that many little lenses are stacked on top of each other. The DNA lenses form a chain that acts a bit like fiber-optic cables [Science News], the researchers suggest.
The researchers then mapped the nuclear architecture of nearly 40 mammal species and found that all the nocturnal animals had the same inverted pattern as mice, also a nocturnal species, whereas all the diurnal animals had the conventional arrangement [The Scientist]. The nocturnal animal’s inverted arrangement with the useful genes scattered around the nucleus’s edges probably comes with disadvantages, the researchers note. They suggest that as soon as certain animals didn’t need the light-focusing advantage, they dumped the backwards system.
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Image: flickr / striatic