DNA Arrangement in Rod Cells Act as Lenses in Nocturnal Mammals

It is well known that DNA is responsible for coding genes, but until recently the importance of the arrangement of DNA within cells was somewhat unknown. Scientists and colleagues at the Ludwig-Maximillians University of Munich recently discovered an unusual arrangement of DNA within the nuclei of rod cells of various nocturnal mammals. Rod cells are located in the retina of the eye and are responsible for receiving light in low-light conditions. These cells are vital to the survival of nocturnal mammals, who must have clear vision in low-light conditions. The discovery has highlighted how DNA arrangement can enhance the functioning of cells in organisms.

Figure 1
The mouse rod cell (top left), ganglion cell (bottom left) and skin cell (right). Heterochromatin is shown in red and purple and active DNA is shown in green.
ScienceNews:
http://www.sciencenews.org/view/generic/id/42846/title/Seemingly_misplaced_DNA_acts__as_lenses

While studying rod cells within the eyes of mice, researchers noticed that the inactive DNA, referred to as heterochromatin, was located in the centre of the nucleus and the active DNA, known as euchromatin, was pushed to the outer edges. This is remarkably different to other cells, such as ganglion cells, skin cells and the rod cells of non-nocturnal animals, which have euchromatin concentrated in the center of the cell, with densely-packed heterochromatin located primarily in the outer edges. When euchromatin is located in the center of the nuclei, it is within close range to organelles so transcription can occur easily. If it is farther away, however, transcription is a much slower process.

Figure 2
Light travelling through a rod cell with euchromatin in the center of its nuclei (left) and a rod cell with heterochromatin in the center of its nuclei (right).
ScienceNews:
http://www.sciencenews.org/view/generic/id/42846/title/Seemingly_misplaced_DNA_acts__as_lenses

The researchers investigated a dozen of other animal rod cells and found that the unusual arrangement of DNA was only found in nocturnal animals, including cats, rats, ferrets, rabbits, opossum and deer. After much study, they came to the conclusion that the densely-packed heterochromatin located within the center of the nuclei functions as a lense, improving the light-receiving abilities of the rod cell by focusing light better. A computer simulation compares the channeling of light through the rod cells with heterochromatin located in the center of the nuclei and heterochromatin located toward the outer edges of the nuclei. It shows that light travelling through rod cells with euchromatin located in the center would scatter. In contrast, light travelling through rod cells with heterochromatin located in the center focus light more effectively. Researchers pinpointed this light-focusing ability to the dense nature of heterochromatin, claiming that the density would raise the refractivity index, slowing down the rate at which the light travels and focusing the light more effectively in the cell.

This is the first known discovery of DNA performing the function of a lens in a cell and further supports the notion that there is still much to discover about the mysterious functions of the molecule that is DNA.

(http://www.sciencenews.org/view/generic/id/42846/title/Seemingly_misplaced_DNA_acts__as_lenses, 2009)


Rebecca Yaun (s24200322)


References

Hesman Saey, T. Seemingly Misplaced DNA Acts As Lenses, 16.04.09, [online], ScienceNews, http://www.sciencenews.org/view/generic/id/42846/title/Seemingly_misplaced_DNA_acts__as_lenses, 2009.