Cell Biology

Nuclear Optics

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Science  15 May 2009:
Vol. 324, Issue 5929, pp. 859
DOI: 10.1126/science.324_859a
Light intensity (high, red/blue; low, green/orange) transmitted by conventional (left) and inverted (right) nuclei. CREDIT: SOLOVEI ET AL., CELL 137, 356 (2009)

In humans, the 3 billion nucleotides of DNA that constitute the genome would take up substantial cellular space if they were all stored in an open configuration; in a remarkable instance of molecular housekeeping, the DNA strands are instead packaged efficiently by the cell with proteins to form chromatin—a compressed material that can be straightforwardly confined to an approximately 10-µm-diameter membrane-bound nucleus. Some of the DNA needs to remain easily accessible to proteins and small molecules that together regulate gene expression and ensure the whole lot can be copied faithfully base by base, once per cell cycle. The vast majority of cells from both unicellular and multicellular organisms package transcriptionally inactive chromatin (heterochromatin) at the nuclear periphery and the more active chromatin (euchromatin) in the center. Although the function of this segregation is debated, the pattern correlates with the timing of replication, which during S phase is generally later for heterochromatin, and is altered by changes in gene activity, as occurs during development.

In mammals, rod photoreceptor cells display the opposite pattern, with euchromatin found at the nuclear periphery. Solovei et al. have analyzed chromatin organization in rod cells from more than 30 mammalian species, including deer, rabbits, and pigs. They found the inverted pattern predominantly in nocturnal animals, and they demonstrated that this inversion has the consequence of improving photon transmission through the retina. In fact, the heterochromatin regions had a higher refractive index than the euchromatin, and the rod cell nuclei acted as converging lenses, indicating that the large-scale organization of euchromatin and heterochromatin can be usefully exploited to achieve specialized cellular functions.

Cell 137, 356 (2009).

  • * Helen Pickersgill is a locum editor in Science's editorial department.

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