Neumann and Nurse report.
How cells determine the dimensions of the nucleus and other membrane-wrapped organelles is a mystery. One hypothesis suggests that nuclear volume depends on variables that include the amount of DNA and how compact it is. However, that hypothesis falters on the question of why different cells within an organism often have different-sized nuclei. Neumann and Nurse went looking for another mechanism.
The scientists measured cells and nuclei in fission yeast that ranged in size from tiny spores to hefty, nondividing mutants. No matter the yeast's girth, the volume of the nucleus remained close to 8% of the cell volume. To determine whether cells were keying on the amount of DNA, the team tested yeast cells that harbored up to 32 times the normal allotment. However, cells with extra DNA didn't sport larger nuclei, indicating that DNA quantity doesn't directly influence nuclear size.
The researchers also found that in multinucleate cells, not all nuclei grew at the same rate. Nuclei that were farther from their neighbors grew faster, suggesting that their size corresponds to the amount of surrounding cytoplasm.
To explore this idea further, the scientists centrifuged yeast with multiple nuclei and spurred them to separate into individual cells. The technique creates imbalances between the size of the cell and the nucleus. If the nucleus was disproportionately large, it stopped growing and waited for the cell to expand. If the nucleus was small, however, it inflated rapidly until it was back to the appropriate size.
Overall, the work suggests that a cell scales the nucleus according to its own size, not to the amount of DNA. Why yeast cells maintain this ratio, and whether mammalian cells do also, are so far unclear. Two more problems to address are how a cell gauges the relative volume of the nucleus and how it bulks up the structure.