Stochastic variations (noise) in gene expression have been extensively characterized, but the ramifications of this gene-level variation for cellular structure and function remain unclear. To what extent are cellular structures subject to noise? We show that flagellar length in Chlamydomonas exhibits significant variation that results from a combination of intrinsic fluctuations within the flagella and extrinsic cell to cell variation. We analyzed a series of candidate genes affecting flagella and found that flagellar length variation is increased in mutations which increase the average flagellar length, an effect that can be explained using a theoretical model for flagellar length regulation. Cells with greater differences in their flagellar lengths show impaired swimming but improved gliding motility, raising the possibility that cells have evolved mechanisms to tune intrinsic noise in length. Taken together our results show that biological noise exists at the level of subcellular structures, with a corresponding effect on cell function.

Meiotic homolog pairing involves associations between homologous DNA regions scattered along the length of a chromosome. When homologs associate, they tend to do so by a processive zippering processive, which apparently results from avidity effects. Using a computational model, we show that this avidity-driven processive zippering reduces the selectivity of pairing. When active random forces are applied to telomeres, this drop in selectivity is eliminated in a force-dependent manner. Further simulations suggest that active telomere forces are engaged in a tug-of-war against zippering, which can be interpreted as a Brownian ratchet with a stall force that depends on the dissociation constant of pairing. When perfectly homologous regions of high affinity compete with homeologous regions of lower affinity, the affinity difference can be amplified through this tug of war effect provided the telomere force acts in a range that is strong enough to oppose zippering of homeologs while still permitting zippering of correct homologs. The degree of unzippering depends on the radius of the nucleus, such that complete unzippering of homeologous regions can only take place if the nucleus is large enough to pull the two chromosomes completely apart. A picture of meiotic pairing thus emerges that is fundamentally mechanical in nature, possibly explaining the purpose of active telomere forces, increased nuclear diameter, and the presence of "Maverick" chromosomes in meiosis.All rights reserved. No reuse allowed without permission.