Meiosis

Direct coupling between meiotic DNA replication and recombination initiation. Borde, V., Goldman, A. S. & Lichten, M. Science 290 , 806?809 (2000) [PubMed]

During meiosis, information is swapped between parental chromosomes by homologous recombination. Initiation of this process requires a double-stranded DNA break (DSB) which, according to this paper, is introduced in a replication-dependent manner. The authors show that by delaying replication of a chromosomal segment, the formation of a DSB can be delayed in that segment.

Chromosome biology

Cleavage of cohesin by the CD clan protease separin triggers anaphase in yeast. Uhlmann, F. et al. Cell 103 , 375?386 (2000) [Contents page]

Disjunction of homologous chromosomes in meiosis I depends on proteolytic cleavage of the meiotic cohesin Rec8 by separin. Buonomo, S. B. C. et al. Cell 103 , 387?398 (2000) [Contents page]

Two distinct pathways remove mammalian cohesin from chromosome arms in prophase and from centromeres in anaphase. Waizenegger, I. C. et al. Cell 103 , 399?410 (2000) [Contents page]

This trio of papers ? two from Kim Nasmyth's lab and one from Jan-Michael Peters and colleagues ? tackle the question of how sister-chromatid cohesion is regulated. The first shows that separin, a conserved protein responsible for cleaving the Scc1 subunit of cohesin, is a cysteine protease related to caspases. Moreover, in vitro it alone is enough to cleave Scc1, an event that triggers sister chromatid separation. The second paper shows that cleavage of Rec8 ? the meiotic equivalent of Scc1 ? by separin at two different sites is necessary for the resolution of chiasmata during meiosis. Finally, Waizenegger et al. propose that, in vertebrates, cohesin is removed from chromosome arms by a different, cleavage-independent mechanism to the one that removes centromeric cohesin and involves cleavage of Scc1.

Cytoskeleton

Dynein, dynactin, and kinesin II's interaction with microtubules is regulated during bidirectional organelle transport. Reese, E. L. & Haimo, L. T. J. Cell Biol. 151, 155 ?165 (2000) [PubMed]

Dynein and kinesin motors transport organelles to opposite ends of microtubules, but it is a mystery what controls the net direction of the vesicles. This paper shows that dynein, dynactin and kinesin II are continuously associated with pigmented organelles in Xenopus melanophores, indicating that association of the motors with the organelle is not regulated. The direction is, in fact, determined by controlling the binding of motors to microtubules, and this probably occurs through phosphorylation.