The large distance between two loci (green) in rec8 cells indicates decompacted chromosomes.

The glue that holds together sister chromatids also compacts meiotic chromosomes, according to Ding et al. on page 499. This cohesin subunit, called Rec8, seems to tie up chromosomes into loops.

The role of Rec8, a meiosis-specific cohesin subunit, in chromosome architecture was identified by the authors in a fission yeast screen for meiosis mutants. During meiotic prophase, the yeast nuclei oscillate via microtubule pulling forces that extend chromosomes along the direction of movement. The abnormal movements of chromosomes in rec8 cells suggested an unusual DNA architecture.

The DNA extension that accompanies oscillation offered the group the opportunity to compare chromosome compaction in wild-type and mutant strains. The authors' measurements of the distance between the telomeres and a particular locus revealed long and loose chromosomes in the absences of Rec8. They found the opposite situation—hypercompaction—in a second strain that was mutated in a Rec8 binding partner called Pds5.

Given this contrast, the authors expected to see more Rec8 on chromosomes in the pds5 mutant. But instead they found fewer Rec8 binding sites, suggesting that Pds5 helps to load Rec8.

To explain the discrepancy, the authors suggest that the meiotic DNA is compacted as loops, with Rec8 sites serving as loop base attachment points along the chromosomal axis. In the absence of Rec8, loops do not form and chromosomes are extended. When Rec8 is present in low quantities, there are fewer attachment points along a shorter than normal axis, resulting in longer loops but more highly compacted chromosomes.

Meiotic compaction was not affected by the loss of condensins, which control mitotic condensation. Perhaps the specific Rec8-mediated arrangement exposes particular loci in a precise orientation that assists homologue pairing.