Cover image

Like origami, DNA is intricately folded inside of a cell's nucleus. DNA compaction differs in distinct cell types, like somatic and germ cells, and is typically tightly controlled by the function of cohesin, a protein complex that mediates sister chromatid cohesion and DNA looping. In this issue, it is shown that in frog and human sperm, genomic DNA is not folded by cohesin, as it is in somatic cells, possibly because in these cells DNA is densely packaged by specialized sperm proteins called protamines. Earlier work by others had reported that in mouse sperm the genome is folded by cohesin but, in a related study (also in this issue), it was found that these data were caused by chromatin contaminations from somatic cells. Together, these two studies suggest that in sperm from all vertebrate species genomic DNA might be packaged very differently from how it is folded in somatic cells. (Cover illustration by Philipp Dexheimer (www.philippdexheimer.com). [For details, see Jessberger et al., pp. 2094–2107 and Yin et al., pp. 2079–2093.])