The top row of images show genetic material (shaded in grey) in male reproductive germ cells from mice, starting from left to right with pachytene stage spermatocytes, then round spermatid, and finally sperm. Called chromatin, the material comprises massive polymers of DNA spooled around proteins. The bottom row shows, from left to right, Hi-C heat maps for a specific chromosome in the nucleus of germ cells at different developmental stages. The heat maps essentially are snapshots of chromatin interactions, shown in various shades of orange. The deeper the orange, the more interactions there are. The images are from a study in Nature Structural and Molecular Biology.