DNA folding by protamine from loops to stacks

Multiple modes of DNA compaction by protamine

“…The extreme compaction protects sperm DNA from damage and facilitates its delivery to the oocyte cytoplasm. A variety of imaging techniques have been used to investigate the shapes of DNA when compacted by protamine, including transmission electron (TEM) (21), atomic force (AFM) (21)(22)(23)(24), and fluorescence (25,26) microscopy. Based on TEM and AFM imaging, Hud et al (21) proposed that protamine condenses sperm DNA into toroids containing up to 60 kilo-base pairs (kb).…”

“…Based on TEM and AFM imaging, Hud et al (21) proposed that protamine condenses sperm DNA into toroids containing up to 60 kilo-base pairs (kb). More recent AFM studies on shorter DNA have revealed a variety of other shapes, including single loops (23), multiple loops emanating from a central position, and stacks of loops (24). Single-molecule fluorescence imaging has shown DNA can be compacted into a near-zero length when fully covered by protamine (25).…”