Nucleosomes

The primary structural unit for chromatin in somatic cells is the nucleosome. Approximately 160 bp of DNA are wrapped 1.65 times around a histone octamer. During spermiogenesis, histones are displaced from nucleosomes to form DNA-protamine toroids, a much denser form of packaging shown below.

DNA-Protamine Toroids

DNA-protamine toroids are shown containing approximately 50 kb of DNA. The toroids are approximately 100 nm in diameter and are imaged using electron microscopy. The scale bar is 100 nm.

This QuickTime Movie video shows the formation of DNA-protamine toroids in real time. A single DNA molecule stained with an intercalating dye YOYO-1 is pulled into protamine by a single optical trap using a two-channel flow cell. The toroid forms starting at the free end of the λ-phage DNA molecule because that is where it is most flexible.

DNA-Protamine Structure

This figure shows how protamine is thought to bind to DNA within the toroid. The footprint of protamine is approximately 15 bp and it binds to DNA in its major groove in a nonspecific manner. Protamine has arginine side-chains and they bind to alternate strands of the DNA helix.

Structure Of Toroids Within The Sperm Nucleus

The higher order structure of toroids within the sperm nucleus is unknown but the DNA is quite densely packed. The fractional packing volume is approximately 13% approaching that of DNA in viruses.

Steps Of Sperm Cell Formation During Spermiogenesis

Spermiogenesis is shown for the mouse as a series of discrete steps. The appearance of the sperm nuclear proteins is shown at the top of the figure and the shape of the sperm nucleus and degree of compaction of the sperm chromatin is shown below. Figure is from a paper by Zhao (1)

1. M. Zhao, C. R. Shirley, S. Mounsey, M. L. Meistrich, Nucleoprotein transitions during spermiogenesis in mice with transition nuclear protein Tnp1 and Tnp2 mutations. Biol Reprod 71, 1016-1025 (2004).