Sperm Chromatin: An Overview

The changes in chromatin organization that occur as mammalian spermatocytes differentiate into mature sperm are among the most extreme that have been observed in biology. The DNA in each spermatocyte chromosome is packaged by histones into a continuous string of nucleosomal subunits that coil into higher-ordered 30 nm fibers. These fibers interact with each other, many other proteins, and the nuclear matrix to provide a predominantly soluble, loosely packed genome similar to that found in all other somatic cells. Genes required for spermatocyte function are actively transcribed and translated up until the cell commits to undergo two meiotic divisions that lead to the production of four spermatids, each containing a single copy of the paternal genome. The physical and functional state of the chromatin in each spermatid is then progressively transformed via a series of DNA-binding protein transitions that shut down the genetic programming required for spermatid function, reprogram (imprint) subsets of genes that will need to be activated at different stages of embryogenesis, and finally condense the entire genome into a genetically inactive, densely packed macrostructure that is not only protected from the majority of environmental influences but is also hydrodynamically optimized for transport into an oocyte (Braun, Nat Genet 28(1):10–12, 2001). The changes in chromatin organization that occur as mammalian spermatocytes differentiate into mature sperm are among the most extreme that have been observed in biology.