Chromatin and gene organization during spermatogenesis. An evolutionary perspective

精子发生是导致雄性配子（精子）形成的细胞分化过程。干细胞的脱氧核糖核酸（DNA）在这种转变的开始，就像身体的其他细胞一样，包括卵子，充满了一组高度保守的基本蛋白质，称为组蛋白。这种蛋白质与dna结合形成的复合体称为染色质。精子包含唯一的细胞类型，其中染色质可能由组蛋白组成，也可能不由组蛋白组成，这取决于生物体类型。例如，在螃蟹中，组蛋白几乎完全被去除，DNA采用裸露的外观（螃蟹型精子），而原始无脊椎动物（如海葵/水母）和一些淡水鱼保留了组蛋白（精子组蛋白H型）。然而，在大量生物体中，组蛋白被高度碱性的精子特异性蛋白取代，如精蛋白（精子型P）或精蛋白样蛋白（精子型PL），它们与DNA结合的亲和力最高。这导致了非常紧密的DNA包装。大多数海洋软体动物，如蛤和一些鱼类，是PL型，而昆虫，鸟类和哺乳动物是P型。无论精子的染色质类型如何，在精子使卵子受精的过程中，整个组蛋白位移/替换过程都是恢复的。

Spermatogenesis is a cell differentiation process leading to the formation of the male gametes (spermatozoa). The deoxyribonucleic acid (DNA) of the stem cells at the onset of this transition, like the rest of the cells in the body, including eggs, is packed with a set of highly conserved basic proteins known as histones. The complex resulting from this protein-DNA association is called chromatin. The spermatozoa encompass the only cell type where chromatin may or may not consist of histones depending on the organism type. For instance, in crabs, histones are almost completely removed and the DNA adopts a naked-like appearance (Crab type sperm) whereas primitive invertebrates (such as anemones/jellyfish) and some fresh water fish retain their histones (sperm histone type H). However, in a large number of organisms, histones are displaced and replaced by highly basic sperm-specific proteins such as protamines (sperm type P) or protamine-like proteins (sperm type PL) that bind to DNA with highest affinity. This leads to a very tight DNA packaging. Most marine molluscs like clams and some fish, are PL type whereas insects, birds and mammals are P type. Regardless of the chromatin sperm type, the overall histone displacement/replacement process is reverted during fertilization of the egg by the sperm.