Functional Aspects of Sperm Chromatin Organization

Abstract

Sperm nuclei present a highly organized and condensed chromatin due to the interchange of histones by protamines during spermiogenesis. This high DNA condensation leads to almost inert chromatin, with the impossibility of conducting gene transcription as in most other somatic cells. The major chromosomal structure responsible for DNA condensation is the formation of protamine-DNA toroids containing 25-50 kilobases of DNA. These toroids are connected by toroid linker regions (TLR), which attach them to the nuclear matrix, as matrix attachment regions (MAR) do in somatic cells. Despite this high degree of condensation, evidence shows that sperm chromatin contains vulnerable elements that can be degraded even in fully condensed chromatin, which may correspond to chromatin regions that transfer functionality to the zygote at fertilization. This chapter covers an updated review of our model for sperm chromatin structure and its potential functional elements that affect embryo development.

Keywords: Chromatin condensation; Double-stranded DNA breaks; Matrix attachment region; Sperm; Toroid linker region.

Figure 1.

The Toroid Loop Model for the sperm chromatin condensation. (A). Each DNA loop domain is coiled into a protamine-condensed toroid, suggesting that both structures are structurally related; (B). Adjacent toroids are linked by nuclease sensitive toroid linker regions (TLR), which are attached to the nuclear matrix; (C). Lists the different aspects of the model that are supported by experimental evidence (right), and the hypothesized aspects yet to be experimentally shown (left).