The average somatic nucleus is only 10 microns in diameter, yet contains 2 meters of DNA coiled within its volume. This compaction must be accomplished in such a way so that the DNA remains accessible to the machinery of transcription, replication and repair. Sperm nuclei also have compacted DNA; however, structurally and functionally the arrangement of DNA in sperm is vastly different from that of somatic DNA. Somatic DNA is initially compacted by coiling around histone octamers, while sperm DNA is bound to elongate, highly basic proteins called protamines. Protamines bind the major groove of the double helix and allow adjacent DNA strands to come in close contact by neutralizing the negative charge of the sugar-phosphate backbone. Figure 1 illustrates the two most widely accepted models of DNA compaction in both somatic and sperm nuclei.

The arrangement of DNA within sperm nuclei is difficult to study because it is so highly compressed.