What is the primary structure of nucleosomes?

The primary structure of nucleosomes is understood to be the "beads on a string" model, where DNA is wrapped around histone proteins. This allows for the compaction of DNA within the nucleus and plays a crucial role in regulating gene expression. In a nucleosome, approximately 147 base pairs of DNA are coiled around a core of eight histone proteins, forming a structure that resembles a bead. This arrangement is fundamental to the higher-order structure of chromatin, influencing accessibility for transcription and replication processes.

Other structures mentioned, such as single strands of DNA or double helices, do not describe nucleosomes but rather the basic configuration of DNA itself. Fragmented pieces of chromatin refer to disorganized elements of DNA and protein complexes that have been broken down, rather than the organized and compact form presented by nucleosomes. The concept of nucleosomes is critical in the broader understanding of how DNA is packaged and managed within the cell.