What is a nucleosome?

A nucleosome is correctly identified as a repeating subunit of chromatin fibers. This structure plays a crucial role in the packaging of DNA within the cell nucleus. Each nucleosome consists of a segment of DNA wrapped around a core of histone proteins, which helps to condense the long strands of DNA into a more manageable form, allowing them to fit within the nucleus and enabling effective regulation of gene expression and DNA replication.

The nucleosome structure is fundamental in defining how DNA is organized and accessed by various cellular machinery. This organization is essential for processes like transcription, replication, and DNA repair, as it allows certain regions of the DNA to be more accessible while keeping others more tightly packed.

In contrast, the other options do not accurately describe a nucleosome. A single DNA molecule refers to the long polymer itself, which can be a part of a nucleosome but is not the nucleosome itself. The enzyme that proofreads DNA relates to DNA polymerase but does not pertain to the structural organization of DNA. Lastly, while RNA plays a significant role in replication, it is distinct from the structural components of DNA such as nucleosomes.