Final answer:
A nucleosome consists of a 146-bp segment of DNA wrapped twice around a core of eight histone proteins, forming part of the chromatin 'beads on a string' structure in eukaryotic cells.
Step-by-step explanation:
The correct answer to the question of what a nucleosome is that it consists of a 146-bp segment of DNA wrapped twice around a core of histone proteins. Nucleosomes play a crucial role in the organization and compaction of DNA in eukaryotic cells. They are composed of a core of eight histone proteins, with DNA tightly wrapped around this core. This arrangement allows for a remarkable degree of DNA compaction, which is necessary for fitting the long DNA molecules into the relatively small nucleus of a cell. Although nucleosomes represent an intermediate level of chromatin structure, they are not the most highly condensed form. That state is reached at the metaphase stage in chromosomes with further packing.
The structure known as a nucleosome is often described using the "beads on a string" analogy, where each "bead" is a nucleosome and the "string" is linker DNA connecting one nucleosome to the next. This is the first level of DNA compaction in the process of forming chromosomes. The 30 nm fiber mentioned in the references is a higher order of compaction involving the coiling of the beads-on-a-string structure. Finally, even more highly condensed states occur during cell division, culminating in the metaphase chromosome which is far more compact than the nucleosome itself.