In the structure of chromatin, a nucleosome is the basic repeating unit. In a nucleosome, the DNA double helix is wrapped around which of the following?

Introduction / Context:
In eukaryotic cells, DNA is packed into the nucleus in a highly organised way. The fundamental packing unit of chromatin is the nucleosome. Understanding what DNA is wrapped around in a nucleosome helps explain how long DNA molecules fit into small nuclei while still remaining accessible for replication and transcription. This question asks you to identify the protein complex that forms the core of the nucleosome.

Given Data / Assumptions:

- The structure in question is the nucleosome, a basic unit of chromatin. - We consider the relationship between DNA and histone proteins. - Options include a histone octamer, a single helix, another DNA helix, and none of the above. - We assume standard eukaryotic chromatin structure.

Concept / Approach:
A nucleosome consists of a core of eight histone proteins, called a histone octamer, with about 146 base pairs of DNA wrapped around it in nearly two turns. The histone octamer is composed of two molecules each of histones H2A, H2B, H3, and H4. This arrangement compacts DNA and helps regulate gene expression. The DNA is not wrapped around another DNA helix and is not simply a bare helix; it is tightly associated with this histone protein core.

Step-by-Step Solution:
1. Recall that chromatin is DNA combined with histone and non histone proteins. 2. At the first level of packaging, DNA wraps around small, positively charged histone protein complexes. 3. Each nucleosome core particle contains a histone octamer made of two molecules each of H2A, H2B, H3, and H4. 4. Approximately 146 base pairs of DNA are wound around this octamer in about 1.65 to 2 turns. 5. This DNA plus histone octamer structure is what is referred to as a nucleosome. 6. Therefore, DNA in a nucleosome is wrapped around a histone octamer, not around itself or without proteins.

Verification / Alternative check:
Electron microscopy and X ray diffraction studies of chromatin show a beads on a string appearance, where each bead corresponds to a nucleosome. Biochemical analysis reveals that each bead contains the eight core histones plus a defined length of DNA. Removing histones from chromatin results in the loss of this beaded structure, which confirms that histone octamers form the core around which DNA is wrapped in nucleosomes.

Why Other Options Are Wrong:
- A single DNA helix without any proteins: This would represent naked DNA, not a nucleosome. Nucleosomes require histones to form their structure. - Another DNA double helix forming a super double helix: DNA does not wrap around another DNA double helix in the nucleosome; it wraps around histone proteins. - None of the above structures: This is incorrect because the histone octamer is the well established core of the nucleosome.

Common Pitfalls:
A common mistake is to think of DNA packing only in terms of coiling and supercoiling without recognising the role of histones. Some students also confuse the histone octamer with a larger complex including histone H1, but H1 is associated with linker DNA between nucleosomes, not the core particle itself. Remember that the nucleosome core is specifically defined as DNA wrapped around a histone octamer composed of eight histone molecules.

Final Answer:
In a nucleosome, the DNA is wrapped around a histone octamer composed of eight histone protein molecules.