In a nucleosome, the DNA double helix is tightly wrapped around which type of protein?

Introduction / Context:
In eukaryotic cells, extremely long DNA molecules must be efficiently packed inside the nucleus while still remaining accessible for replication and transcription. This packaging is accomplished by organizing DNA into repeating units called nucleosomes. Understanding what forms the core of a nucleosome is vital in genetics, molecular biology, and epigenetics because it explains how DNA is compacted and how gene expression can be regulated by changing chromatin structure.

Given Data / Assumptions:
• The question is about the structure of the nucleosome. • Options include ribosomal proteins, thymine dimers, histone proteins, and DNA polymerase enzymes. • Standard eukaryotic chromatin structure is assumed.

Concept / Approach:
A nucleosome consists of a segment of DNA wound around a core of histone proteins. Specifically, about 146 base pairs of DNA wrap around an octamer of histones, which includes two molecules each of H2A, H2B, H3, and H4. Histones are small, positively charged proteins that interact strongly with the negatively charged DNA backbone. Ribosomal proteins are associated with ribosomes, thymine dimers are DNA damage lesions, and DNA polymerase is an enzyme involved in DNA replication. None of these fit the description of the nucleosome core, so the correct answer must be histone proteins.

Step-by-Step Solution:
Step 1: Recall that chromatin is the complex of DNA and proteins found in eukaryotic nuclei. Step 2: Remember that the basic repeating unit of chromatin is the nucleosome. Step 3: Understand that a nucleosome core is formed by an octamer of histone proteins with DNA wrapped around it. Step 4: Compare this with each option and eliminate those that are not structural DNA wrapping proteins. Step 5: Select histone proteins as the only option that correctly forms the core of a nucleosome.

Verification / Alternative check:
Textbook diagrams of chromatin consistently depict DNA wrapped around histone cores as bead like structures, representing nucleosomes. Epigenetic regulation often involves chemical modifications of histone tails, further confirming that histones are central to nucleosome structure. These consistent references strongly support histone proteins as the correct answer, while the other listed options have different biological roles.

Why Other Options Are Wrong:
Option A (Ribosomal proteins) is incorrect because these proteins are part of ribosomes in the cytoplasm where protein synthesis occurs. Option B (Thymine dimers) is incorrect because these are lesions formed in DNA due to ultraviolet radiation and are not proteins around which DNA wraps. Option D (DNA polymerase enzymes) is incorrect because these enzymes synthesize new DNA strands but do not serve as structural cores for nucleosomes.

Common Pitfalls:
Some students may confuse any protein associated with DNA as being part of the nucleosome. It is important to distinguish between histone proteins, which have a structural role in packaging DNA, and enzymes such as DNA polymerase that interact with DNA transiently during replication. Always remember that in a nucleosome, DNA is wrapped specifically around histone proteins.

Final Answer:
In a nucleosome, the DNA double helix is wrapped around histone proteins.