In the structure of chromatin, a nucleosome is formed when DNA is tightly wrapped around which specific protein core?

Introduction / Context:
This question tests your understanding of how DNA is packaged inside the nucleus of eukaryotic cells. The basic repeating unit of chromatin packing is the nucleosome, which helps to coil long DNA molecules into a compact form. Knowing which proteins form the core around which DNA is wrapped in a nucleosome is an essential concept in genetics, molecular biology, and exam questions about chromatin organisation.

Given Data / Assumptions:
- The question asks about the protein core of the nucleosome. - Four possible structures are listed as answer options. - Standard school and college level descriptions of chromatin and nucleosomes are assumed.

Concept / Approach:
In eukaryotic cells, DNA does not float loosely in the nucleus. Instead, it is associated with small basic proteins called histones. A nucleosome consists of about 146 base pairs of DNA wrapped almost two turns around an octamer of histone proteins. This octamer contains two molecules each of histones H2A, H2B, H3, and H4. This arrangement helps in compaction and regulation of gene expression. Other structures like ribosomes, thymine dimers, and polymerases have different roles and do not serve as the nucleosome core.

Step-by-Step Solution:
Step 1: Recall that chromatin in eukaryotic cells is composed of DNA and histone proteins. Step 2: Remember that the nucleosome is the fundamental unit of chromatin packing and is described as DNA wrapped around a histone octamer. Step 3: Consider ribosomes. They are involved in protein synthesis in the cytoplasm or on rough endoplasmic reticulum, not in DNA packing inside the nucleus. Step 4: Consider thymine dimers. These are DNA lesions caused by ultraviolet radiation and are not normal structural components of nucleosomes. Step 5: Consider polymerase. DNA polymerase is an enzyme that synthesises DNA, and RNA polymerase synthesises RNA, but neither forms the structural core of nucleosomes. Step 6: Conclude that histones are the proteins around which DNA is wrapped to form nucleosomes.

Verification / Alternative check:
As an alternative check, recall textbook diagrams of nucleosomes showing DNA spiralled around a bead like structure. These beads are labelled as histone cores. The presence of histones H2A, H2B, H3, and H4 in an octameric arrangement is a standard detail in molecular biology. No diagrams or descriptions ever show ribosomes or polymerases as part of the nucleosome core. This consistent representation confirms that histones must be the correct answer.

Why Other Options Are Wrong:
Ribosomes: These structures synthesise proteins using messenger RNA templates and are found in the cytoplasm or attached to membranes, not as DNA wrapping cores. Thymine dimer: This is a type of DNA damage where two adjacent thymine bases become covalently linked due to ultraviolet radiation, and it is not a normal structural protein. Polymerase: Enzymes like DNA polymerase and RNA polymerase catalyse nucleic acid synthesis but do not provide the scaffold for DNA packing into nucleosomes.

Common Pitfalls:
A frequent confusion arises because students know many proteins associate with DNA, including polymerases and repair enzymes. However, only histones form the stable core around which DNA is wrapped to create the characteristic beads on a string structure. Another pitfall is to assume ribosomes handle all functions related to nucleic acids, but their role is limited to translation. Carefully distinguishing between structural DNA packing proteins and enzymes that work on DNA or RNA helps avoid these mistakes.

Final Answer:
In a nucleosome, DNA is wrapped around a core of Histones.