**Question:** Nucleosome core protein is made up of -

A. Histone H1
B. Histone H2A
C. Histone H3
D. Histone H4

**Correct Answer:** C. Histone H3

**Core Concept:**
Nucleosome is the fundamental unit of chromatin, composed of DNA wrapped around a histone octamer. Histones are proteins that play a crucial role in the structure and function of chromatin. Histones are categorized into two main types: Histone H1 (linker histone) and Histone H2A, H2B, H3, and H4 (core histones). The histone octamer is formed by two copies of each of the core histones (H2A, H2B, H3, and H4). The linker histone H1 binds to the linker DNA between nucleosomes, regulating chromatin structure and gene expression.

**Why the Correct Answer is Right:**
Histone H3 is one of the four core histones (along with H2A, H2B, and H4) that form the nucleosome core. The nucleosome core is the basic repeating unit of chromatin, and it plays a crucial role in determining the overall structure and stability of chromatin fibers. The histone octamer (H2A, H2B, H3, and H4) is responsible for wrapping DNA around the histone core, which influences gene expression and DNA replication.

**Why Each Wrong Option is Incorrect:**
Option A (Histone H1) is incorrect because it represents the linker histone, which binds to the linker DNA between nucleosomes, regulating chromatin structure and gene expression.

Option B (Histone H2B) is incorrect because it is another core histone, but H3 is more specific to the nucleosome core.

Option D (Histone H4) is incorrect because it is another core histone, but H3 is more specific to the nucleosome core.

**Clinical Pearl:**
Understanding the role of histones in chromatin structure and gene regulation is essential for understanding cellular processes like DNA replication, transcription, and DNA repair. Knowledge of histone variants and their functions can help in interpreting genetic disorders, such as hereditary disorders caused by mutations in histone genes.