Question

The nucleosome

• A. contains DNA and nonhistone proteins2
• B. has a core of histones with DNA wound around it
• C. is fully responsible for DNA packaging into chromosomes
• D. surrounds nuclear pore

Hint:

Remember the "Beads on a string" model. The "bead" is the nucleosome (DNA + histone octamer), and the "string" is the linker DNA associated with histone H1.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The nucleosome is the fundamental structural unit of chromatin in eukaryotic organisms. It represents the first level of DNA compaction, allowing the long strands of genomic DNA to fit inside the relatively small nucleus while remaining accessible for processes like transcription and replication.

Step 2: Detailed Explanation:
A nucleosome consists of a segment of DNA wound around a protein core. This core is an octamer composed of two molecules each of four core histone proteins: H2A, H2B, H3, and H4. Approximately 146 to 147 base pairs of DNA wrap around this histone octamer in roughly 1.67 left-handed superhelical turns. Histones are highly basic proteins, rich in lysine and arginine, which allows them to bind tightly to the negatively charged phosphate backbone of DNA.
Looking at the other options: Option 1 is incorrect because nucleosomes contain histone proteins, not primarily non-histone proteins (though non-histone proteins are involved in higher-order chromatin structure). Option 3 is partially true but misleading; while nucleosomes are the first step, several other levels of folding (solenoids, looped domains, scaffolds) are required to form a condensed chromosome. Option 4 is factually incorrect as nuclear pores are complex protein structures in the nuclear envelope, unrelated to the internal DNA-wrapping units. The description of DNA winding around a histone core is the precise biochemical definition of a nucleosome.

Step 3: Final Answer:
A nucleosome is defined by its core of eight histone proteins around which a specific length of DNA is coiled.