Chromatin higher-order structure: a ~30 nm fiber in which each helical turn contains about five to six nucleosomes is called what?

Introduction / Context:Eukaryotic DNA wraps around histone octamers to form nucleosomes (~10–11 nm fiber). These units can pack into higher-order fibers near ~30 nm diameter. One classic model with five to six nucleosomes per helical turn is the solenoid model. This question checks recognition of that terminology within chromatin organization.

Given Data / Assumptions:

• Fiber diameter is approximately 30 nm.
• Each helical turn contains roughly five to six nucleosomes.
• Asks for the conventional name of this arrangement.

Concept / Approach:Two principal 30 nm packing models are described: the solenoid (one-start helix with bent linker DNA) and the zigzag/two-start model (straighter linker DNA crossing the fiber). The stem specifies five to six nucleosomes per helical turn, aligning with the solenoid description.

Step-by-Step Solution:

Verification / Alternative check:Texts often contrast solenoid vs zigzag. The “five to six nucleosomes per turn” cue specifically signals the solenoid arrangement.

Why Other Options Are Wrong:

• Halonoid: not a standard chromatin fiber term.
• Kinetoplast: a mitochondrial DNA network in kinetoplastids, unrelated to nucleosomal fibers.
• None of these: incorrect because a standard term exists.
• Zigzag two-start fiber: alternative model, but the turn/packing cue favors solenoid.

Common Pitfalls:Assuming all 30 nm fibers are identical; in reality, multiple packing modes may exist and depend on ionic conditions and linker histone presence. Here, the numeric cue resolves the correct term.

Final Answer:Solenoid