Approximately what is the packing ratio (fold compaction) of DNA in a nucleosome fiber relative to naked DNA?

Introduction / Context:
Chromatin organization starts with wrapping DNA around histone octamers to form nucleosomes, creating the “beads-on-a-string” fiber. The question asks for the approximate compaction achieved at this level.

Given Data / Assumptions:

• Nucleosome core particle wraps about 147 bp of DNA around histones.
• Linker DNA connects adjacent nucleosomes to form a 10-nm fiber.

Concept / Approach:
Packing ratio is the linear compaction of DNA length after nucleosome formation compared to naked DNA. Empirically, the beads-on-a-string fiber yields roughly 6–7-fold compaction; the commonly cited value is about 7.

Step-by-Step Solution:
1) Each nucleosome wraps ~1.7 turns of DNA.2) Considering linker segments, average compaction of the 10-nm fiber is near 7-fold.3) Therefore, the best approximate answer is 7.

Verification / Alternative check:
Biophysical measurements and electron microscopy of reconstituted chromatin consistently report ~7-fold compaction at the primary nucleosome level.

Why Other Options Are Wrong:
Values 6, 8, or 9 deviate from the commonly accepted approximation; 7 is the standard textbook number.

Common Pitfalls:
Confusing higher-order compaction (e.g., 30-nm fiber) with the primary nucleosome fiber; those yield greater packing ratios.

Final Answer:
7