Chromosome Condensation in Prophase

Steps:

• Identify histones P and R as proteins that DNA (Q) wraps around to form nucleosomes.
• Recognize that in prophase, these nucleosomes coil into chromatin fibers.
• Note that fibers further fold and twist to condense into visible chromatids.
• Match this process to the option describing histone-DNA groups forming twisted strands into chromatids.

Why D is correct:

• In prophase, histone octamers with wrapped DNA form 30-nm fibers that loop and supercoil into chromatids, per the hierarchical model of chromosome structure.

Why the others are wrong:

• A: DNA does not form clusters labeled P, Q, and R; P and R are histones, with Q as DNA.
• B: Histones do not cluster around DNA; DNA wraps around histone cores to form nucleosomes.
• C: While true for nucleosome basics, it ignores prophase-specific condensation into chromatids.

Final answer: D