Collagen's Triple Helical Structure

Steps:

• Recall collagen's basic unit: a tropocollagen molecule with three polypeptide chains forming a right-handed triple helix.
• Identify key feature: each chain is a left-handed helix rich in glycine every third residue for tight packing.
• Distinguish hierarchy: individual triple helices aggregate into fibrils, then fibers.
• Compare options: select the one accurately describing chains, glycine role, helix formation, and fiber assembly.

Why D is correct:

• Matches collagen definition: three chains with glycine enable tight triple helix coiling; multiple helices aggregate into fibers.

Why the others are wrong:

• A: Mislabels triple helix as "fiber"; fibers are higher-order structures from many helices.
• B: Omits glycine's essential role in stabilizing the structure via its small size.
• C: Fails to note each chain forms a helix before triple coiling.

Final answer: D