Which statement best describes alpha helices?

• A. Tight coils
• B. Flat, sheet-like arrangements
• C. Random coils
• D. Extended linear strands

Answer: (A)

Alpha helices are a common protein secondary structure that forms a right-handed coil. The backbone carbonyl of each amino acid donates a hydrogen bond to the amide hydrogen of the amino acid four residues ahead, creating a stable, repeating pattern. This hydrogen-bonding scheme makes the chain wind into a tight, regular spiral (about 3.6 residues per turn, with side chains pointing outward), which is why describing alpha helices as tight coils is the best match. The other descriptions don’t fit: flat, sheet-like arrangements refer to beta sheets; random coils describe unstructured segments without a defined shape; and extended linear strands describe noncoiled stretches, not the compact coil of an alpha helix.