Which statement correctly describes the constituents and boundaries of a sarcomere?

• A. Z-disc marks the sarcomere boundaries; M-line runs through the center; A-band contains thick filaments; I-band contains thin filaments; H-zone is the center with only thick filaments.
• B. Z-disc marks the center of the sarcomere; M-line marks the boundary; A-band contains thin filaments; I-band contains thick filaments.
• C. Z-disc anchors thick filaments; M-line anchors thin filaments; A-band is the boundary; H-zone contains no filaments.
• D. Z-disc is where actin and myosin overlap most; M-line isolates the I-band; A-band is the length of thin filaments.

Answer: (A)

At the sarcomere level, boundaries and filament organization are defined by distinct landmarks: the Z-discs mark the ends of each sarcomere and anchor actin (thin) filaments, the M-line sits at the center and stabilizes the thick filaments, and the A-band spans the entire length of the thick filaments, including the overlap with thin filaments. The I-band contains only thin filaments and is bisected by the Z-disc, while the central part of the A-band where there are no thin filaments is the H-zone. This description matches the statement because it correctly assigns Z-discs to sarcomere boundaries, the M-line to the center, the A-band to thick filaments (with overlap zones), the I-band to thin filaments, and the H-zone as the region with only thick filaments. The other descriptions mix up which filaments are in the bands or where the boundaries lie, leading to mismatches like misplacing the Z-disc, misidentifying the A- and I-bands, or stating the M-line anchors the wrong filament.