What is the dark band in a sarcomere known as?

• A. I Band
• B. H Band
• C. Z Disc
• D. A Band

Answer: (D)

The dark band in a sarcomere is the A band. It spans the length of the thick filaments (myosin) and includes regions where thick and thin filaments overlap. It looks darker because this area has a high density of contractile proteins and cross-bridges, increasing its optical density under light microscopy. The surrounding lighter region, the I band, contains only thin actin filaments and thus appears lighter. The Z disc marks the boundary between sarcomeres, and the H band is the central part within the A band where thick filaments exist without overlapping thin filaments. During contraction, the A band stays the same length, while the I band and H zone shorten as actin slides toward the center.