Question

in a sarcomere, cross - bridge attachment occurs specifically in the
h band.
i band.
m line.
zone of overlap.
a band.

Explanation:

• H band: Contains only thick filaments (myosin), no thin filaments (actin), so no cross - bridge attachment (cross - bridges form between myosin and actin).
• I band: Contains only thin filaments (actin), no thick filaments (myosin), so no cross - bridge attachment.
• M line: A structural element in the middle of the sarcomere, not a site of cross - bridge attachment.
• Zone of overlap: In this region, thick (myosin) and thin (actin) filaments overlap. Cross - bridges (myosin heads) attach to actin in this zone during muscle contraction.
• A band: The entire length of the thick filament (myosin). But cross - bridge attachment specifically occurs where there is overlap with actin (i.e., the zone of overlap within the A band).

Answer:

zone of overlap.