Final answer:
I bands in a sarcomere are light bands containing thin actin filaments and are bisected by a Z disc. They shorten during muscle contraction as thin filaments are pulled inward, reducing the distance between Z discs. Option A is correct.
Step-by-step explanation:
The I bands in a sarcomere are characterized by their location and contents. They are light bands that contain only thin filaments, primarily made of the protein actin. These I bands are bisected by a dense vertical line called a Z disc or Z line, which serves to anchor the thin filaments and demarcate the boundaries of a sarcomere.
The sarcomere is the functional unit of skeletal muscle contraction, running from one Z disc to the next. During muscle contraction, the I bands shorten as the thin filaments are pulled towards the center of the sarcomere by the thick filaments, increasing the zone of overlap and reducing the distance between adjacent Z discs.
The I bands in a sarcomere are characterized by the presence of a dense line called the Z disc or Z line, which runs vertically through the middle of each I band. The Z discs mark the border of sarcomeres, which are the functional units of skeletal muscle. Each sarcomere contains one entire A band and two halves of an I band.