The cells that allow your bones to move contain bundles of long cylinders. These cells have alternating light and dark bands called striations. The striations are due to the presence and arrangement of two different types of filaments in the cells.

Explanation:

The cells that allow your bones to move, the movement of thick (myosin) and thin (actin) filaments during contraction .

During a contraction thick and thin filaments do not shorten but increase their overlap  of each other.

Thin filaments slide past thick filaments extending more deeply into the A band.

The I bands and H bands decrease in lenght as Z discs are come closer together .

Sarcomere represents area between two Z disc, so the sarcomere gets smaller during a contraction .

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a compound is a molecule made of atoms from different elements.

a. by the combination of an unlimited number of elements