Explain how it is that actin and myosin in the sarcomere never actually shorten and yet the muscle as a whole does.

The muscle fibers are pulled by the myosin and sarcomere together, resulting in the shortening of the fibres, but the sarcomere and myosin just move.

Explanation:

During muscle contraction, each sarcomere shortens, bringing the Z discs close together. There is no change in the width of the A band but both I bands and the H zone almost completely disappear. These changes are explained by actin and myosin filaments sliding past one another, so that the actin filaments move into A band and H zone. Muscle contraction thus results from an interaction between the actin and the mosin filaments that generates their movement relative to one another. The molecular basis for this interaction is the binding of the myosin to actin filaments that generate their movement relative to one another.

sunlight and water

explanation:

a tetrad is the foursome during meiosis made by two homologous chromosomes that have each already replicated into a pair of sister chromatids.