Final answer:
A sarcomere is the smallest contractile unit of muscle, comprised of myosin and actin protein filaments that interact during muscle contraction.
Step-by-step explanation:
The smallest contractile unit of a muscle, which is composed of proteins, is known as a sarcomere. Within the sarcomere, muscle contraction occurs as a result of the sliding filament mechanism involving two types of myofilaments: the thick filaments, which are made of the protein myosin, and the thin filaments, composed of the protein actin, along with regulatory proteins such as troponin and tropomyosin. During muscle contraction, the myosin heads bind to actin, pulling the actin filaments closer together and thus shortening the sarcomere.