Final answer:
The striated appearance of skeletal muscle fibers is due to cross-striations formed by alternating A and I bands along the myofibrils, with Z lines marking the borders of sarcomeres.
Step-by-step explanation:
The characteristic feature of skeletal muscle fibers formed by alternating A and I bands is known as cross-striations. This striated appearance is due to the arrangement of the myofilaments of actin and myosin in myofibrils. These repeating bands are observable through high-resolution electron microscopy, displaying a pattern of dark A bands and light I bands along the myofibrils. The alignment of myofibrils within the muscle cell generates the overall striated or banded look. Furthermore, each I band features a dense line known as a Z disc or Z line, which delineates the border of the sarcomere, the functional unit of skeletal muscle. A sarcomere spans from one Z line to the next and comprises one entire A band and two halves of I bands, one on each side of the A band.