Which region of the sarcomere contains only actin thin filaments or microfilaments?

I bands

Explanation:

The contractile cells of skeletal muscles are very long and multinucleated. The cell membrane is called sarcolemma. Much of the cell is occupied by contractile elements, the myofibrils, which are arranged bundles parallel to the axis of contraction. Each myofibril is about 1 mm in diameter. Myofibrils are immersed in a cytosol called sarcoplasma that contains glycogen, glycolytic enzymes ATP, ADP, AMP, phosphocreatine, creatine, various amino acids and inorganic electrolytes.

In the red muscles are many mitochondria arranged regularly.

An electron mycograph of a myofibril presents a functional unit called a sarcomere that is repeated every 2, 3 uM along the axis of the myofibril. There is a regular alternation of a dark band A and a light band I. The central part of band A, called zone H, is less dense than the rest of the band. In the middle of zone H there is a dark line M. Band I is divided in two parts by a very dense Z line.

The cross section of the myofibrils reveals two kinds of filaments: thick filaments of around 150 Aº in diameter and other thin ones of about 70 Aº in diameter. The thick filaments are myosin and the thin ones contain actin, tropomyosin and troponin. Band I consists only of thin filaments (actin) while in zone H of band A only thick filaments are present.

fertilization, pictured in figure 24.23a is the process in which gametes (an egg and sperm) fuse to form a zygote. the egg and sperm each contain one set of chromosomes. to ensure that the offspring has only one complete diploid set of chromosomes, only one sperm must fuse with one egg. in mammals, the egg is protected by a layer of extracellular matrix consisting mainly of glycoproteins called the zona pellucida. when a sperm binds to the zona pellucida, a series of biochemical events, called the acrosomal reactions, take place. in placental mammals, the acrosome contains digestive enzymes that initiate the degradation of the glycoprotein matrix protecting the egg and allowing the sperm plasma membrane to fuse with the egg plasma membrane, as illustrated in figure 24.23b. the fusion of these two membranes creates an opening through which the sperm nucleus is transferred into the ovum. the nuclear membranes of the egg and sperm break down and the two haploid genomes condense to form a diploid genome.

explanation: