PDQ PHYSIOLOGY may not be homogeneous throughout the cytosol, and the importance of this for normal function is not yet fully evident. Cytoskeleton The cytoskeleton, an arrangement of intracellular structural elements, (1) helps maintain cell shape, (2) permits motion of one part of a cell relative to other parts, and(3) provides the machinery for the locomotion of the whole cell. the skeletal elements are, in descending order of size, microtubules, intermediate filaments, and actin (or microfilaments Microtubules, centrioles, and ciliae. Microtubules are hollow, cylindrical arrangements of the proteins a- and B-tubulin, 20 to 30 nm in diameter and 10 to 25 um in length. They grow from one end(the plus end)by polymerization of tubulin, whereas the minus end tends to disintegrate by hydrolysis unless it is stabilized. Microtubules are present in almost all mammalian cells and have three main functions: (1)control of the mitotic process,(2)movements of ciliae and flagellae, and (3)guided intracellular transport of proteins or vesicles. Control of the mitotic process. In most cells, with the notable exception of nerve cells, the negative end of most microtubules is anchored and sta- bilized in the centrosome. The plus ends, as long as they are free, grow from the pericentriolar material of the centrosome along an arbitrary path During the S phase of the cell replication cycle, when DNA replicates, the centrosome duplicates and divides into two equal parts, each containing a centriole pair. When mitosis begins, the two centrosomes move to opposite sides of the nucleus and form the two poles of the mitotic spindle, an array of microtubules that aligns chromosomes and holds them in place for the subsequent steps of cell division. These aspects are described more fully below(see The Cell Cycle) In the long phase preceding mitosis, the configuration of microtubules attached to a centrosome changes continually as new microtubules grow by tubulin polymerization at the plus end and old ones disintegrate by tubu lin hydrolysis at the minus end. A variety of chemical agents can inhibit microtubule formation and, with that, inhibit cell division. Examples of such chemical agents, all of which bind a-and B-tubulin, are colchicin, vin- blastin, and vincristine. n that lies near the nucleus. The centrosome contains amorphous pericentriolar and two centrioles(see Figure 1-4), each a pair of cylindrical bodies, positionedmay not be homogeneous throughout the cytosol, and the importance of this for normal function is not yet fully evident. Cytoskeleton The cytoskeleton, an arrangement of intracellular structural elements, (1) helps maintain cell shape, (2) permits motion of one part of a cell relative to other parts, and (3) provides the machinery for the locomotion of the whole cell. The primary skeletal elements are, in descending order of size, microtubules, intermediate filaments, and actin (or microfilaments). Microtubules, centrioles, and ciliae. Microtubules are hollow, cylindrical arrangements of the proteins α- and β-tubulin, 20 to 30 nm in diameter and 10 to 25 µm in length. They grow from one end (the plus end) by polymerization of tubulin, whereas the minus end tends to disintegrate by hydrolysis unless it is stabilized. Microtubules are present in almost all mammalian cells and have three main functions: (1) control of the mitotic process, (2) movements of ciliae and flagellae, and (3) guided intracellular transport of proteins or vesicles. Control of the mitotic process. In most cells, with the notable exception of nerve cells, the negative end of most microtubules is anchored and sta￾bilized in the centrosome.* The plus ends, as long as they are free, grow from the pericentriolar material of the centrosome along an arbitrary path. During the S phase of the cell replication cycle, when DNA replicates, the centrosome duplicates and divides into two equal parts, each containing a centriole pair. When mitosis begins, the two centrosomes move to opposite sides of the nucleus and form the two poles of the mitotic spindle, an array of microtubules that aligns chromosomes and holds them in place for the subsequent steps of cell division. These aspects are described more fully below (see The Cell Cycle). In the long phase preceding mitosis, the configuration of microtubules attached to a centrosome changes continually as new microtubules grow by tubulin polymerization at the plus end and old ones disintegrate by tubu￾lin hydrolysis at the minus end. A variety of chemical agents can inhibit microtubule formation and, with that, inhibit cell division. Examples of such chemical agents, all of which bind α- and β-tubulin, are colchicin, vin￾blastine, and vincristine. 6 PDQ PHYSIOLOGY *A region that lies near the nucleus. The centrosome contains amorphous pericentriolar material and two centrioles (see Figure 1–4), each a pair of cylindrical bodies, positioned at right angles to each other