Each tubulin molecule is a dimer consisting of two subunits A microtubule

Each tubulin molecule is a dimer consisting of two

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Each tubulin molecule is a dimer consisting of two subunits. A microtubule changes in length by adding or removing tubulin dimers.
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Microtubules shape and support the cell and serve as tracks to guide motor proteins carrying organelles to their destination. Microtubules are also responsible for the separation of chromosomes during cell division. In many cells, microtubules grow out from a centrosome near the nucleus. These microtubules resist compression to the cell. In animal cells, the centrosome has a pair of centrioles , each with nine triplets of microtubules arranged in a ring. Before a cell divides, the centrioles replicate. A specialized arrangement of microtubules is responsible for the beating of cilia and flagella. Many unicellular eukaryotic organisms are propelled through water by cilia and flagella. Cilia or flagella can extend from cells within a tissue layer, beating to move fluid over the surface of the tissue. For example, cilia lining the windpipe sweep mucus carrying trapped debris out of the lungs. Cilia usually occur in large numbers on the cell surface. They are about 0.25 microns in diameter and 2–20 microns long. There are usually just one or a few flagella per cell. Flagella are the same width as cilia, but 10–200 microns long. Cilia and flagella differ in their beating patterns. A flagellum has an undulatory movement that generates force in the same direction as the flagellum’s axis. Cilia move more like oars with alternating power and recovery strokes that generate force perpendicular to the cilium’s axis. In spite of their differences, both cilia and flagella have the same ultrastructure. Both have a core of microtubules sheathed by the plasma membrane. Nine doublets of microtubules are arranged in a ring around a pair at the center. This “9 + 2” pattern is found in nearly all eukaryotic cilia and flagella. Flexible “wheels” of proteins connect outer doublets to each other and to the two central microtubules. The outer doublets are also connected by motor proteins. The cilium or flagellum is anchored in the cell by a basal body, whose structure is identical to a centriole. The bending of cilia and flagella is driven by the arms of a motor protein, dynein. Addition and removal of a phosphate group causes conformation changes in dynein. Dynein arms alternately grab, move, and release the outer microtubules. Protein cross-links limit sliding. As a result, the forces exerted by the dynein arms cause the doublets to curve, bending the cilium or flagellum. Microfilaments are solid rods about 7 nm in diameter. Each microfilament is built as a twisted double chain of actin subunits. Microfilaments can form structural networks due to their ability to branch. The structural role of microfilaments in the cytoskeleton is to bear tension, resisting pulling forces within the cell.
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