4.11
In eukaryotic cells, microtubules are the thickest cytoskeletal elements in cells.
They are hollow tubes about 25 nanometers wide. They are usually made of 13 protofilaments made from repeating alpha- and beta-tubulin heterodimers.
Each microtubule has structural polarity, with a plus end and a minus end. At the plus end, where beta-tubulin is exposed, cells usually add new dimers. The minus end has alpha-tubulin, is usually less dynamic, and is often attached to a microtubule-organizing center that anchors many microtubules.
Microtubules can rapidly grow and shrink. This behavior is known as dynamic instability.
Microtubule-associated proteins bind along the sides of microtubules and help control their stability and organization.
Microtubule polarity supports directional transport, where motor proteins such as kinesin and dynein move along microtubules and carry cargo, such as vesicles, through the cytoplasm.
In eukaryotic cells such as epithelial cells and sperm cells, microtubules form the core of cilia and flagella, which have a characteristic 9+2 arrangement. These structures move fluidly across cell surfaces or propel cells through their environment.
There are three types of cytoskeletal structures in eukaryotic cells—microfilaments, intermediate filaments, and microtubules. With a diameter of about 25 nm, microtubules are the thickest of these fibers. Microtubules carry out a variety of functions that include cell structure and support, transport of organelles, cell motility (movement), and the separation of chromosomes during cell division.
Microtubules are hollow tubes whose walls are made up of globular tubulin proteins. Each tubulin molecule is a heterodimer, consisting of a subunit of α-tubulin and a subunit of β-tubulin. The dimers are arranged in linear rows called protofilaments. A microtubule usually consists of 13 protofilaments, arranged side by side, wrapped around the hollow core.
Because of this arrangement, microtubules are polar, meaning that they have different ends. The plus end has β-tubulin exposed, and the minus end has α-tubulin exposed. Microtubules can rapidly assemble—grow in length through polymerization of tubulin molecules—and disassemble. The two ends behave differently in this regard. The plus end is typically the fast-growing end or the end where tubulin is added, and the minus end is the slow-growing end or the end where tubulin dissociates—depending on the situation.
This process of dynamic instability, where microtubules rapidly grow and shrink, is important for functions such as the remodeling of the cytoskeleton during cell division and the extension of axons from growing neurons.
Microtubules also can be stable, often by binding to microtubule-associated proteins, which help the cell to maintain its shape. Other proteins, called motor proteins, can interact with microtubules to transport organelles in a particular direction. For example, many neurotransmitters are packaged into vesicles in the cell body of a neuron and are then transported down the axon along a “track” of microtubules, delivering the vesicles to where they are needed. Finally, microtubules can also protrude outside of the cell—making up the filamentous flagella and cilia that move to push cells (such as sperm) along, or to move fluid across their surfaces, such as in the lungs.
In eukaryotic cells, microtubules are the thickest cytoskeletal elements in cells.
They are hollow tubes about 25 nanometers wide. They are usually made of 13 protofilaments made from repeating alpha- and beta-tubulin heterodimers.
Each microtubule has structural polarity, with a plus end and a minus end. At the plus end, where beta-tubulin is exposed, cells usually add new dimers. The minus end has alpha-tubulin, is usually less dynamic, and is often attached to a microtubule-organizing center that anchors many microtubules.
Microtubules can rapidly grow and shrink. This behavior is known as dynamic instability.
Microtubule-associated proteins bind along the sides of microtubules and help control their stability and organization.
Microtubule polarity supports directional transport, where motor proteins such as kinesin and dynein move along microtubules and carry cargo, such as vesicles, through the cytoplasm.
In eukaryotic cells such as epithelial cells and sperm cells, microtubules form the core of cilia and flagella, which have a characteristic 9+2 arrangement. These structures move fluidly across cell surfaces or propel cells through their environment.
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