18: The Cytoskeleton and Cell Motility
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- 18.1: Introduction
- The cell as it appears in a microscope was long thought to be a bag of liquid surrounded by a membrane. The electron microscope revealed a cytoskeleton composed of thin and thick rods, tubes and filaments. Other intracellular structures and organelles are enmeshed in these microfilaments, intermediate filaments and microtubules. We will compare the molecular compositions of these structures and their subunit proteins.
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- 18.2: Cytoskeletal Components
- Most eukaryotic cells look like a membrane-bound sac of cytoplasm containing a nucleus and assorted organelles in a light microscope. In the late 19th century, microscopists described a dramatic structural re-organization of dividing cells. In mitosis, duplicated chromosomes (i.e., chromatids) condense in the nucleus just as the nuclear membrane dissolves. Spindle fibers emerge and then seem to pull the chromatids apart to opposite poles of the cell.
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- 18.3: The Molecular Structure and Sub-Cellular Organization of Cytoskeletal Components
- Of the three main cytoskeletal fibers, intermediate filaments serve a mainly structural role in cells. Microtubules and microfilaments have dual functions, dynamically maintaining cell shape and enabling cell motility. For example, when attached to the plasma membrane, microfilaments maintain cell shape. However, by interacting with motor proteins (e.g., myosin), they can pull or push against a muscle cell membrane, changing the shape of the cell.
Thumbnail: Image of a human cell showing microtubules in green, chromosomes (DNA) in blue, and kinetochores in pink (Public Domain; Afunguy ).