actin filaments
Summary
Actin filaments (microfilaments) are the thinnest cytoskeletal fibers, polymers of globular G-actin that form polar F-actin strands. They shape the cell, drive crawling and cytokinesis, and with myosin power muscle contraction. Testable: ATP-actin adds faster at the plus end.
Detail
Meaning: Actin filaments, also called microfilaments, are the thinnest cytoskeletal fibers, about 7 nm across. ATP-bound globular actin (G-actin) polymerizes into filamentous F-actin, a twisted double strand. The filament is polar: subunits add faster at the plus (barbed) end than at the minus (pointed) end, and cells assemble and disassemble it quickly. Actin supports the cell cortex and microvilli, drives crawling at the leading edge, forms the contractile ring of cytokinesis with myosin II, and makes up the thin filaments of muscle.
On the MCAT: A passage may describe a compound that blocks actin polymerization and ask what fails: cytokinesis, cell migration or muscle contraction, but not chromosome separation by the spindle. Expect to match fibers to roles: actin works with myosin; microtubules (tubulin) work with kinesin and dynein and form the spindle and cilia; intermediate filaments (keratins, lamins) give mechanical strength.
Common confusion: Students give all of cell division to microtubules. Spindle microtubules separate the chromosomes; the actin–myosin ring then pinches the animal cell in two. Also, actin is the thin filament of muscle, while the thick filament is myosin.
Sources
- Molecular Biology of the Cell
- Lehninger Principles of Biochemistry
- Campbell Biology
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.