Actin is made of:
**Core Concept**
Actin is a type of protein that forms microfilaments, which are the thinnest components of the cytoskeleton. It plays a crucial role in cell motility, shape determination, and muscle contraction. Actin filaments are composed of two strands of actin subunits that are twisted together.
**Why the Correct Answer is Right**
Actin filaments are formed by the polymerization of actin monomers, which are globular proteins. The actin subunits are arranged in a helical fashion, with each subunit having a specific orientation. This unique arrangement allows actin filaments to interact with other cytoskeletal components, such as myosin, to facilitate muscle contraction and cell movement. The actin subunits are linked together by weak ionic bonds, which can be broken and reformed to allow for dynamic changes in filament structure.
**Why Each Wrong Option is Incorrect**
* **Option A:** Actin is not composed of tubulin subunits, which are the building blocks of microtubules, another component of the cytoskeleton.
* **Option B:** Actin filaments are not formed by the polymerization of myosin subunits, which are responsible for muscle contraction but are not part of the actin filament structure.
* **Option D:** Actin filaments are not composed of keratin subunits, which are found in intermediate filaments and provide structural support to cells.
**Clinical Pearl / High-Yield Fact**
Actin filaments are dynamic structures that can be rapidly assembled and disassembled in response to cellular signals. This property allows actin filaments to play a key role in various cellular processes, including muscle contraction, cell migration, and cytokinesis.
**Correct Answer: C. Actin monomers. Actin monomers.**