Which of the following is true of microtubules but not actin? Select one: Microtubules are nucleated by proteins like Arp2/3 and tormin. Microtubule subunits bind to and hydrolyze ATP. Microtubules are formed from multiple protofilaments in a cylindrical arrangement. Microtubules do not have plus or minus ends. Microtubules are involved in muscle contraction.
The Correct Answer and Explanation is:
Correct answer:
Microtubules are formed from multiple protofilaments in a cylindrical arrangement.
Explanation:
Microtubules and actin filaments are both components of the cytoskeleton, but they differ significantly in structure, dynamics, and function. One key difference lies in their architecture. Microtubules are composed of tubulin subunits (alpha and beta tubulin) that polymerize into linear chains called protofilaments. Typically, thirteen of these protofilaments align side by side to form a hollow cylindrical tube. This cylindrical arrangement gives microtubules both strength and rigidity, making them well suited for their roles in maintaining cell shape, intracellular transport, and chromosome separation during cell division.
In contrast, actin filaments (also known as microfilaments) are thinner and composed of actin monomers that form a helical structure. Actin filaments are more flexible and are important for cell motility, shape changes, and muscle contraction.
The other choices in the question are incorrect for the following reasons:
- Microtubules are nucleated by proteins like Arp2/3 and tormin – This is incorrect. Arp2/3 and formins are involved in actin filament nucleation, not microtubules. Microtubules are nucleated by the gamma-tubulin ring complex (γ-TuRC).
- Microtubule subunits bind to and hydrolyze ATP – This is incorrect. Microtubule subunits bind to and hydrolyze GTP, not ATP. Actin subunits bind and hydrolyze ATP.
- Microtubules do not have plus or minus ends – This is incorrect. Like actin filaments, microtubules have polarity, with a plus end that typically grows faster and a minus end that grows more slowly or is anchored.
- Microtubules are involved in muscle contraction – This is false. Muscle contraction involves actin and myosin filaments, not microtubules.
Therefore, the defining structural feature of microtubules—the cylindrical arrangement of multiple protofilaments—is unique and correctly identifies the true statement.
