Question

Which of the given choices are cytoplasmic filaments?
(A) Microtubules
(B) Actin filaments
(C) Myosin filaments
(D) Intermediate filaments
Choose the correct answer from the options given below:

• A. (B) and (D) only.
• B. (A), (B) and (D) only.
• C. (A), (B) and (C) only.
• D. (A), (B), (C) and (D).

Hint:

Remember the "cytoskeletal trio": Microtubules ($25\,\text{nm}$), Intermediate Filaments ($10\,\text{nm}$), and Actin/Microfilaments ($7\,\text{nm}$).
These three fibers are universally present in eukaryotic cells to maintain cell architecture.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The eukaryotic cytoskeleton is a dynamic structural network of protein filaments that extends throughout the cytoplasm.
It provides mechanical support, maintains cell shape, coordinates cell division, and facilitates intracellular transport.

Step 2: Detailed Explanation:
Standard cell biology textbooks define the cytoskeleton as being composed of three primary classes of cytoplasmic filaments:
1. Microtubules (A): Hollow cylinders measuring approximately $25\,\text{nm}$ in diameter, composed of polymerized $\alpha$- and $\beta$-tubulin dimers.
2. Actin filaments Microfilaments (B): Double-helical polymers of globular actin monomers, measuring about $7\,\text{nm}$ in diameter.
3. Intermediate filaments (D): Tough, rope-like fibers measuring about $10\,\text{nm}$ in diameter, composed of various tissue-specific fibrous proteins (such as keratins, vimentin, desmin, and lamins).
These three filaments form the structural framework of virtually all eukaryotic cells.
While myosin filaments (C) are important thick filaments involved in muscle contraction, they are specialized motor assembly structures found predominantly in muscle cells, rather than representing the universal, primary classes of cytoplasmic cytoskeletal filaments.

Step 3: Final Answer:
Therefore, the three primary cytoplasmic filaments are microtubules, actin filaments, and intermediate filaments, corresponding to option (B).