Which of the following statements about the cytoskeleton in cells is true according to modern cell biology?

Introduction / Context:
The cytoskeleton is a complex network of protein filaments that provides structural support, enables movement, and organizes the interior of cells. Modern research has shown that the cytoskeleton is highly dynamic rather than static. This question asks which statement correctly reflects current understanding of the cytoskeleton.

Given Data / Assumptions:

• The cytoskeleton exists in both eukaryotic and prokaryotic cells.
• It is composed of several types of protein filaments.
• Filaments assemble and disassemble as needed.
• We assume current cell biology knowledge.

Concept / Approach:
In eukaryotic cells, the cytoskeleton consists mainly of microfilaments actin filaments, intermediate filaments, and microtubules. These filaments are built from protein subunits held together largely by noncovalent interactions, allowing rapid assembly and disassembly. Bacteria also possess cytoskeletal proteins that are important for cell shape, division, and chromosome segregation. The cytoskeleton is constantly remodeling in response to internal and external signals, for example during cell migration or mitosis. Therefore, the statement emphasizing cytoskeletal dynamics in response to the environment is true, while statements describing it as unnecessary for division or as a rigid unchanging structure are false.

Step-by-Step Solution:
Step 1: Examine option A, which claims that the bacterial cytoskeleton is not required for cell division and DNA segregation. This is incorrect because bacterial cytoskeletal proteins such as FtsZ and MreB play key roles in these processes. Step 2: Examine option B, which states that the cytoskeleton is made up of only two types of protein filament. In fact, eukaryotic cells have three major filament systems microfilaments, intermediate filaments, and microtubules, so this statement is false. Step 3: Examine option C, claiming that protein monomers are held together mainly by covalent bonds. Cytoskeletal filaments are assembled by noncovalent interactions, allowing dynamic remodeling, so this is false. Step 4: Examine option D, which states that the cytoskeleton can change dynamically in response to signals and the environment. This matches the modern view of the cytoskeleton as a highly dynamic structure. Step 5: Examine option E, which describes the cytoskeleton as rigid and unchanging, which contradicts abundant evidence of cytoskeletal remodeling. Step 6: Conclude that option D is the only true statement among the choices.

Verification / Alternative check:
Live cell imaging and fluorescence microscopy show continuous growth and shrinkage of microtubules, polymerization and depolymerization of actin filaments, and reorganization of intermediate filaments. During cell division, the mitotic spindle forms and disassembles. During cell migration, actin filaments assemble at the leading edge and disassemble at the rear. External signals such as growth factors or mechanical stress trigger cytoskeletal rearrangements. These observations confirm that the cytoskeleton is dynamic and responsive to environmental cues.

Why Other Options Are Wrong:
Option A is wrong because bacterial cytoskeletal proteins are essential for cell division and chromosome partitioning. Option B is incorrect because it underestimates the number of filament types. Option C misstates the nature of filament assembly; covalent bonds would make the structure too rigid and hard to remodel. Option E is directly opposite to the known dynamic behavior of the cytoskeleton. Only option D correctly highlights the ability of the cytoskeleton to respond to signals and environmental changes.

Common Pitfalls:
Students may still picture the cytoskeleton as a fixed scaffold that simply holds the cell shape, based on older simplified diagrams. It is important to update this view and recognize that cytoskeletal elements are constantly turning over. Another pitfall is to think that only eukaryotes have a cytoskeleton, ignoring the bacterial cytoskeleton, which also contributes to essential processes.

Final Answer:
The true statement about the cytoskeleton is that the cytoskeleton of a cell can change dynamically in response to signals and the environment.