# The Cytoskeleton

Biology I · Cells and Tissues · https://tryals.app/learn/biology-i/the-cytoskeleton

## Scaffolding That Rebuilds Itself

The cytoskeleton is not a static frame. It is three filament systems in continuous assembly and disassembly, and that dynamism is the point, a static scaffold could not divide a cell or crawl it across a surface.

| Filament | Diameter | Subunit | Main roles |
|---|---|---|---|
| Microfilaments | ~7 nm | Actin | Shape, movement, cytokinesis |
| Intermediate filaments | ~10 nm | Various | Mechanical strength |
| Microtubules | ~25 nm | Tubulin | Transport, cilia, mitotic spindle |

**Microfilaments** are actin polymers, the thinnest of the three. They form the cortex just beneath the membrane, drive cell crawling, pinch the cell in two during cytokinesis, and, with myosin, produce muscle contraction.

**Intermediate filaments** are the only one of the three that is not dynamic. They are rope-like, mechanically strong, and built from tissue-specific proteins: keratins in epithelia, lamins lining the nuclear envelope. Their job is purely to resist mechanical stress, and the genetic diseases affecting them produce fragile tissues.

**Microtubules** are hollow tubes of tubulin, the widest and stiffest. They radiate from the **centrosome** and act as the cell's roads: **kinesin** walks toward the plus end, generally outward, and **dynein** toward the minus end, generally inward. They also build cilia and flagella in the **9+2** arrangement, nine doublets around a central pair, and form the spindle that separates chromosomes.

Microtubules are so central to mitosis that they are a major drug target. **Colchicine** blocks their polymerisation and **taxol** blocks their disassembly; both arrest dividing cells, which is why taxol is a chemotherapy agent.

Motor proteins take discrete steps: kinesin advances about 8 nm per ATP hydrolysed, so moving a vesicle 1 µm costs roughly 125 steps and 125 ATP.

> **Common pitfall:** thinking a drug that *stabilises* microtubules would help a dividing cell. It does the opposite. Mitosis needs microtubules to shorten as well as grow, so freezing them in place is just as lethal as preventing their assembly, which is exactly how taxol works.

## Practice questions

6 of this lesson's 11 practice questions, with answers. The full set is in the app.

### 1. Why do both colchicine, which prevents microtubule assembly, and taxol, which prevents disassembly, arrest cell division?

A. Both drugs bind directly to nuclear DNA and inhibit replication during S phase
B. Both compounds trigger the rapid chemical degradation of centrosome structures
C. Mitosis requires microtubules to both grow and shrink, so freezing either process blocks it
D. Taxol only disrupts spindle dynamics after colchicine has pre-treated the cell

**Answer:** C. Mitosis requires microtubules to both grow and shrink, so freezing either process blocks it

**Why:** The spindle must both assemble and then shorten to pull chromosomes apart. Blocking either half of that cycle stops mitosis, which is why a stabiliser and a destabiliser have the same clinical effect, and why taxol is a chemotherapy drug.

Page: https://tryals.app/practice/biology-i/the-cytoskeleton/why-do-both-colchicine-which-prevents-microtubule-assembly-and

### 2. Microtubules are about 25 nm in diameter and microfilaments about 7 nm. How many times wider is a microtubule, to one decimal place?

**Answer:** 3.6 (within ±0.15)

**Why:** $25/7 = 3.6$ times wider. The difference matters mechanically: a wider hollow tube is far stiffer in bending, which is why microtubules serve as tracks and spindle fibres while thin actin filaments handle deformation.

Page: https://tryals.app/practice/biology-i/the-cytoskeleton/microtubules-are-about-25-nm-in-diameter-and-microfilaments-about-7

### 3. Intermediate filaments are the most dynamic of the three cytoskeletal systems.

**Answer:** False

**Why:** False, intermediate filaments are the *least* dynamic. Microfilaments and microtubules turn over continuously, which is what lets them remodel; intermediate filaments are stable ropes whose whole purpose is to resist mechanical stress.

Page: https://tryals.app/practice/biology-i/the-cytoskeleton/intermediate-filaments-are-the-most-dynamic-of-the-three-cytoskeletal

### 4. Which processes depend on microtubules?

A. Beating of cilia and flagella
B. Separation of chromosomes in mitosis
C. Pinching the cell in two during cytokinesis
D. Movement of vesicles through the cytoplasm

**Answer:** A. Beating of cilia and flagella; B. Separation of chromosomes in mitosis; D. Movement of vesicles through the cytoplasm

**Why:** Microtubules handle transport, ciliary beating and chromosome separation. Cytokinesis is actin’s job, a contractile ring of actin and myosin pinches the cell, which is why the two systems are needed in sequence.

Page: https://tryals.app/practice/biology-i/the-cytoskeleton/which-processes-depend-on-microtubules

### 5. Intermediate filaments provide mechanical strength without dynamic turnover, unlike actin or tubulin. What consequence follows from this structural difference?

A. They rely on subunit turnover to resist tension
B. Their loss disrupts chromosome division in mitosis
C. They cannot support force-generating motor proteins
D. Their defects make tissues fragile rather than weak

**Answer:** C. They cannot support force-generating motor proteins

**Why:** Static scaffolds cannot sustain directional motor transport or rapid remodelling. Confusing filament stability with active force production misses that motors require polarised tracks; intermediate filaments lack dynamic polarity and tissue fragility stems directly from loss of tensile integrity.

Page: https://tryals.app/practice/biology-i/the-cytoskeleton/intermediate-filaments-provide-mechanical-strength-without-dynamic

### 6. Sort each motor protein or direction by the system it belongs to.

**Answer:**

- Moves along microtubules: Kinesin, walking toward the plus end, Dynein, walking toward the minus end
- Moves along actin filaments: Myosin, driving muscle contraction, Myosin, driving cytoplasmic streaming

**Why:** Kinesin and dynein are microtubule motors running in opposite directions; myosin is the actin motor. The pairing is strict, a motor cannot switch track types, which is what makes directional transport possible.

Page: https://tryals.app/practice/biology-i/the-cytoskeleton/sort-each-motor-protein-or-direction-by-the-system-it-belongs-to