The three principal filament systems of the eukaryotic cytoskeleton are
Answer: A
They differ in diameter, in the protein from which each is built and in the jobs they do. Collagen and elastin lie outside the cell in the extracellular matrix rather than within the cytoplasm, so they are not part of the cytoskeleton at all.
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Microfilaments are composed of
Answer: B
Globular actin monomers polymerise into a two-stranded helix about seven nanometres across. Myosin is the motor protein that walks along actin rather than a component of the filament itself, which is a distinction worth keeping clear.
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Microtubules are built from
Answer: C
The dimers stack into thirteen protofilaments arranged as a hollow tube some twenty-five nanometres across. Their hollow construction gives great stiffness for the quantity of protein used, which is why they can serve as compression-bearing struts within the cell.
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Which cytoskeletal element has the largest diameter?
Answer: D
Microtubules are about 25 nanometres across, intermediate filaments about 10 and microfilaments about 7. The intermediate filaments take their name from sitting between the other two in size, which is the only thing the name records about them.
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State whether the following is true or false.
Intermediate filaments are so named because they are intermediate in diameter between microfilaments and microtubules.
Answer: A
At around ten nanometres they lie between the seven of a microfilament and the twenty-five of a microtubule. The name is purely descriptive of size and says nothing about their function, which is chiefly mechanical rather than to do with movement or transport.
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The chief function of intermediate filaments is to
Answer: B
They resist tension in the way a rope does, which is why they are abundant in tissues under mechanical stress such as skin and the lining of blood vessels. Transport and force generation belong to microtubules and microfilaments respectively.
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Assertion (A): Intermediate filaments are especially abundant in epithelial cells of the skin.
Reason (R): Such cells are subject to repeated mechanical stress and must resist tearing.
Answer: C
Both are true and the reason accounts for the abundance: keratin filaments anchored at desmosomes distribute stress across a whole sheet of cells, and inherited faults in them cause skin that blisters at slight friction.
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Microtubules grow from an organising centre which in animal cells is the
Answer: D
The centrosome contains a pair of centrioles surrounded by material rich in gamma-tubulin, which nucleates new microtubules from their minus ends. Plant cells organise their microtubules perfectly well without any such discrete centre, so the centrosome is not universal.
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Dynamic instability of microtubules refers to their
Answer: A
An individual microtubule grows for a time and then catastrophically depolymerises, which lets the cell search space rapidly and rebuild its array. The behaviour depends on whether the GTP cap at the growing end is retained.
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State whether the following is true or false.
A microtubule that loses its GTP cap tends to depolymerise rapidly.
Answer: B
The cap stabilises the growing end, and once hydrolysis catches up with growth the tubule begins to peel apart from the end. This is the molecular basis of dynamic instability rather than an accident of handling, and it is what lets the array be rebuilt quickly.
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