The nuclear envelope consists of
Answer: A
The outer membrane is continuous with the endoplasmic reticulum and carries ribosomes, while the inner one is lined by the lamina. The space between them is continuous with the lumen of the reticulum. The nucleus is therefore part of the endomembrane system rather than separate from it.
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Material passes between nucleus and cytoplasm through
Answer: B
Each pore is a large assembly of proteins that spans both membranes and controls what crosses. Small molecules diffuse through freely while large cargo requires a signal and a carrier. The cut-off lies around forty kilodaltons for unaided passage.
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State whether the following is true or false.
Proteins large enough to require assistance can enter the nucleus only if they carry a nuclear localisation signal.
Answer: C
The signal is recognised by an importin that escorts the protein through the pore. Without it a large protein remains in the cytosol however abundant it may be there. Adding such a signal experimentally will redirect a protein into the nucleus.
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Assertion (A): A protein made in the cytosol can accumulate to high concentration inside the nucleus.
Reason (R): Nuclear import is an active process that can work against a concentration gradient.
Answer: D
Both are true and the reason gives the mechanism: energy from the Ran GTPase system drives directional transport through the pore, so equilibrium is not what determines the final distribution. The gradient is maintained by hydrolysis rather than by simple partitioning.
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The nucleolus is the site where
Answer: A
It forms around the clusters of genes encoding ribosomal RNA and is not bounded by any membrane of its own. Its size reflects how heavily a cell is making ribosomes. Cells growing rapidly show conspicuously large nucleoli under the microscope.
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State whether the following is true or false.
The nucleolus is a membrane-bound compartment within the nucleus.
Answer: B
It is a dense region formed by the material gathering around ribosomal RNA genes, with no membrane separating it from the nucleoplasm. Compartments of this kind are held together by molecular association rather than a boundary.
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Chromatin is best described as
Answer: C
Histones make up the bulk of the protein, and the combination is what actually exists in a living nucleus. Naked DNA of the kind drawn in diagrams is not found there. Every process acting on DNA must therefore contend with the packaging first.
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The basic repeating unit of chromatin is the
Answer: D
About 147 base pairs of DNA wrap nearly twice around a core of eight histone proteins. Strung together these give the beads-on-a-string appearance seen when chromatin is spread out. Further coiling of that fibre produces the more compact forms.
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The histone octamer at the core of a nucleosome contains two copies each of
Answer: A
Histone H1 is the linker histone and sits outside the core, binding where the DNA enters and leaves. Confusing it with a core histone is a common error. The octamer is two copies each of four different histones, not eight different ones.
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If a nucleosome holds about 147 base pairs and linkers average about 53, roughly how many nucleosomes occupy 20,000 base pairs?
Answer: B
Each repeat covers about 200 base pairs once the linker is included, and 20,000 divided by 200 gives 100. Using the 147 alone would overcount, since the linker DNA also has to be accommodated. The repeat length varies somewhat between cell types and organisms.
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