# The Cell Nucleus Biology I · Cells and Tissues · https://tryals.app/learn/biology-i/the-cell-nucleus ## Packing and Gatekeeping A human cell holds about 2 m of DNA in a nucleus roughly 6 µm across, a packing ratio of over 300,000 to 1. The packing is hierarchical, and crucially it is **reversible**, because DNA that cannot be reached cannot be read. The first level wraps 147 base pairs around a histone octamer to form a **nucleosome**, the "beads on a string" arrangement. Nucleosomes coil into a 30 nm fibre, which loops onto a protein scaffold, and in mitosis condenses further into the visible chromosome. Packing state is functional, and the two states are visibly different: **euchromatin** is loosely packed and transcriptionally active; **heterochromatin** is densely packed and silent. A cell regulates genes partly by moving regions between the two. The **nuclear envelope** is a double membrane continuous with the endoplasmic reticulum, perforated by **nuclear pore complexes**: large assemblies of around 30 different proteins. Small molecules diffuse through freely; anything above roughly 40 kDa needs a **nuclear localisation signal** and active, energy-dependent transport. Traffic is heavy: a typical pore handles hundreds of molecules a second, in both directions at once. The **nucleolus** is not a membrane-bound organelle but a dense region where ribosomal RNA is transcribed and ribosomal subunits are assembled before export. Its size tracks how much protein synthesis a cell is doing, so a rapidly growing cell has a conspicuously large nucleolus. The envelope is also structural: the **nuclear lamina**, a mesh of intermediate filaments, lines its inner face, gives the nucleus its shape and anchors chromatin. Mutations in lamins cause a striking set of diseases including progeria, evidence of how much depends on the nucleus being mechanically sound. > **Common pitfall:** thinking chromosomes are always the X-shaped objects of textbook diagrams. That form exists only during division. For most of a cell's life the DNA is decondensed chromatin, and it has to be, because condensed DNA cannot be transcribed. ## Practice questions 7 of this lesson's 11 practice questions, with answers. The full set is in the app. ### 1. A nuclear pore allows free diffusion below about 40 kDa, which is why a larger protein needs a signal and energy to get in. A protein of 70 kDa needs that help. Set the ratio of that protein’s mass to the diffusion limit, to one decimal place. **Answer:** 1.75 (within ±0.2) **Why:** $70/40 = 1.75$ times the limit, so this protein needs a nuclear localisation signal and active transport. The cut-off is soft rather than sharp, but the principle is strict: large cargo requires a signal and energy. Page: https://tryals.app/practice/biology-i/the-cell-nucleus/a-nuclear-pore-allows-free-diffusion-below-about-40-kda-which-is-why ### 2. The nucleolus is a membrane-bound organelle within the nucleus. **Answer:** False **Why:** False, the nucleolus has no membrane. It is a dense region where ribosomal RNA is transcribed and ribosomal subunits assembled, held together by the concentration of activity itself rather than by a boundary. Page: https://tryals.app/practice/biology-i/the-cell-nucleus/the-nucleolus-is-a-membrane-bound-organelle-within-the-nucleus ### 3. Chromatin packing is dynamic rather than fixed. Why must a differentiated cell continuously shift specific regions of DNA between euchromatin and heterochromatin? A. To enable selective access to genes required for changing metabolic demands B. To protect the structural stability of the nuclear envelope against mechanical stress C. To permit the complete replication of all genetic material prior to cell division D. To allow ribosomal subunit assembly to occur throughout the entire nucleoplasm **Answer:** A. To enable selective access to genes required for changing metabolic demands **Why:** Packing regulates transcription by physical accessibility; silent regions are condensed, not permanently locked away. Structural support relies on the lamina rather than chromatin state, while replication and ribosome assembly involve distinct, specialised nuclear subdomains. Page: https://tryals.app/practice/biology-i/the-cell-nucleus/chromatin-packing-is-dynamic-rather-than-fixed-why-must-a ### 4. Match each nuclear structure to its function. **Answer:** - Nuclear envelope → A double membrane separating nucleus from cytoplasm - Nuclear pore complex → Controls what crosses the envelope - Nucleolus → Assembles ribosomal subunits - Nuclear lamina → Gives the nucleus mechanical shape and anchors chromatin **Why:** The envelope separates, the pores regulate, the nucleolus manufactures and the lamina supports. Lamin mutations cause progeria, which shows how much depends on the nucleus being mechanically sound. Page: https://tryals.app/practice/biology-i/the-cell-nucleus/match-each-nuclear-structure-to-its-function ### 5. Arrange these levels of DNA packing from least to most condensed. **Answer:** 1. Naked DNA double helix 2. Nucleosomes, the beads-on-a-string arrangement 3. The 30 nm chromatin fibre 4. Looped domains on a protein scaffold 5. The fully condensed mitotic chromosome **Why:** Packing is hierarchical, with each level built from the one below. The whole hierarchy is reversible, because a permanently packed genome would be a permanently silent one. Page: https://tryals.app/practice/biology-i/the-cell-nucleus/arrange-these-levels-of-dna-packing-from-least-to-most-condensed ### 6. Which statements about nuclear transport are correct? A. Small molecules diffuse through pores freely B. The envelope is a single membrane C. Large proteins need a nuclear localisation signal D. Import of large cargo requires energy **Answer:** A. Small molecules diffuse through pores freely; C. Large proteins need a nuclear localisation signal; D. Import of large cargo requires energy **Why:** Small molecules pass freely, large ones need a signal and energy. The envelope is a *double* membrane, and its continuity with the endoplasmic reticulum is what allows newly made membrane protein to reach it. Page: https://tryals.app/practice/biology-i/the-cell-nucleus/which-statements-about-nuclear-transport-are-correct ### 7. Complete the account of chromatin states. **Answer:** Loosely packed chromatin is called **euchromatin** and is actively transcribed, while densely packed chromatin is called **heterochromatin** and is largely **silenced**. A cell can regulate its genes by moving regions between these two states, which means the packing must be **reversible**. **Why:** Reversibility is the essential point: packing is a regulatory mechanism, not just storage. A gene silenced by condensation can be reactivated by decondensing the region, and much of development works this way. Page: https://tryals.app/practice/biology-i/the-cell-nucleus/complete-the-account-of-chromatin-states