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Q11:
What are nuclear pores? State their function.
Solution :
Definition of Nuclear Pores
Nuclear pores are complex, protein-lined channels that span the double-membrane structure of the nuclear envelope. These are not merely passive holes but are large, sophisticated multi-protein assemblies known as Nuclear Pore Complexes (NPCs). Each NPC is composed of approximately 30 different proteins, collectively referred to as nucleoporins, which regulate the traffic between the nucleoplasm and the cytoplasm. [Ref: Cell Biology - Nucleus Structure & Transport Mechanisms]
Structural Dimensions and Characteristics
- Diameter: The pores have an estimated diameter of approximately $80-100\text{ nm}$.
- Composition: They consist of a central scaffold, a cytoplasmic ring with fibrils, and a nuclear ring with a basket-like projection extending into the nucleoplasm.
- Density: The number of pores varies significantly depending on the metabolic activity of the cell; highly active cells (e.g., oocytes) contain thousands, while quiescent cells contain fewer.
Functional Analysis of Nuclear Pores
The nuclear pore complex serves as the primary regulatory gateway for all molecular exchange between the nucleus and the cytoplasm, governed by the following critical functions:
| Function | Mechanism |
|---|---|
| Selective Transport | NPCs utilize active transport mechanisms to selectively move macromolecules such as proteins and RNA. Smaller molecules (ions, water, glucose) move via passive diffusion. |
| Exporting Genetic Transcripts | They facilitate the export of mature $mRNA$, $tRNA$, and ribosomal subunits from the nucleus to the cytoplasm for protein synthesis. [Per Central Dogma of Molecular Biology] |
| Importing Nuclear Proteins | They import proteins synthesized in the cytoplasm (e.g., histones, DNA polymerases, transcription factors) that are required for nuclear function and genetic regulation. |
| Signal-Mediated Gating | Proteins intended for nuclear import carry a Nuclear Localization Signal (NLS), which is recognized by transport receptors (importins), ensuring specificity in compartmentalization. |
Conclusion on Biological Significance
By mediating the nucleocytoplasmic exchange, nuclear pores ensure that the genetic material remains sequestered and protected within the nuclear envelope while maintaining essential communication with the cytoplasmic machinery. This compartmentalization is fundamental to eukaryotic cell viability and gene expression control.
Final Solution: Nuclear pores are multi-protein complexes in the nuclear envelope that function as highly selective, gated channels for the transport of mRNA, ribosomal subunits, proteins, and signaling molecules between the nucleus and the cytoplasm.
More Questions from Class 11 Biology Cell – The Unit of Life EXERCISES
- Q1: Which of the following is not correct?
- Q2: New cells generate from
- Q3: Match the following Column I Column II (a) Cristae (i) Flat membranous sacs in stroma (b) Cisternae (ii) Infoldings in mitochondria (c) Thylakoids (iii) Disc-shaped sacs in Golgi apparatus
- Q4: Which of the following is correct:
- Q5: What is a mesosome in a prokaryotic cell? Mention the functions that it performs.
- Q6: How do neutral solutes move across the plasma membrane? Can the polar molecules also move across it in the same way? If not, then how are these transported across the membrane?
- Q7: Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles? State their functions and draw labelled diagrams of both.
- Q8: What are the characteristics of prokaryotic cells?
- Q9: Multicellular organisms have division of labour. Explain.
- Q10: Cell is the basic unit of life. Discuss in brief.
- Q12: Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.
- Q13(i): Describe the structure of the following with the help of labelled diagrams. (i) Nucleus
- Q13(ii): Describe the structure of the following with the help of labelled diagrams. (ii) Centrosome
- Q14: What is a centromere? How does the position of centromere form the basis of classification of chromosomes. Support your answer with a diagram showing the position of centromere on different types of chromosomes.
CBSE Solutions for Class 11 Biology Cell – The Unit of Life
Chapters in CBSE - Class 11 Biology
- The Living World
- Biological Classification
- Plant Kingdom
- Animal Kingdom
- Morphology of Flowering Plants
- Anatomy of Flowering Plants
- Structural Organisation in Animals
- Biomolecules
- Cell Cycle and Division
- Transport in Plants
- Mineral Nutrition
- Photosynthesis
- Respiration in Plants
- Plant Growth and Development
- Digestion and Absorption
- Breathing and Exchange of Gases
- Body Fluids and Circulation
- Excretory Products and Elimination
- Locomotion and Movement
- Neural Control and Coordination
- Chemical Coordination and Integration
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