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Q13(i):
Describe the structure of the following with the help of labelled diagrams.
(i) Nucleus
Solution :
Initial Structural Definition: The Nucleus
The nucleus is a specialized, membrane-bound organelle that serves as the administrative center and genetic repository of the eukaryotic cell. It is responsible for orchestrating cellular activities, including growth, intermediary metabolism, protein synthesis, and reproduction (cell division) through the regulation of gene expression.
Step 1: Component Analysis
The structure of the nucleus comprises several distinct functional components:
- Nuclear Envelope: A double-membrane structure enclosing the nuclear content. The outer membrane is often continuous with the Rough Endoplasmic Reticulum (RER).
- Nuclear Pores: Perforations in the envelope that allow for the regulated movement of molecules (e.g., mRNA, ribosomal subunits) between the nucleoplasm and cytoplasm [Per Nucleocytoplasmic Transport Theory].
- Nucleoplasm (Nuclear Matrix): A dense, protein-rich gel containing chromatin, enzymes, and nucleotides.
- Nucleolus: A non-membrane-bound spherical body that serves as the primary site for rRNA transcription and ribosomal assembly.
- Chromatin: A complex of DNA and histone proteins which condenses into chromosomes during prophase [Per Chromatin Condensation Model].
Step 2: Visual Representation
Step 3: Functional Summary Table
| Component | Primary Function |
|---|---|
| Nuclear Envelope | Maintains homeostasis of the nuclear environment; acts as a selective barrier. |
| Nucleolus | Synthesis of $18S$, $5.8S$, and $28S$ rRNA [Per Ribosome Biogenesis Pathway]. |
| Nucleoplasm | Medium for DNA replication and RNA transcription enzymes. |
| Nuclear Pore Complex | Regulated traffic of macromolecules (proteins and RNAs) via NLS (Nuclear Localization Signals). |
Step 4: Theoretical Justification
The nucleus functions according to the Central Dogma of Molecular Biology: $DNA \rightarrow RNA \rightarrow Protein$. By compartmentalizing the genome, the eukaryotic cell protects the DNA from cytoplasmic metabolic byproducts and facilitates the spatial separation of transcription (in the nucleus) and translation (in the cytoplasm) [Per Eukaryotic Cellular Organization Principles].
Final Solution: The nucleus is a double-membrane-bound organelle comprising the nuclear envelope with pores, nucleoplasm, nucleolus, and chromatin, serving as the regulatory hub for genetic material and cellular metabolic control.
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.
- Q11: What are nuclear pores? State their function.
- Q12: Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.
- 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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