Que. 1. a) Write a brief note on the origins of the chloroplast and mitochondria.
Ans- The origins of chloroplasts and mitochondria are explained by the endosymbiotic theory, which suggests that these organelles evolved from free-living prokaryotic cells that were engulfed by a primitive eukaryotic cell.
- Mitochondria are believed to have originated from an aerobic (oxygen-using) proteobacterium that was engulfed by an ancestral eukaryotic cell. This symbiotic relationship provided the host cell with efficient ATP production, giving it an evolutionary advantage.
- Chloroplasts are thought to have evolved from an oxygenic photosynthetic cyanobacterium that was similarly engulfed by a eukaryotic cell, allowing it to perform photosynthesis.
Que. 1. B) Describe the biological significance of the cytoskeleton.
The cytoskeleton is a dynamic network of protein filaments that provides structural support, shape, and organization to cells. It plays a crucial role in various cellular processes, making it biologically significant in multiple ways:
- Structural Support and Shape – The cytoskeleton maintains the shape of the cell, providing mechanical support and preventing deformation.
- Intracellular Transport – It acts as a network for transporting organelles, vesicles, and molecules within the cell, facilitated by motor proteins like kinesin and dynein.
- Cell Division – The cytoskeleton is essential for mitosis and meiosis. Microtubules form the mitotic spindle, ensuring accurate chromosome segregation.
- Cell Motility – It enables cell movement through structures like cilia, flagella, and pseudopodia, crucial for processes like immune response and embryonic development.
- Endocytosis and Exocytosis – The cytoskeleton regulates the uptake and release of materials, playing a role in nutrient absorption and neurotransmitter release.
- Signal Transduction – It helps in transmitting signals from the extracellular environment to the cell interior, influencing cell behavior, division, and response to stimuli.
- Tissue Integrity and Connectivity – In multicellular organisms, cytoskeletal components link cells together and contribute to tissue formation, stability, and function.
Que. 2. a) Briefly discuss the structure of intermediate filaments.
Intermediate filaments are a key component of the cytoskeleton, providing structural support and mechanical stability to cells. They have a rope-like structure composed of elongated fibrous proteins. Their assembly follows a hierarchical process:
- Monomers – Composed of a central α-helical rod domain flanked by variable N-terminal (head) and C-terminal (tail) domains.
- Dimers – Two monomers coil around each other to form a parallel coiled-coil dimer.
- Tetramers – Two dimers align in an antiparallel, staggered manner to form a tetramer.
- Protofilaments and Protofibrils – Tetramers further associate laterally and longitudinally into protofilaments and then into thicker protofibrils.
- Final Filament – Multiple protofibrils assemble into a mature intermediate filament, typically around 10 nm in diameter.
Que. 2. b) How is the MAPK pathway activated? Explain.
The MAPK (Mitogen-Activated Protein Kinase) pathway is a key signaling cascade involved in cell growth, differentiation, survival, and apoptosis. It is activated by various extracellular stimuli, such as growth factors, cytokines, and stress signals. The activation process occurs through a highly conserved three-tiered kinase cascade.
Steps of MAPK Pathway Activation:
- Extracellular Signal Binding
- A ligand, such as epidermal growth factor (EGF) or platelet-derived growth factor (PDGF), binds to its specific receptor tyrosine kinase (RTK) on the cell membrane.
- This binding leads to dimerization and autophosphorylation of the receptor, creating docking sites for adaptor proteins.
- Recruitment of Adaptor Proteins
- The phosphorylated receptor recruits adaptor proteins such as Grb2 (Growth factor receptor-bound protein 2) and Sos (Son of Sevenless).
- These adaptor proteins facilitate the exchange of GDP for GTP on Ras, a small GTPase, thereby activating it.
- Activation of MAPK Kinase Cascade
- Ras-GTP activates Raf (MAPKKK or MAP3K), a serine/threonine kinase.
- Raf phosphorylates and activates MEK (MAPKK or MAP2K).
- MEK then phosphorylates and activates ERK (MAPK).
- ERK Translocation & Cellular Responses
- Activated ERK translocates to the nucleus and phosphorylates various transcription factors (e.g., c-Myc, Elk-1, Fos, Jun).
- This leads to changes in gene expression, promoting cell proliferation, differentiation, or survival depending on the context.
Regulation and Feedback Mechanisms
- Negative feedback: Activated ERK can phosphorylate upstream components, such as Raf, leading to pathway inhibition.
- Phosphatases (e.g., MKPs – MAP kinase phosphatases): Dephosphorylate MAPKs to terminate signaling.
- Scaffold Proteins (e.g., KSR – Kinase Suppressor of Ras): Organize components to ensure pathway specificity and efficiency.
3. Write difference between following pairs:
i) Centrosomes and Centrioles
Centrosome
- The centrosome is an organelle found in animal cells that functions as the main microtubule-organizing center (MTOC).
- It plays a crucial role in organizing the microtubule network, particularly during cell division.
- Typically located near the nucleus, the centrosome is responsible for the formation of the mitotic spindle during mitosis.
- It consists of two centrioles surrounded by a dense matrix called the pericentriolar material (PCM), which contains proteins essential for microtubule nucleation and organization.
Centrioles
- Helping in the organization of microtubules in interphase cells.
- Centrioles are cylindrical structures composed of microtubules arranged in a 9+0 triplet pattern (nine sets of microtubule triplets arranged in a circle).
- A pair of centrioles is found within the centrosome, positioned perpendicular to each other.
- Functions of centrioles include:
- Organizing the mitotic spindle during cell division.
- Aiding in the formation of cilia and flagella (by giving rise to basal bodies).
ii) Intrinsic and extrinsic apoptotic pathways
1. Intrinsic (Mitochondrial) Pathway:
- Initiated inside the cell in response to internal stress signals like DNA damage, oxidative stress, or lack of survival factors.
- Controlled by Bcl-2 family proteins, which regulate mitochondrial membrane permeability.
- Pro-apoptotic proteins (e.g., Bax, Bak) promote mitochondrial outer membrane permeabilization (MOMP).
- This releases cytochrome c into the cytoplasm.
- Cytochrome c binds to Apaf-1, forming the apoptosome, which activates caspase-9.
- Caspase-9 activates executioner caspases (e.g., caspase-3, caspase-7), leading to cell death.
2. Extrinsic (Death Receptor) Pathway:
- In some cells, caspase-8 can activate Bid, a pro-apoptotic Bcl-2 protein, linking the extrinsic pathway to the intrinsic pathway.
- Triggered by external signals, such as binding of death ligands (e.g., FasL, TNF-α, TRAIL) to death receptors (e.g., Fas/CD95, TNFR1) on the cell membrane.
- Death receptor activation recruits FADD (Fas-associated death domain) and pro-caspase-8, forming the death-inducing signaling complex (DISC).
- Caspase-8 is activated and directly activates executioner caspases (caspase-3, caspase-7), leading to apoptosis.