PYQ – MZO-001: Molecular Cell Biology (Solved Q&A) | MZO-001 | MSCZOO | M.Sc.Zoology | IGNOU | December 2024
M.Sc. (Zoology) (MSCZOO)
Term-End Examination
December, 2024
MZO-001 : MOLECULAR CELL BIOLOGY
Time : 2 Hours| Maximum Marks : 50
Note: (i) Attempt any five questions.
(ii) All questions carry equal marks.
1. (a) Describe the structure and function of Endoplasmic reticulum and Golgi apparatus. (5 Marks)
The endoplasmic reticulum (ER) and Golgi apparatus are two important membrane-bound organelles found in all eukaryotic cells. They work together in the synthesis, modification, and transport of proteins and lipids.
Endoplasmic Reticulum (ER)
Endoplasmic Reticulum (ER) is divided into two types – Rough ER (RER) and Smooth ER (SER). Rough ER has ribosomes attached to its outer surface which gives it a rough appearance. It plays a key role in the synthesis of proteins, especially those that are secreted, inserted into membranes, or delivered to lysosomes. After synthesis, these proteins are folded and packed into vesicles and sent to the Golgi apparatus. Smooth ER lacks ribosomes and is mainly involved in lipid and steroid synthesis, detoxification of drugs and poisons, and storage of calcium ions, especially in muscle cells. In liver cells, SER helps in metabolism of carbohydrates and detoxification of chemicals.
Golgi Apparatus
Golgi Apparatus is made up of stacked, flattened membrane sacs called cisternae. It has a cis face which receives vesicles from the ER and a trans face from which vesicles exit. The Golgi modifies proteins by adding carbohydrates (glycosylation), phosphates etc. It then sorts and packages them into vesicles for transport to their final destinations like lysosomes, plasma membrane, or for secretion. It also plays a role in the formation of lysosomes and secretion of enzymes and hormones.
(b) Describe the Fuid mosaic model of membrane using a suitable diagram. (5 Marks)
The fluid mosaic model is the most widely accepted explanation for the structural organization of the plasma membrane in all living cells. It was proposed by S.J. Singer and Garth L. Nicolson in 1972. This model explains how membranes are both flexible and functionally active, which is essential for processes like transport, signaling, recognition and cell communication.
According to this model, the membrane is made of a phospholipid bilayer where lipids and proteins are arranged in a dynamic and mosaic-like pattern. The term fluid refers to the lateral movement of lipids and proteins within the layer, which gives the membrane its flexibility. The term mosaic refers to the scattered distribution of different types of proteins that either float freely or are partially or fully embedded in the lipid bilayer.
The main components of the membrane include:
- Phospholipids
- Phospholipids, forming the basic bilayer with hydrophilic heads facing outwards and hydrophobic tails facing inwards, creating a semi-permeable barrier.
- Proteins
- Proteins, which are of two types:
- Integral proteins span the membrane and help in transport and signaling.
- Peripheral proteins are loosely attached and assist in support and enzymatic activity.
- Cholesterol
- Cholesterol, present between phospholipids in animal cells, stabilizes membrane fluidity.
- Carbohydrates
- Carbohydrates, usually bound to proteins (glycoproteins) or lipids (glycolipids), help in cell recognition and signaling.
2. (a) Explain the process of meiosis in oogenesis and spermatogenesis in animals using suitable diagrams. (5 Marks)
Oogenesis and spermatogenesis are the two types of gametogenesis processes that occur in female and male animals respectively. Both involve meiosis, but they differ in timing, process and outcome.
Process of Meiosis in Spermatogenesis
Spermatogenesis takes place in the seminiferous tubules of testes. It begins at puberty and continues throughout life. A diploid spermatogonium undergoes mitosis to form a primary spermatocyte. This primary spermatocyte undergoes meiosis I to form two haploid secondary spermatocytes. Each secondary spermatocyte undergoes meiosis II to form two haploid spermatids, resulting in four spermatids from one primary spermatocyte. These spermatids undergo maturation to form functional spermatozoa. Hence, meiosis in spermatogenesis is continuous and results in four equal-sized sperm cells.
Process of Meiosis in Oogenesis
Oogenesis occurs in the ovaries and starts during fetal development. A diploid oogonium undergoes mitosis to form a primary oocyte, which enters meiosis I but gets arrested at prophase I until puberty. After puberty, during each menstrual cycle, one primary oocyte resumes meiosis I and forms one secondary oocyte and one polar body (small non-functional cell). The secondary oocyte begins meiosis II but arrests at metaphase II. Meiosis II completes only if fertilization occurs, forming one ovum and another polar body. So, oogenesis is discontinuous and produces only one functional ovum and three polar bodies from one oogonium.
(b) Discuss the various applications of animal cell cultare. (5 Marks)
Animal cell culture refers to the process of growing animal cells in artificial conditions outside the body. These cells are maintained in nutrient-rich media under strictly controlled conditions of temperature, pH and oxygen. This technique plays a vital role in many areas of biological research and biotechnology. The wide applications of animal cell culture span from basic research to industrial production, clinical therapy and diagnostics.
There are following major applications of animal cell culture which are:
1. Vaccine Production
Animal cell culture is commonly used for producing vaccines. Viruses are grown in cultured cells to prepare safe and pure vaccines in large quantity. For example, Vero cells were used for Covaxin during COVID-19. Vaccines for polio, rabies, hepatitis B, etc. are also made using this method.
2. Production of Monoclonal Antibodies and Therapeutic Proteins
CHO (Chinese Hamster Ovary) cells and hybridoma cells are used to produce monoclonal antibodies and proteins like insulin and interferons. These are used in treating cancer, infections and other diseases.
3. Drug Testing and Toxicity Studies
New drugs are tested on cultured cells to check their effect and safety. HepG2 cells are used to test liver toxicity. This helps reduce animal usage.
4. Research on Cancer and Genetic Disorders
Cancer cell lines like HeLa and MCF-7 are used to study tumor growth and treatment. Cells with genetic problems help in understanding diseases like muscular dystrophy.
5. Tissue Engineering and Regenerative Medicine
Cultured cells are used to grow tissues like skin and cartilage for replacement or transplant.
6. Virology and Viral Pathogenesis Studies
Cultured animal cells help study virus infection and development of antiviral drugs.
3. (a) Discuss the intrinsic pathway of apoptosis with suitable diagrams. (5 Marks)
The intrinsic pathway of apoptosis, also known as the mitochondrial pathway, is an internal cell death mechanism that plays an essential role in development, immune system regulation and removal of damaged or unwanted cells. It is mainly triggered by internal stress signals such as DNA damage, oxidative stress, nutrient deficiency or withdrawal of growth factors.
This pathway is regulated by Bcl-2 family proteins. When a cell receives death signals, pro-apoptotic members like Bax and Bak get activated and create pores in the mitochondrial outer membrane. This leads to the release of cytochrome c from mitochondria into the cytoplasm. Cytochrome c binds to Apaf-1 and dATP, forming a complex called the apoptosome.
The apoptosome activates procaspase-9, converting it into caspase-9, which then activates downstream executioner caspases such as caspase-3 and caspase-7. These executioner caspases break down various cellular proteins, leading to nuclear condensation, chromatin fragmentation, membrane blebbing and ultimately controlled cell death.
Anti-apoptotic proteins like Bcl-2 and Bcl-XL prevent apoptosis by inhibiting Bax and Bak activation. The final outcome depends on the balance between pro-apoptotic and anti-apoptotic signals inside the cell. Disruption in this balance is linked with cancer, neurodegeneration and autoimmune diseases.
(b) Explain the role of Cadherin in cell-cell adhesions. (5 Marks)
Cadherins are a large family of calcium-dependent transmembrane glycoproteins that play a key role in forming and maintaining cell-to-cell adhesions in animal tissues. These proteins are most active in epithelial, neural and cardiac tissues where strong cell-cell interactions are needed.
Cadherins are not only essential for sticking cells together but also for helping maintain the structural organization of tissues. Here are the three main roles of cadherin in cell-cell adhesion:
1. Strong Cell-Cell Adhesion (Direct Role):
Cadherins form strong and specific adhesive bonds between adjacent cells by homophilic binding (same cadherin type binds with the same cadherin type on another cell). This selective binding creates tight and stable junctions that prevent cells from pulling apart, especially in tissues that undergo mechanical stress, such as skin or cardiac muscles. The strength and specificity of cadherin-mediated adhesion help in maintaining tissue organization.
2. Linking to the Cytoskeleton:
In addition to forming cell-cell adhesions, cadherins are connected to the cytoskeleton, particularly the actin filaments in adherens junctions. This interaction helps cells maintain their shape and resist external mechanical forces. In desmosomes, cadherins link to intermediate filaments like keratin, providing further structural support and resistance to mechanical stress.
3. Maintaining Tissue Architecture and Polarity
Although slightly broader, this function still falls under cell adhesion. By forming stable junctions between neighboring cells, cadherins help in defining tissue boundaries and maintain the polarity of epithelial cells. This ensures that cells know which side is the basal side and which is the apical, which is crucial for tissue functioning. Cadherins also play a role in tissue morphogenesis during embryonic development through organized cell adhesion.
4. Write short notes on the following: (2.5 × 4 = 10 Marks)
(a) Intermediate Filament Associated Proteins (IFAPs)
Intermediate Filament Associated Proteins (IFAPs) are a group of proteins that bind to intermediate filaments (IFs) and help in their organization, stabilization and integration with other cellular components. Intermediate filaments (IFs) are one of the three main components of the cytoskeleton and provide mechanical strength to cells. Intermediate filament associated proteins (IFAPs) connect intermediate filaments to microtubules, actin filaments, plasma membrane and organelles. Examples include plectin, which crosslinks intermediate filaments with microtubules and actin filaments, and filaggrin, which bundles keratin filaments in epithelial cells. Intermediate Filament Associated Proteins (IFAPs) are important in maintaining cell shape, integrity and resilience, especially in cells exposed to mechanical stress like epithelial and muscle cells.
(b) Pinocytosis
Pinocytosis is a type of endocytosis where cells take in extracellular fluid and dissolved substances through small vesicles. It is a non-specific, continuous process and is often called "cell drinking." In this process, the plasma membrane invaginates to form a vesicle that encloses extracellular fluid. This vesicle then fuses with endosomes and lysosomes for digestion and absorption. Pinocytosis is important for nutrient uptake, especially in cells that lack direct access to blood supply, like in early embryos or cells lining the intestine. It also plays a role in maintaining membrane surface area and fluid balance within the cell.
(c) Role of mannose 6-phosphate residues in protein sorting
Mannose 6-phosphate (M6P) residues are key signals for the transport of lysosomal enzymes. In the cis-Golgi, enzymes that are destined for lysosomes receive M6P tags. These tagged proteins are recognized by M6P receptors in the trans-Golgi network, which bind to them and package them into clathrin-coated vesicles. These vesicles are then directed to late endosomes and eventually to lysosomes. Once inside the acidic environment of endosomes, the enzymes dissociate from the receptor. This system ensures that lysosomal enzymes are accurately delivered to lysosomes and not secreted outside the cell. Defects in M6P tagging cause diseases like I-cell disease.
(d) Gap junction
Gap junctions are specialized intercellular connections that allow direct communication between adjacent animal cells. They are made of protein units called connexins, which form channels called connexons. Each connexon aligns with a connexon from a neighboring cell, forming a continuous aqueous channel that permits passage of ions, small metabolites and signaling molecules (up to 1 kDa). These junctions are especially important in cardiac and smooth muscle tissues, where they help in rapid electrical signal transmission. Gap junctions also maintain tissue homeostasis and coordinate cell responses. Dysfunction in gap junctions is linked to heart defects, deafness and neuropathies.
5. Differentiate between the following pair of terms: (2.5 × 4 = 10 Marks)
(a) Excitatory and inhibitory postsynaptic potentials
Excitatory postsynaptic potentials (EPSPs) are small depolarizations in the postsynaptic membrane caused by the inflow of positive ions like sodium (Na⁺) or calcium (Ca²⁺). These bring the membrane potential closer to the threshold and increase the likelihood of action potential generation. They are usually caused by neurotransmitters like glutamate or acetylcholine binding to excitatory receptors.
Inhibitory postsynaptic potentials (IPSPs) are small hyperpolarizations due to the inflow of chloride ions (Cl⁻) or outflow of potassium ions (K⁺), which make the membrane potential more negative. This reduces the chances of firing an action potential. Neurotransmitters like GABA and glycine generate IPSPs. Thus, EPSPs promote neuronal activity while IPSPs suppress it.
(b) G-protein coupled receptors and tyrosine kinase receptors
G-protein coupled receptors (GPCRs) have seven transmembrane domains and interact with G-proteins on the inner surface of the membrane. When a ligand binds, the receptor activates the G-protein, which in turn activates or inhibits downstream effectors like adenylyl cyclase or phospholipase C, leading to the production of second messengers such as cAMP or IP₃. GPCRs are used widely in neurotransmission, vision, smell and hormonal signaling.
Tyrosine kinase receptors (RTKs) have a single transmembrane domain and possess intrinsic kinase activity. Upon ligand binding (often growth factors), the receptors dimerize and autophosphorylate their tyrosine residues in the cytoplasmic region. This creates docking sites for other signaling proteins and activates pathways like MAPK and PI3K-Akt. RTKs are involved in cell growth, differentiation and survival.
(c) Adherent and suspension cell culture
Adherent cultures require a solid surface for cells to attach and grow. They are mostly derived from tissue cells like fibroblasts and epithelial cells. These are ideal for microscopy and morphological studies. They need regular subculturing as cells reach full growth.
Suspension cultures do not require attachment and cells grow freely in the medium. Examples include blood cells and some transformed or cancer cell lines. These cultures are preferred for large-scale industrial processes such as monoclonal antibody production. Stirred-tank bioreactors are commonly used for suspension cultures.
(d) Secretary and endocytic pathways of protein sorting
Secretory pathway handles proteins synthesized in the rough ER. These are packaged into vesicles, transported to the Golgi apparatus for modifications and then delivered to the plasma membrane or secreted out. This includes both constitutive secretion (continuous) and regulated secretion (requires signal like hormones or neurotransmitters).
Endocytic pathway is responsible for internalizing materials from the cell surface. It includes formation of endocytic vesicles, sorting into early endosomes, maturation into late endosomes and fusion with lysosomes for degradation. Receptor-mediated endocytosis helps in nutrient uptake, downregulation of receptors and controlling cell signaling.
6. (a) Describe how ATP is utilized during actin treadmilling. (5 Marks)
Actin treadmilling is a process in which actin filaments (F-actin) maintain a dynamic steady state by adding actin monomers (G-actin) at one end (the plus or barbed end) and losing them from the other end (the minus or pointed end). This is important for cell movement, shape changes and cytoskeletal rearrangement. The process requires ATP at multiple steps because only ATP-bound G-actin can efficiently polymerize and ATP hydrolysis promotes depolymerization at the minus end, which helps in regulating filament stability.
There are five key steps where ATP is used either directly or indirectly:
ATP Binding to G-actin
Before an actin monomer (G-actin) can join a filament, it must first bind ATP. ATP binding changes the shape of the actin monomer, making it ready to polymerize. This ATP-bound G-actin has a higher affinity for the filament's plus end, enabling efficient addition.
This is the first direct use of ATP because ATP is essential for preparing the monomer for filament growth.
2. Polymerization of ATP-Actin at the Plus End (Barbed end)
The ATP-bound actin monomers add to the plus end (barbed end) of the filament, causing the filament to grow. ATP-actin subunits fit well at this end and create a stable structure.
This step directly uses ATP since only ATP-actin can efficiently polymerize. This polymerization is energy-dependent and essential for filament elongation.
3. ATP Hydrolysis Within the Filament
Once the ATP-actin is incorporated into the filament, the bound ATP is hydrolyzed to ADP and inorganic phosphate (Pi). This hydrolysis weakens the binding between actin subunits inside the filament and makes the filament more dynamic. Hydrolysis does not cause immediate loss but the filament can easily depolymerize at the minus end.
This is the third direct ATP-related step as ATP is chemically broken down inside the filament.
4. Disassembly of ADP-Actin at the Minus End (Pointed end)
At the minus end, actin subunits bound to ADP are less stable and dissociate more easily from the filament. This causes the filament to shrink at this end while growing at the plus end. The disassembly is indirectly controlled by ATP hydrolysis since the ADP state promotes subunit loss.
This is the fourth direct effect linked to ATP use in the treadmilling process.
5. Nucleotide Exchange on Actin Monomers by Profilin
After dissociation, ADP-actin monomers are inactive and cannot polymerize immediately. Profilin helps these monomers release ADP and bind fresh ATP, regenerating ATP-actin monomers ready for a new round of polymerization.
This step uses ATP indirectly because it restores the active form of actin for filament growth.
(b) How do the cell organelles maintain their optimum pH? (5 Marks)
Different cell organelles perform specific biochemical functions that work only at a particular pH level. For example, lysosomes need an acidic environment to activate hydrolytic enzymes, while the nucleus and cytoplasm function around neutral pH. Maintaining this pH is necessary to avoid enzyme inactivation or damage to cell processes. Cells use ATP-driven proton pumps, ion exchangers or channels and buffering systems to regulate the internal pH of each organelle.
Here are the different organelles that use different types of systems to regulate their internal pH:
Lysosomes and Endosomes
They maintain an acidic pH around 4.5 to 5.5 using V-type H⁺-ATPases. These are ATP-driven proton pumps that directly transport H⁺ ions into the lumen. To balance the charge difference, chloride ion channels allow Cl⁻ ions to enter, which supports the acidification.
Golgi Apparatus
The pH here is slightly acidic, around 6.0. It also uses proton pumps but works at a lower rate compared to lysosomes. This pH is required for enzyme activities like glycosylation.
Mitochondria
Here, pH regulation happens indirectly during oxidative phosphorylation. The electron transport chain pumps protons across the inner membrane into the intermembrane space, creating a proton gradient. The matrix remains alkaline and this gradient is used to make ATP, not consume it.
Endoplasmic Reticulum (ER)
The ER maintains a neutral pH using ion exchangers like Na⁺/H⁺ antiporters, along with leak channels. It does not use ATP-driven proton pumps but instead relies on passive ion flow and buffering systems, especially bicarbonate.
Cytoplasm and Peroxisomes
These maintain pH through buffer systems like phosphate and bicarbonate, and by regulated movement of ions across membranes. No direct ATP usage is involved here.
7. (a) Give a brief account of lysosomal proteolysis. (5 Marks)
Lysosomal proteolysis is the process by which proteins are broken down inside lysosomes. Lysosomes are membrane-bound organelles that contain hydrolytic enzymes like cathepsins, which become active only in acidic pH (around 4.5 to 5). This low pH is maintained by ATP-dependent proton pumps (V-type H⁺-ATPase). The function of lysosomal proteolysis is to recycle damaged or unneeded proteins, remove waste and maintain cellular homeostasis. It acts not only on internal cellular components but also on materials taken up from outside the cell.
There are three main pathways through which proteins are targeted to lysosomes for degradation.
1. Endocytosis Pathway
This pathway is used to degrade proteins that come from outside the cell. These proteins bind to receptors on the plasma membrane and enter the cell via receptor-mediated endocytosis or other endocytic processes. The protein-containing vesicles first form early endosomes, which later mature into late endosomes. These late endosomes finally fuse with lysosomes, where the proteins are digested. This pathway is important for nutrient uptake, receptor turnover and clearing extracellular debris.
2. Autophagy Pathway
Autophagy mainly degrades intracellular components, especially during stress, starvation, or when organelles get damaged. In macroautophagy, the cell forms a double-membrane structure called an autophagosome, which surrounds the target material (like damaged mitochondria or aggregated proteins). This autophagosome then fuses with a lysosome, where its contents are broken down. Other forms include microautophagy, where lysosomes directly engulf small cytoplasmic parts and selective autophagy, where specific targets are degraded.
3. Chaperone-Mediated Autophagy (CMA)
CMA is a highly selective pathway that degrades specific cytosolic proteins. These proteins must have a special amino acid sequence called a KFERQ motif. This motif is recognized by a chaperone protein, mainly Hsc70, which guides the protein to the lysosome. There, it binds to the LAMP-2A receptor, gets unfolded and is transported directly into the lysosomal lumen for degradation. This pathway becomes more active during oxidative stress and prolonged nutrient deficiency.
(b) Describe any five diseases resulting from mutations in intermediate filament genes. (5 Marks)
Intermediate filaments (IFs) are important cytoskeletal elements that provide strength and support to cells. When genes that code for intermediate filament proteins get mutated, they disrupt cellular integrity and cause dysfunction in tissues like skin, muscles and nerves.
Here are five well-known diseases caused by mutations in intermediate filament genes:
1. Epidermolysis Bullosa Simplex (EBS):
This condition is caused by mutations in KRT5 or KRT14, which code for keratins in skin cells. These mutations make the skin fragile and easily blistered due to minor mechanical stress. The basal cells of the epidermis become weak and separate from each other.
2. Alexander Disease:
This is a rare brain disorder caused by mutations in GFAP (Glial Fibrillary Acidic Protein), which is found in astrocytes. The mutated protein forms clumps called Rosenthal fibers that disturb normal brain function. Patients usually show delayed development, seizures and enlargement of the head.
3. Amyotrophic Lateral Sclerosis (ALS):
Some familial cases of ALS are linked to mutations in the NEFH (Neurofilament Heavy Polypeptide) gene. This gene makes the heavy chain of neurofilaments. Mutation leads to protein buildup in motor neurons, blocking axonal transport and causing muscle weakness.
4. Dilated Cardiomyopathy:
This condition is caused by mutations in DES, the gene for desmin found in heart muscles. It leads to misaligned muscle fibers and weak heart contractions, eventually resulting in heart failure.
5. Charcot–Marie–Tooth Disease Type 2E (CMT2E):
This is a nerve disorder caused by mutations in NEFL (Neurofilament Light Polypeptide), which affects neurofilament light chain. It leads to nerve damage, causing muscle weakness and sensory loss in limbs.
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