Question 1(a) [2 Marks]

Write two features of eukaryotic cell.

• Presence of a true nucleus: The genetic material is enclosed within a well-defined nuclear envelope.
• Membrane-bound organelles: They contain specialized structures like mitochondria, endoplasmic reticulum, and Golgi bodies.

Question 1(b) [2 Marks]

What is archaeobacteria? Give example of one eubacteria.

Archaebacteria: A group of microorganisms that are considered to be some of the oldest living forms on Earth, often found in extreme environments like hot springs and salt lakes. They lack a peptidoglycan cell wall.

Example of Eubacteria: Escherichia coli (E. coli).

Question 1(c) [2 Marks]

Write two ecological adaptation of cell in high altitude.

• Increased concentration of hemoglobin: To facilitate better oxygen transport in low-pressure environments.
• Increased number of mitochondria: To maximize energy production efficiency under hypoxic conditions.

Question 2 [10 Marks]

Write short notes on: (a) Prions (b) Viroids

(a) Prions (5 Marks)

Prions are infectious agents composed entirely of protein material. Unlike viruses or bacteria, they do not contain nucleic acids (DNA or RNA).

• Nature: They are misfolded forms of normal cellular proteins (PrP).
• Mechanism: When a prion enters a healthy organism, it induces existing, properly folded proteins to convert into the disease-associated, misfolded conformation.
• Diseases: They cause neurodegenerative diseases such as Mad Cow Disease (BSE) and Creutzfeldt-Jakob Disease (CJD) in humans.

(b) Viroids (5 Marks)

Viroids are the smallest known infectious pathogens, consisting solely of a short strand of circular, single-stranded RNA.

• Structure: They lack a protein coat (capsid) which distinguishes them from viruses.
• Host: They primarily infect higher plants.
• Function: They do not code for any proteins but replicate using the host cell's RNA polymerase, often interfering with the host's regulatory RNAs.
• Example: Potato spindle tuber viroid (PSTVd).

Question 2 [OR] Option A [5 Marks]

"Cell is the basic unit of life." Explain.

This statement is the core of Cell Theory. It implies:

• Structural Unit: Every living organism is composed of one or more cells. It is the smallest structure capable of performing all life functions.
• Functional Unit: All metabolic processes (energy conversion, chemical synthesis) occur within the cell.
• Reproductive Unit: New cells arise only from pre-existing cells through division.

Question 2 [OR] Option B [5 Marks]

Write about the cellular structure of mycoplasma.

Mycoplasmas are the smallest independent living organisms known.

• Cell Wall: They characteristically lack a cell wall, making them naturally resistant to antibiotics like penicillin.
• Plasma Membrane: Contains sterols (like cholesterol) which provide stability and flexibility.
• Genetic Material: Consists of a double-stranded DNA molecule and ribosomes for protein synthesis.
• Shape: Due to the lack of a wall, they are pleomorphic (can change shape).

Question 3(a) [2 Marks]

What is cell signalling?

Cell Signalling: The complex communication process that governs basic cellular activities and coordinates cell actions through the perception and response to environmental stimuli.

Question 3(b) [2 Marks]

Who proposed fluid mosaic model of plasma membrane?

The model was proposed by S.J. Singer and G.L. Nicolson in 1972.

Question 3(c) [2 Marks]

Write two functions of lysosomes.

• Intracellular Digestion: Breaking down macromolecules like proteins, lipids, and carbohydrates.
• Autophagy: Digestion of worn-out cellular organelles to recycle components.

Question 4 [10 Marks]

With the help of suitable diagram, describe the structure of mitochondria. Add a note on the importance of mitochondria.

Structure (6 Marks):

• Outer Membrane: Smooth and contains porins that allow small molecules to pass.
• Inner Membrane: Thrown into numerous folds called Cristae to increase surface area.
• Matrix: The central fluid-filled space containing enzymes, DNA, and ribosomes.
• Intermembrane Space: The area between the inner and outer membranes.

Importance (4 Marks):

• ATP Production: Known as the "Powerhouse of the Cell," they generate energy via oxidative phosphorylation.
• Apoptosis: They play a critical role in programmed cell death.
• Calcium Storage: They help maintain the concentration of calcium ions within the cell.

Question 4 [OR] Option A [5 Marks]

Fluid mosaic model

The fluid mosaic model describes the plasma membrane as a tapestry of several types of molecules (phospholipids, cholesterols, and proteins) that are constantly moving. This movement helps the cell membrane maintain its role as a barrier between the inside and outside of the cell environments.

Question 4 [OR] Option B [5 Marks]

Endoplasmic reticulum

The ER is a network of membranous tubules and sacs called cisternae.

• Rough ER: Studded with ribosomes; involved in protein synthesis and transport.
• Smooth ER: Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

Question 5(a) [2 Marks]

Write the functions of rRNA and mRNA.

• mRNA (Messenger RNA): Carries genetic information from DNA to the ribosomes for protein synthesis.
• rRNA (Ribosomal RNA): Forms the structural and catalytic core of ribosomes.

Question 5(b) [2 Marks]

Write the chemical composition of nucleic acids.

Nucleic acids are composed of Nucleotides, each consisting of:

1. A Pentose Sugar (Ribose or Deoxyribose).
2. A Phosphate Group.
3. A Nitrogenous Base (Adenine, Guanine, Cytosine, and Thymine/Uracil).

Question 5(c) [2 Marks]

Write the functions of DNA polymerase.

• Polymerization: Adding nucleotides to the growing DNA strand in the 5' to 3' direction.
• Proofreading: Correcting errors during replication to maintain genetic integrity.

Question 6 [10 Marks]

With the help of suitable model, describe the process of DNA replication in prokaryotes. What are lagging and leading strands?

DNA Replication Process (8 Marks):

1. Initiation: Starts at the Origin of Replication (OriC). Helicase unwinds the double helix.
2. Elongation: Primase adds RNA primers. DNA Polymerase III synthesizes new DNA by adding complementary nucleotides.
3. Termination: Occurs when replication forks meet or at specific termination sequences.

Strands (2 Marks):

• Leading Strand: Synthesized continuously in the 5' to 3' direction toward the replication fork.
• Lagging Strand: Synthesized discontinuously in short segments called Okazaki fragments away from the fork.

Question 6 [OR] Option A [5 Marks]

Give a comparative elaboration of A, B and Z forms of DNA.

Question 6 [OR] Option B [5 Marks]

Describe the double helical structure of DNA with the help of a diagram.

Proposed by Watson and Crick:

• Two polynucleotide chains coiled in a right-handed screw.
• Backbone: Sugar-Phosphate.
• Base Pairing: A pairs with T (2 H-bonds), G pairs with C (3 H-bonds).
• Antiparallel: One strand runs 5' to 3', the other 3' to 5'.

Question 7(a) [2 Marks]

What is kinetochore? What is synapse?

• Kinetochore: A protein structure on chromatids where the spindle fibers attach during cell division.
• Synapse: The pairing of two homologous chromosomes that occurs during meiosis.

Question 7(b) [2 Marks]

What are the check-point of cell cycle?

The three main checkpoints are:

1. G1 Checkpoint: Ensures cell is ready for DNA synthesis.
2. G2 Checkpoint: Ensures DNA replication is complete and undamaged.
3. M Checkpoint (Spindle Checkpoint): Ensures chromosomes are attached to spindles.

Question 7(c) [2 Marks]

Write two functions of centromere.

• Attachment: Provides the site for kinetochore formation.
• Segregation: Ensures the equal distribution of genetic material to daughter cells.

Question 8 [10 Marks]

Write notes on the following: (a) Cell senescence (b) Apoptosis

(a) Cell senescence (5 Marks)

The process by which a cell loses its ability to divide and grow, often due to age or DNA damage. It acts as a protective mechanism against cancer by stopping the division of damaged cells.

(b) Apoptosis (5 Marks)

Often called "Programmed Cell Death." It is a highly regulated process where the cell essentially commits suicide to benefit the whole organism (e.g., removing infected cells or shaping fingers during embryonic development).

Question 8 [OR] [10 Marks]

With the help of suitable diagram, describe the meiotic cell division. Differentiate between mitotic and meiotic cell divisions.

Meiosis (7 Marks):

A two-step division (Meiosis I and II) that reduces chromosome number by half, resulting in four haploid daughter cells. Key phases include Prophase I (crossing over), Metaphase I, Anaphase I, and Telophase I.

Differences (3 Marks):

Question 9(a) [2 Marks]

What do you mean stationary phase in chromatography? What is Rf value?

• Stationary Phase: The substance that stays fixed inside the chromatography column or on the plate.
• Rf Value: Retention factor; the ratio of the distance traveled by the substance to the distance traveled by the solvent.

Question 9(b) [2 Marks]

What do you mean by centrifugation?

Centrifugation: A technique used for the separation of particles from a solution according to their size, shape, density, and viscosity using centrifugal force.

Question 9(c) [2 Marks]

What are the basic differences between SEM and TEM?

• SEM (Scanning Electron Microscope): Scans the surface to provide a 3D image.
• TEM (Transmission Electron Microscope): Passes through a thin section to provide detailed 2D internal structures.

Question 10 [10 Marks]

Write about the principle of chromatography. Add a note on TLC.

Principle (2 Marks):

Chromatography is based on the principle of differential distribution of components of a mixture between two phases: a stationary phase and a mobile phase.

Thin Layer Chromatography (TLC) (8 Marks):

• Technique: Uses a glass or plastic plate coated with a thin layer of adsorbent (silica gel).
• Procedure: Sample is spotted, plate is placed in solvent, and components separate as the solvent rises.
• Applications: Identifying purity of compounds, monitoring reaction progress.

Question 10 [OR] Option A [5 Marks]

Autoradiography and its applications

A technique using X-ray film to visualize the distribution of radioactive substances in biological samples.

Applications: Tracking DNA replication, metabolic pathways, and drug localization.

Question 10 [OR] Option B [5 Marks]

Fluorescence microscopy

Uses fluorescence to study properties of organic or inorganic substances. It specifically targets fluorescently labeled molecules within the cell to observe specific structures like the nucleus or cytoskeleton.