Continuity of Life: WBBSE Class 10 Life Science Chapter 2 Solutions & Question Bank

Explanation: Human somatic cells are diploid ($2n = 46$), whereas gametes (sperm and ovum) are haploid ($n = 23$) formed through meiosis reduction division.

Explanation: Amitosis is called direct cell division because the nucleus elongates, constricts, and pinches directly into two daughter nuclei without spindle apparatus formation or chromosome condensation.

Explanation: Meiosis (specifically Meiosis-I) reduces the maternal and paternal chromosome number to half ($2n \rightarrow n$) in daughter cells, earning the designation of reductional division.

Explanation: At late prophase, the nuclear envelope, nucleolus, Golgi complex, and endoplasmic reticulum disintegrate and disappear, allowing spindle fibres to interact with chromosomes.

Explanation: Telophase is the reverse of prophase; chromosome groups uncoil at opposite poles, and the nuclear envelope reforms around each cluster.

Explanation: One diploid mother cell undergoes two successive nuclear divisions (Meiosis I and Meiosis II) with only a single DNA replication cycle, producing four haploid daughter cells.

Explanation: During anaphase, centromeres split longitudinally, and chromosomal traction fibres contract, pulling sister chromatids toward opposite spindle poles.

Explanation: In a metacentric chromosome, the centromere lies at the exact median centre, forming two equal-length arms that appear 'V'-shaped during anaphase migration.

Explanation: A telocentric chromosome has its centromere located at the extreme tip (telomere end), exhibiting an 'I'-shaped rod profile.

Explanation: A gene is a functional, heritable physical unit of heredity consisting of a specific sequence of nucleotides within a DNA molecule.

Explanation: Out of 46 chromosomes in somatic cells, 44 (22 pairs) are autosomes regulating somatic traits, while 2 (1 pair) are allosomes (sex chromosomes).

Explanation: A knob-like terminal chromosomal body called a satellite (trabant) is located beyond the secondary constriction (nucleolar organizer region, NOR).

Explanation: Non-histone acidic proteins in chromosomes are rich in tryptophan and tyrosine, whereas basic histone proteins are rich in lysine and arginine.

Explanation: In sexual reproduction, meiosis is required to produce haploid gametes, and mitosis is required for subsequent zygotic cleavage, embryo development, and organismal growth.

Explanation: Mitosis maintains an identical chromosome number and genetic constitution between mother and daughter cells ($2n \rightarrow 2n$ or $n \rightarrow n$), hence known as equational division.

Explanation: The filamentous green alga Spirogyra reproduces asexually by fragmentation, where accidental filament breakage yields fragments that grow into new filaments via mitosis.

Explanation: Horticultural artificial vegetative propagation through grafting (joining scion and stock) is widely used in fruit trees such as Guava, Mango, Apple, and Citrus.

Explanation: The calyx is the outermost floral whorl, and its individual free or united green leaf-like segments are called sepals.

Explanation: In Mirabilis jalapa (Four O'clock plant), bisexual flowers readily undergo autogamy (self-pollination) through spontaneous bending of stamens onto the stigma.

Explanation: Maize (Zea mays) is a classic wind-pollinated (anemophilous) plant possessing dry, lightweight pollen and feathery silk stigmas.

Explanation: Shimul (Bombax ceiba, Red Silk Cotton) bears large, bright red, nectar-rich flowers pollinated primarily by birds (ornithophily).

Explanation: Human developmental milestones categorize birth up to 1-2 years as infancy, followed by childhood spanning from approx. 10 months/1 year to about 12-13 years before puberty begins.

Explanation: Human females are homogametic, bearing two identical X chromosomes ($44A + XX$), while males are heterogametic ($44A + XY$).

Explanation: In anaphase, centromeres split simultaneously, allowing the two sister chromatids of each chromosome to disjoin and migrate toward opposite poles.

Explanation: Unicellular fungus Yeast undergoes asexual reproduction predominantly through unequal external outgrowth called budding (torulation).

Explanation: During adolescence (puberty), sex hormones trigger secondary sexual characteristics and functional maturation of gonads (testes and ovaries).

Explanation: Uracil ($U$) is found exclusively in RNA in place of Thymine ($T$). DNA comprises Deoxyribose, Phosphoric acid, Adenine, Guanine, Cytosine, and Thymine.

Explanation: Spirogyra reliably reproduces asexually via fragmentation. (Hydra reproduces by budding, Planaria by true regeneration).

Explanation: Because cross-pollination relies on external abiotic/biotic carriers, a large quantity of pollen grains is inevitably wasted during transfer.

Explanation: In a newly divided daughter cell (during $G_1$ phase), each unreplicated chromosome consists of exactly one single, continuous, double-stranded DNA molecule highly coiled around histone octamers.

A chromosome is a highly condensed, organized nuclear thread-like structure composed of a single long double-stranded DNA molecule associated with basic histone and non-histone proteins, responsible for carrying hereditary genes from generation to generation.

A gene is the fundamental functional and physical unit of heredity. Structurally, it is a defined segment of DNA occupying a fixed chromosomal locus that encodes the synthesis of a specific polypeptide chain or functional RNA.

Autosomes are chromosomes that regulate general somatic (body) morphology and physiological functions, but do not play a primary role in determining biological sex. Human diploid cells possess 44 autosomes (22 pairs).

Sex chromosomes (Allosomes or Heterosomes) are the specialized chromosomes that directly determine the biological sex of an organism. In humans, they comprise 1 pair: XX in females and XY in males.

During prophase in animal cells, the centrosome duplicates and migrates to opposite cell poles. Centrioles organize tubulin monomers to form astral rays and the mitotic spindle apparatus, which aligns and separates chromosomes during anaphase.

Mitosis is the type of eukaryotic equational cell division in which a diploid or haploid parent cell divides once to yield two genetically identical daughter cells with the exact same chromosome number and qualitative characteristics as the parent cell.

• Growth and Development: Multiplies cell count from the single-celled zygote into a multicellular organism.
• Tissue Regeneration & Repair: Replaces worn-out, injured, or dead cells (e.g., wound healing, skin epidermis, and RBC turnover).

Meiosis is a specialized reductional cell division in sexually reproducing organisms wherein a diploid germ mother cell ($2n$) undergoes two sequential nuclear divisions with one round of replication to produce four genetically diverse haploid daughter cells ($n$).

• Chromosome Number Constancy: Halves the chromosome count in gametes, ensuring the species-specific diploid number is preserved upon subsequent fertilization.
• Genetic Variation: Crossing over (pachytene) and random independent assortment generate novel gene combinations, driving organic evolution.

• Equatorial Alignment: Chromosomes attain maximal coiling, condense to their shortest lengths, and align along the equatorial plane (metaphase plate).
• Kinetochore Attachment: Spindle chromosomal fibres attach firmly to the proteinaceous kinetochores located on both sides of each centromere.

• Centromere Splitting: Each centromere splits longitudinally, converting paired sister chromatids into independent daughter chromosomes.
• Poleward Movement: Contracting spindle fibres haul daughter chromosomes toward opposite poles, assuming characteristic V, L, J, or I shapes according to centromere position.

Cytokinesis is the physical division of the parent cell's cytoplasm into two distinct daughter cells following karyokinesis. It occurs by centripetal cleavage furrowing in animal cells and centrifugal cell plate formation via phragmoplast in plant cells.

A bivalent (or tetrad) is a physical pairing complex of two homologous chromosomes, each containing two sister chromatids (total 4 chromatids), formed during the zygotene stage of prophase-I through synapsis.

Euchromatin is the genetically active, loosely coiled, lightly staining region of chromatin during interphase that is rich in active genes and undergoes early replication during the S phase.

Heterochromatin is the tightly condensed, deeply staining portion of chromatin during interphase. It consists mostly of transcriptionally inactive, non-coding repetitive DNA sequences that undergo late replication.

Asexual reproduction is a uniparental mode of reproduction wherein new individuals arise from somatic cells or mitotic spore units without the formation or fusion of gametes, resulting in genetically identical clones.

Sexual reproduction is a biparental (or hermaphroditic) biological process involving the formation of haploid male and female gametes through meiosis and their subsequent amphimixis (fertilization) to produce a genetically diverse diploid zygote.

Cutting is an artificial vegetative propagation technique in which a vegetative portion of a plant (stem, root, or leaf with axillary buds) is excised and placed in moist soil or treated with rooting hormones (e.g., IBA) to regenerate adventitious roots and shoot systems (e.g., Rose, Sugarcane).

Grafting is an artificial method where vegetative parts of two separate plants are joined so their cambium tissues unite and grow as a composite plant. The rooted lower portion is called stock, and the shoot portion of high economic yield is termed scion (e.g., Mango, Apple).

Micropropagation is the modern in vitro tissue culture technique of rapidly propagating plants using microscopic explants (shoot apical meristems, cells) under sterile aseptic conditions on a synthetic nutrient medium enriched with auxins and cytokinins.

Alternation of generations (Metagenesis) is the regular cyclic alternation between the sexual haploid gametophyte ($n$) phase and the asexual diploid sporophyte ($2n$) phase throughout the biological life cycle of an organism, as seen in ferns and bryophytes.

Pollination is the biological transfer of pollen grains from the ripe anther of a stamen to the receptive stigma of a carpel in gymnosperms and angiosperms.

Self-pollination is the transfer of pollen grains from an anther to the stigma of either the same flower (autogamy) or another flower on the exact same plant (geitonogamy), such as in Pea and Mirabilis.

Cross-pollination (Xenogamy / Allogamy) is the transfer of pollen grains from the anther of one flower to the stigma of another flower situated on a genetically distinct plant of the same species, requiring external biotic or abiotic agents.

• Orderly Genetic Duplication: Ensures precise replication of DNA and distribution of chromosomes to maintain species stability.
• Growth Regulation & Quality Control: Checkpoints prevent propagation of mutated cells; loss of cycle regulation leads to malignant tumors.

• Paddy (Oryza sativa): Wind (Anemophily).
• Hydrilla (Hydrilla verticillata): Water (Hydrophily).
• Shimul (Bombax ceiba): Birds (Ornithophily).
• Mango (Mangifera indica): Insects / Flies (Entomophily).

• Vision Changes: Loss of eye lens elasticity causing presbyopia (inability to focus nearby) and lens clouding (cataract).
• Bone Changes: Progressive reduction of bone calcium density and osteoid matrix, leading to osteoporosis and brittle, fracture-prone bones.

• Significance of Chromosome Reduction ($2n \rightarrow n$): Prevents exponential doubling of chromosome count each generation; gametes are haploid so syngamy restores the stable diploid status.
• Significance of Exchange of Segments (Crossing Over): Non-sister chromatid exchange shuffles maternal and paternal alleles, giving rise to novel genetic combinations (variations), the raw material for adaptation and evolution.

What is Chromosome? (2 Marks):
A chromosome is a distinct nuclear structure composed of a single giant linear double-stranded DNA molecule wound around basic histone octamer cores into nucleosomes, forming a 30 nm chromatin fibre. During cell division, it condenses further into microscopically visible, rod-like bodies that carry genetic instructions.

Inter-relationship Among Chromosome, DNA and Gene (3 Marks):

• Chemical Composition: A chromosome is essentially a macromolecular nucleoprotein complex comprising ~40% DNA, ~50% histone/non-histone proteins, and ~10% RNA.
• Hierarchical Organization: DNA is the chemical master molecule. The 2-metre long DNA double helix in human cells wraps around histone octamers to form beads-on-a-string nucleosomes, which compact into chromatin threads that further fold into the chromosome.
• Functional Relationship: Specific functional segments of the genomic DNA that contain the coding sequence for a specific protein or functional RNA molecule are called genes. Therefore: $$\text{Gene} \subset \text{DNA} \subset \text{Chromatin Reticulum} \xrightarrow{\text{condensation}} \text{Chromosome}$$ Hence, genes are located linearly on DNA, and DNA constitutes the core physical backbone of each chromosome.

Types of Chromosome Based on Centromere Position (2 Marks):

1. Metacentric: Centromere lies median in the exact centre, creating two equal arms ($p = q$); 'V'-shaped during anaphase.
2. Sub-metacentric: Centromere is placed slightly off-centre, resulting in one slightly shorter 'p' arm and a longer 'q' arm; 'L'-shaped during anaphase.
3. Acrocentric: Centromere is sub-terminal, yielding an extremely short arm and a very long arm; 'J'-shaped during anaphase.
4. Telocentric: Centromere sits at the terminal tip; rod-like or 'I'-shaped during anaphase.

Difference between Centrosome and Centromere (3 Marks):

During the mitotic metaphase, each fully duplicated chromosome exhibits the following morphological structures:

1. Chromatids: Each metaphase chromosome consists of two longitudinally symmetric parallel identical halves termed sister chromatids, linked at the centromere.
2. Centromere (Primary Constriction): The narrow pale-staining constricted region of the chromosome. It divides the chromosome into two arms. On both exterior faces, disc-shaped proteinaceous structures called kinetochores assemble, where spindle fibres attach.
3. Chromonema and Chromomeres: Throughout each chromatid runs a thin spirally coiled microfibrillar thread called chromonema. Along its length, bead-like dense chromatin granules appear as chromomeres.
4. Secondary Constriction (NOR): An additional constriction other than the primary one. In specific chromosomes, it serves as the Nucleolar Organizer Region (NOR) responsible for synthesizing ribosomal RNA (rRNA) and reforming the nucleolus.
5. Satellite (Trabant): A distinct spherical or knob-like terminal segment beyond the secondary constriction. Chromosomes having satellites are designated SAT-chromosomes.
6. Telomeres: The specialized, stable polar non-sticky extremities of a chromosome characterized by repetitive hexanucleotide sequences ($TTAGGG$), preserving chromosomal structural integrity and preventing end-to-end fusion.

Chromosomes are nucleoprotein complexes composed of nucleic acids, proteins, and metallic ions:

1. Nucleic Acids (~45%):
   • DNA (~40%): The hereditary genetic material constructed of deoxyribose sugar, phosphoric acid, and four nitrogenous bases ($A, T, G, C$).
   • RNA (~1.5%–5%): Interspersed single-stranded mRNA, tRNA, and rRNA synthesized by the chromosomal DNA template.
2. Proteins (~55%):
   • Basic Histone Proteins (~80% of protein): Low-molecular-weight positively charged proteins rich in basic amino acids (Lysine and Arginine). Five types occur: $H_1$ (linker histone) and core histones $H_2A, H_2B, H_3, H_4$ forming the octamer core around which DNA coils.
   • Acidic Non-Histone Proteins (~20% of protein): High-molecular-weight proteins rich in acidic amino acids (Tryptophan and Tyrosine), functioning as structural scaffold proteins, polymerases, and gene regulatory factors.
3. Metallic Cations ($Ca^{2+}, Mg^{2+}, Fe^{3+}$): Trace divalent/trivalent cations that stabilize DNA-histone compaction and protect chromosomal integrity.

Definitions (2 Marks):
During interphase, chromatin exists in two distinct structural states:

• Euchromatin: The lightly packed, uncoiled chromosomal region that contains transcriptionally active genes.
• Heterochromatin: The tightly compacted, densely coiled region containing genetically inactive, non-transcribing DNA.

Individual Characteristics & Differences (3 Marks):

In 1953, James Watson and Francis Crick proposed the double helix model of DNA based on X-ray diffraction data by Rosalind Franklin and Maurice Wilkins:

1. Double Stranded & Anti-parallel: DNA consists of two polynucleotide chains wound helically around a central imaginary axis in a right-handed coil. The two strands run in opposite polarities: one runs $5' \rightarrow 3'$ and the other runs $3' \rightarrow 5'$.
2. Sugar-Phosphate Backbone: The external hydrophilic structural backbone of each strand is composed of alternating Deoxyribose sugars linked by $3',5'$-phosphodiester bonds.
3. Complementary Base Pairing: Nitrogenous bases project perpendicularly inward into the centre of the helix:
   • Adenine ($A$) always pairs with Thymine ($T$) via 2 hydrogen bonds ($A = T$).
   • Guanine ($G$) always pairs with Cytosine ($C$) via 3 hydrogen bonds ($G \equiv C$).
4. Dimensions:
   • Helix diameter is uniform at 2.0 nm ($20 \text{ \AA}$).
   • One complete helical pitch (turn) measures 3.4 nm ($34 \text{ \AA}$) and contains approximately 10 base pairs.
   • The axial distance between two consecutive base pairs is 0.34 nm ($3.4 \text{ \AA}$) with a $36^\circ$ rotational angle.

What is RNA? (2 Marks):
Ribonucleic Acid (RNA) is a single-stranded biological polymer composed of ribose nucleotides (ribose sugar, phosphoric acid, Adenine, Guanine, Cytosine, and Uracil). It plays primary roles in gene expression, carrying genetic messages (mRNA), decoding amino acids (tRNA), and catalyzing peptide bond formation (rRNA).

Difference between DNA and RNA (3 Marks):

1. Role of Nucleus (2 Marks):
   • Houses the chromosomes and genetic material; during the S-phase, its chromatin oversees accurate DNA replication.
   • Controls synthesis of cell-cycle regulatory proteins and cyclins.
   • Undergoes karyokinesis, cleanly segregating duplicated chromosome sets into two daughter nuclei.
2. Role of Centrosome (2 Marks):
   • Duplicates during interphase and migrates to opposite poles during prophase in animal cells.
   • Functions as the primary Microtubule Organizing Centre (MTOC), polymerizing tubulin into astral rays and spindle fibres.
   • Coordinates chromosomal alignment along the equatorial plate and pulls sister chromatids apart in anaphase.
3. Role of Mitochondria (1 Mark):
   • Acts as the cellular powerhouse, generating abundant ATP through oxidative phosphorylation. High ATP levels are mandatory for spindle assembly, chromosome movement, DNA polymerization, and contractile ring/cell plate cytokinesis.

What is Cell Division? (2 Marks):
Cell division is the biological process by which a mature parent cell divides its nucleoplasm and cytoplasm into two or more daughter cells, serving as the basis for growth, repair, and reproduction across all living organisms.

Significance of Cell Division (3 Marks):

1. Growth and Development: Mitosis increases cell numbers, enabling single-celled zygotes to transform into complex multicellular organisms with specialized tissues.
2. Repair and Regeneration: Replaces worn-out, injured, or dead cells (e.g., healing skin incisions, replacing intestinal epithelial lining, generating 2-3 million RBCs every second).
3. Reproduction and Species Continuity: Asexual organisms reproduce via mitotic divisions, while sexual organisms utilize meiosis to produce gametes and preserve constant chromosome counts across generations.

What is Cell Cycle? (2 Marks):
The cell cycle is the coordinated, orderly series of events through which a eukaryotic cell replicates its DNA, synthesizes required proteins and organelles, and divides into two daughter cells.

Phases of the Cell Cycle (3 Marks):

1. Interphase (Preparatory Phase, ~95% of cycle duration):
   • $G_1$ Phase (Gap 1): Active RNA and protein synthesis, cell growth, organelle duplication.
   • $S$ Phase (Synthesis): Complete replication of nuclear DNA ($2C \rightarrow 4C$) and centrosome duplication.
   • $G_2$ Phase (Gap 2): Synthesis of spindle tubulin proteins, ATP accumulation, and preparation for division.
   • $G_0$ Phase (Quiescent): Inactive exit stage where differentiated cells (e.g. neurons) stop dividing.
2. M-Phase (Mitotic Division Phase, ~5% of cycle duration):
   • Karyokinesis: Nuclear division through Prophase, Metaphase, Anaphase, Telophase.
   • Cytokinesis: Division of cytoplasm yielding two daughter cells.

Significance of Cell Cycle (3 Marks):

1. Precise Genomic Conservation: Ensures that genomic DNA is duplicated with fidelity during S-phase and segregated symmetrically, preserving chromosome numbers across generations.
2. Cell Size and Mass Homeostasis: Balances periods of cell growth and metabolic expansion with physical division, maintaining nuclear-cytoplasmic ratio.
3. Surveillance & Cancer Prevention: $G_1/S$, $G_2/M$, and Metaphase checkpoints detect replication errors or double-strand breaks, triggering enzymatic repair or programmed apoptosis if DNA is unrepairable.

Why Mitosis is Called Equational Division (2 Marks):
During mitosis, the longitudinal division of centromeres ensures that the two daughter cells receive the exact same number and identical types of chromosomes as the parent cell ($2n \rightarrow 2n$). Because chromosome ploidy remains equal and unchanged, it is termed an equational division.

Metaphase Description (2 Marks):

• Chromosomes attain their highest condensation state and become clearly visible under light microscopy.
• Chromosomes align along the central equatorial plane of the spindle apparatus, forming the metaphase plate (equatorial plate).
• Each centromere is attached to spindle fibres originating from opposite poles via bilateral kinetochores.

Anaphase Description (2 Marks):

• Centromeres split simultaneously, releasing paired sister chromatids as independent daughter chromosomes.
• Spindle chromosomal fibres shorten while interzonal fibres elongate, drawing daughter chromosomes toward opposite poles.
• Depending upon centromere locus, chromosomes assume distinct kinematic shapes: Metacentric (V), Sub-metacentric (L), Acrocentric (J), and Telocentric (I).

What is Meiosis? (2 Marks):
Meiosis is a double nuclear division of diploid reproductive cells ($2n$) producing four haploid daughter gametes ($n$), reducing chromosome numbers by half.

Key Features of Meiosis-I (Heterotypic / Reductional Division) (3 Marks):

1. Prophase-I Sub-stages:
   • Leptotene: Chromatin condenses into beaded threads.
   • Zygotene: Homologous chromosomes pair up side-by-side in synapsis, forming bivalents.
   • Pachytene: Non-sister chromatids form chiasmata and perform crossing over, recombining maternal and paternal genes.
   • Diplotene: Synaptonemal complex dissolves; homologous chromosomes repel but remain linked at chiasmata.
   • Diakinesis: Terminalization of chiasmata and breakdown of nuclear membrane.
2. Metaphase-I: Bivalents arrange in double rows along the equatorial plane.
3. Anaphase-I: Whole homologous chromosomes (not chromatids) disjoin and migrate to opposite poles, halving the ploidy ($2n \rightarrow n$).

Differences Between Plant and Animal Mitosis (3 Marks):

What is Prophase? (2 Marks):
Prophase is the initial, longest stage of karyokinesis characterized by gradual chromatin dehydration and spiralization into distinct chromosomes, duplication and polar migration of centrosomes (in animals), and progressive disassembly of the nucleolus and nuclear envelope.

Features of Meiosis-II (Homotypic / Equational Division) (3 Marks):

• Resembles standard mitosis but operates simultaneously in two haploid ($n$) daughter cells without preceding DNA replication.
• In Metaphase-II, univalent chromosomes align singly at the equator.
• In Anaphase-II, centromeres split, separating sister chromatids into daughter chromosomes.
• Concludes with four distinct haploid nuclei, each with $n$ chromosomes.

What is Sister Chromatid? (2 Marks):
Sister chromatids are the two identical copies of a single replicated chromosome produced during the S-phase, joined together at a common centromere and bearing identical nucleotide sequences.

What is Crossing Over? (2 Marks):
Crossing over is the mutual exchange of corresponding genetic segments between non-sister chromatids of homologous chromosomes during the pachytene stage of prophase-I in meiosis, creating recombinant chromosomes.

Process of Crossing Over (3 Marks):

1. Synapsis: Homologous chromosomes pair intimately in zygotene via a proteinaceous synaptonemal complex, forming bivalents.
2. Breakage: Endonuclease enzymes create matching cuts at identical levels in non-sister chromatids during pachytene.
3. Chiasma Formation: The cut segments cross over and swap positions, forming an X-shaped junction called a chiasma.
4. Ligation: DNA ligase enzymes covalently seal the swapped segments into new recombinant chromatids.

1. Fission: A unicellular parent divides into two or more equal daughter cells.
   • Example: Binary fission in Amoeba; Multiple fission in Plasmodium.
2. Budding: A small daughter outgrowth (bud) develops on the parent body, enlarges, and detaches to live independently.
   • Example: Yeast and Hydra.
3. Fragmentation: The multi-cellular parental body breaks accidentally or mechanically into fragments, each growing into a complete individual by mitosis.
   • Example: Spirogyra.
4. Spore Formation (Sporogenesis): The parent produces microscopic, single-celled, specialized reproductive units called spores within sporangia that disperse and germinate.
   • Example: Rhizopus (Bread mould), Ferns.
5. Regeneration: The capacity of an organism to reconstruct its entire lost body or organs from small cut body fragments through cell dedifferentiation and proliferation.
   • Example: Planaria (Flatworm).

What is Vegetative Propagation? (2 Marks):
An asexual reproductive strategy in plants wherein new offspring develop from specialized non-reproductive somatic structures (vegetative propagules such as roots, stems, or leaves) without seeds or spores.

Vegetative Propagation by Roots and Stems (3 Marks):

• By Roots: Tuberous and tap roots develop adventitious vegetative buds that sprout into leafy shoots when planted in soil.
  • Examples: Sweet potato (Ipomoea batatas), Dahlia, Guava.
• By Stems: Modified underground and sub-aerial stems bear axillary buds that produce new independent plants:
  • Underground Tuber: Potato (via vegetative 'eyes').
  • Rhizome: Ginger and Turmeric.
  • Offset / Runner: Water Hyacinth (Eichhornia) and Grass.

1. Cutting: Stems, roots, or leaves with nodes are cut from a healthy plant and planted in moist soil. Hormones like Auxin (IBA, NAA) induce rapid adventitious rooting. (e.g., Rose, Sugarcane, Hibiscus).
2. Grafting: The shoot system of a desired high-yielding variety (Scion) is inserted and tied onto the rooted base (Stock) of a disease-resistant plant. Vascular cambium layers align and fuse. (e.g., Mango, Apple, Citrus).
3. Layering: A lower flexible branch is bent down, partially defoliated/notched, and pinned beneath moist soil while remaining attached to the mother plant. Once roots appear, it is cut away. (e.g., Jasmine, Lemon).
4. Micropropagation (Tissue Culture): Shoot-tip explants are grown aseptically on synthetic culture media with auxins/cytokinins to produce callus, embryoids, and thousands of clonal plantlets rapidly. (e.g., Orchids, Banana, Potato).

What is Alternation of Generation? (2 Marks):
Alternation of generation is the regular, rhythmic alternation between the asexual spore-bearing diploid phase (Sporophyte, $2n$) and the sexual gamete-bearing haploid phase (Gametophyte, $n$) in the life cycle of plants.

Alternation of Generation in Fern (*Dryopteris*) (3 Marks):

1. Sporophytic Phase ($2n$ - Dominant): The familiar leafy fern plant is a diploid sporophyte. On the underside of mature fronds, sporangia grouped in sori produce haploid spores ($n$) via meiosis.
2. Gametophytic Phase ($n$): Dispersed spores germinate on moist soil into a heart-shaped green photosynthetic thallus called a Prothallus.
3. Gamete Fusion: The prothallus bears male antheridia (producing flagellated antherozoids) and female archegonia (containing an egg). Water is required for fertilization to form a diploid zygote ($2n$).
4. Return to Sporophyte: The zygote undergoes mitosis to form a young sporophyte, completing the cycle.

Inter-relationship Among Chromosome, DNA, and Gene (3 Marks):
DNA is the primary double-helical nucleic acid polymer storing genetic information. Around histone octamers, DNA winds into nucleosomes that coil into chromatin fibers and ultimately condense into the chromosome during division. A gene is a specific nucleotide sequence on this DNA encoding an RNA or polypeptide. Thus, genes reside on DNA, and organized DNA forms chromosomes.

Euchromatin vs Heterochromatin on Specified Aspects (2 Marks):

How Micropropagation is Done (2 Marks):

1. Explant Selection: A healthy shoot-tip, bud, or meristematic tissue (explant) is excised and surface-sterilized.
2. Callus Induction: The explant is placed on sterile agar medium with balanced auxins and cytokinins to induce an undifferentiated dividing cell mass (callus).
3. Organogenesis / Embryoid Formation: Shifting hormone ratios induces shoot and root differentiation (organogenesis) or bipolar embryoids.
4. Hardening & Potting: Plantlets are transferred to a greenhouse for acclimatization (hardening) and then transplanted into field soil.

Three Phases of Sexual Reproduction in Plants (3 Marks):

1. Production of Gamete or Reproductive Cell:
   • In anthers, microspore mother cells ($2n$) undergo meiosis to form haploid microspores that mature into pollen grains carrying two non-motile male gametes ($n$).
   • In ovules, a megaspore mother cell ($2n$) undergoes meiosis to form an embryo sac containing the haploid egg cell ($n$) and diploid secondary nucleus ($2n$).
2. Fertilization (Double Fertilization & Triple Fusion):
   • The pollen tube discharges two male gametes into the embryo sac: $$\text{Male Gamete } (n) + \text{Egg } (n) \rightarrow \text{Zygote } (2n) \quad [\text{Syngamy}]$$ $$\text{Male Gamete } (n) + \text{Secondary Nucleus } (2n) \rightarrow \text{Primary Endosperm Nucleus } (3n) \quad [\text{Triple Fusion}]$$
3. Formation of Embryo and New Plant:
   • The diploid zygote divides mitotically to develop into an organized embryo with plumule, radicle, and cotyledons.
   • The ovule turns into a seed, and the ovary transforms into a fruit. Upon seed germination under favorable water, oxygen, and temperature conditions, the embryo grows into a new photosynthetic seedling.