Cell Biology and Genetics (Mendelism) BBO2T04

Syllabus topics (official RTMNU)

• 1. Definition of cell, brief account of Schleiden and Schwann Cell theory.
• 2. Comparison of prokaryotic and eukaryotic cell
• 3. Ultra-structure of typical plant cell
• 4. Ultra-structure and functions of: a. Cell wall, b. Cell membrane (Fluid Mosaic model), c. Endoplasmic reticulum

Definition of cell, brief account of Schleiden and Schwann Cell theory

Cell: The cell is the basic structural and functional unit of life. It is a mass of protoplasm bounded by a plasma membrane (and in plants also by a cell wall), containing a nucleus or nuclear material, and capable of independent existence and self-reproduction.

• The word "cell" was coined by Robert Hooke (1665) after observing dead cork cells under his microscope (book Micrographia).
• A. van Leeuwenhoek (1674) first observed living cells (bacteria, protozoa, sperms).
• Robert Brown (1831) discovered the nucleus in orchid cells.

Cell theory: Proposed by the German botanist M.J. Schleiden (1838) for plants and the German zoologist Theodor Schwann (1839) for animals. Schwann gave the term "cell theory". Main postulates:

1. All living organisms are made up of cells and cell products.
2. The cell is the basic structural and functional unit of life.
3. The body of plants and animals is an aggregate of cells arranged according to definite laws.

Modification: Schleiden and Schwann could not explain how new cells form (Schleiden thought cells arose from the nucleus by "free cell formation"). Rudolf Virchow (1855) stated "Omnis cellula e cellula" – all cells arise from pre-existing cells. This completed the modern cell theory, which also states that cells contain hereditary material (DNA) passed on during division, and that all metabolic reactions of life occur inside cells.

Exceptions: Viruses (acellular), coenocytic organisms such as Vaucheria and Rhizopus (multinucleate without cross walls), and mature RBCs and sieve tubes that lack a nucleus.

Comparison of prokaryotic and eukaryotic cell

Prokaryotic cells (pro = before, karyon = nucleus) lack a true nucleus and membrane-bound organelles, e.g. bacteria and cyanobacteria. Eukaryotic cells (eu = true) have a true nucleus with a nuclear envelope and membrane-bound organelles, e.g. plants, animals, fungi, protists.

Ultra-structure of typical plant cell

Under the electron microscope a typical plant cell (e.g. mesophyll cell) shows:

• Cell wall – outer rigid non-living layer with middle lamella, primary wall, and plasmodesmata connecting neighbouring cells.
• Plasma membrane – thin, trilaminar, selectively permeable membrane just inside the wall.
• Cytoplasm – the cytosol with organelles: ER (rough and smooth), Golgi bodies (dictyosomes), ribosomes, mitochondria, plastids (chloroplasts), peroxisomes, microtubules and microfilaments.
• Nucleus – with double nuclear envelope, nuclear pores, chromatin and nucleolus.
• Large central vacuole – bounded by tonoplast, filled with cell sap; pushes cytoplasm to the periphery.

Features special to plant cells: cell wall, plastids, large central vacuole, plasmodesmata, glyoxysomes (in fatty seeds). Centrioles are absent in higher plants, and lysosomes are few.

Ultra-structure and functions of: a. Cell wall, b. Cell membrane (Fluid Mosaic model), c. Endoplasmic reticulum

a. Cell wall: A rigid, non-living outer covering of plant cells secreted by the protoplast.

• Middle lamella – first formed, shared between two cells; made of calcium and magnesium pectates; cements cells together.
• Primary wall – thin, elastic, formed during cell growth; made of cellulose microfibrils embedded in a matrix of hemicellulose, pectin and glycoproteins.
• Secondary wall – formed inside the primary wall after growth stops; thick, with layers S1, S2, S3; contains cellulose and often lignin (e.g. xylem, fibres).
• Pits (thin areas) and plasmodesmata (cytoplasmic channels) allow communication between cells.

Functions: gives shape, rigidity and mechanical support; protects against pathogens and osmotic bursting; allows passage of water and solutes; plasmodesmata help transport between cells.

b. Cell membrane (Plasma membrane): A thin (about 7.5 nm) living, selectively permeable membrane. Earlier the unit membrane model of Robertson (1959) showed a trilaminar dark–light–dark structure. The accepted Fluid Mosaic Model was proposed by S.J. Singer and G.L. Nicolson (1972).

• Lipid bilayer: phospholipids with hydrophilic heads facing outward and hydrophobic tails facing inward. The bilayer is fluid; lipids move sideways (lateral diffusion) and rarely flip-flop.
• Proteins as a mosaic: integral (intrinsic) proteins pass partly or completely through the bilayer (transmembrane proteins form channels/carriers); peripheral (extrinsic) proteins are loosely attached to the surface.
• Carbohydrates: present on the outer surface as glycolipids and glycoproteins; act in cell recognition.
• Sterols (in plants phytosterols) regulate fluidity.

Functions: selective permeability; transport by diffusion, osmosis, active transport, endocytosis and exocytosis; cell recognition and signal reception; enclosing the protoplasm.

c. Endoplasmic reticulum (ER): Discovered by Porter, Claude and Fullam (1945); named by Porter (1953). It is a network of membrane-bound channels continuous with the outer nuclear membrane. It has three forms: cisternae (flat parallel sacs), vesicles (round) and tubules (branched).

• Rough ER (RER): ribosomes attached on outer surface (by ribophorins); mostly cisternae; synthesises proteins for secretion and membranes.
• Smooth ER (SER): no ribosomes; mostly tubules; synthesises lipids and steroids, detoxification, glycogen metabolism.

Functions: forms the intracellular transport system; provides mechanical support (cytoskeleton-like); synthesis of proteins and lipids; forms the nuclear envelope after division; gives rise to Golgi vesicles, vacuoles and peroxisomes; detoxification.

5 likely exam questions

Long answer

1. State the cell theory of Schleiden and Schwann. Give a comparative account of prokaryotic and eukaryotic cells.
2. Describe the Fluid Mosaic model of plasma membrane with a labelled diagram and mention its functions.

Short answer / short notes

3. Draw a labelled diagram of the ultra-structure of a typical plant cell.
4. Describe the structure of the plant cell wall.
5. Differentiate between rough ER and smooth ER.

Syllabus topics (official RTMNU)

• d. Golgi complex
• e. Ribosomes
• f. Mitochondria
• g. Chloroplast
• h. Lysosome
• i. Vacuole
• j. Peroxisomes and Glyoxysomes

d. Golgi complex

Discovered by Camillo Golgi (1898) in nerve cells of owl. In plant cells it occurs as many separate units called dictyosomes.

• Cisternae: 4–8 flat, curved, membrane-bound sacs stacked in parallel. The convex cis (forming) face faces the ER; the concave trans (maturing) face faces the plasma membrane.
• Tubules: branched network at the cisternal edges.
• Vesicles: small transport vesicles and larger secretory vesicles budding from the trans face.

Functions: packaging, modification (glycosylation) and secretion of proteins received from RER; synthesis of non-cellulosic polysaccharides (hemicellulose, pectin) for the cell wall; formation of cell plate during cytokinesis in plants; formation of lysosomes; secretion of mucilage and nectar.

e. Ribosomes

Discovered by Robinson and Brown (1953) in plant cells; named by Roberts (1958); G.E. Palade (1955) described them in animal cells. Ribosomes are non-membranous, granular particles (about 15–25 nm) made of rRNA and proteins.

• Each ribosome has a large and a small subunit, joined in the presence of Mg²⁺.
• 70S (50S + 30S) in prokaryotes, mitochondria and chloroplasts; 80S (60S + 40S) in eukaryotic cytoplasm. (S = Svedberg unit of sedimentation.)
• They occur free in cytoplasm or attached to RER. Several ribosomes on one mRNA form a polysome.

Function: site of protein synthesis ("protein factories"); free ribosomes make cytoplasmic proteins and bound ribosomes make secretory and membrane proteins.

f. Mitochondria

First seen by Kolliker (1880); named by Benda (1898). Called the "power house of the cell". Rod-shaped or oval, 0.5–1 µm wide and 2–10 µm long.

• Outer membrane: smooth, permeable (has porins).
• Inner membrane: folded into cristae, bearing stalked F₁ particles (oxysomes / F₀-F₁ ATP synthase) and the electron transport chain.
• Intermembrane space (outer chamber) and matrix (inner chamber) containing enzymes of Krebs cycle, circular DNA, 70S ribosomes.

Functions: aerobic respiration – Krebs cycle in matrix and oxidative phosphorylation on cristae producing ATP; semi-autonomous (own DNA and ribosomes, make some proteins); cytoplasmic inheritance (e.g. cytoplasmic male sterility in maize).

g. Chloroplast

Green plastids containing chlorophyll; lens-shaped, 4–10 µm long in higher plants. Named by Schimper (1883).

• Envelope: double membrane (outer and inner).
• Stroma: colourless matrix with enzymes of the Calvin cycle (RuBisCO), circular DNA, 70S ribosomes, starch grains and plastoglobuli.
• Thylakoids: flattened membrane sacs; stacks of thylakoids form grana; grana are connected by stroma lamellae (intergranal thylakoids).
• Thylakoid membranes contain photosystems I and II (quantasomes), electron carriers and ATP synthase.

Functions: photosynthesis – light reaction in thylakoids (ATP, NADPH, O₂ evolution) and dark reaction (CO₂ fixation) in stroma; storage of starch; semi-autonomous organelle.

h. Lysosome

Discovered by Christian de Duve (1955); called "suicidal bags". Small vesicles (0.2–0.8 µm) bounded by a single membrane, containing about 40 acid hydrolases (acid phosphatase – marker enzyme, proteases, nucleases, lipases) working at pH about 5. In plants lysosome-like activity is mainly carried out by vacuoles, spherosomes and aleurone grains.

Types (polymorphism): primary lysosome (newly formed from Golgi), secondary lysosome / digestive vacuole (fused with food vacuole), residual body, autophagic vacuole.

Functions: intracellular digestion; autophagy (removal of old organelles); autolysis of cells (e.g. during differentiation of xylem vessels and senescence); breakdown of reserve food during seed germination.

i. Vacuole

A membrane-bound sac filled with cell sap, bounded by a single membrane called the tonoplast. In mature plant cells one large central vacuole occupies 80–90% of cell volume.

• Cell sap contains water, sugars, salts, organic acids, amino acids, pigments (anthocyanin), tannins, alkaloids, crystals and enzymes.

Functions: maintains turgor pressure and rigidity; osmoregulation; storage of food and wastes; gives colour to flowers (anthocyanins); lytic (lysosome-like) digestion; contractile vacuoles in some algae expel excess water.

j. Peroxisomes and Glyoxysomes

Both are microbodies – small (0.5–1.5 µm) single-membrane vesicles with fine granular matrix, often with a crystalline core.

Peroxisomes (de Duve, 1965): found in green leaves of C₃ plants, close to chloroplasts and mitochondria. Contain oxidases and catalase. Functions: photorespiration (glycolate → glyoxylate → glycine; this oxidation produces H₂O₂); catalase breaks toxic H₂O₂ into water and O₂.

Glyoxysomes (Breidenbach and Beevers, 1967): found in germinating fatty seeds (castor, groundnut, sunflower). Contain enzymes of β-oxidation and the glyoxylate cycle (isocitrate lyase, malate synthase). Function: convert stored fats into carbohydrates (sucrose) for the growing seedling.

5 likely exam questions

Long answer

1. Describe the ultra-structure and functions of mitochondria with a neat labelled diagram.
2. Describe the ultra-structure of chloroplast and give its functions. Why are chloroplasts called semi-autonomous organelles?

Short answer / short notes

3. Describe the structure and functions of the Golgi complex.
4. Write a note on lysosomes as suicidal bags.
5. Differentiate between peroxisomes and glyoxysomes.

Syllabus topics (official RTMNU)

• k. Nucleus
• 5. Morphology of typical eukaryotic chromosome
• 6. Karyotype and idiogram
• 7. Molecular organization of chromosome – Nucleosome model
• 8. Sex chromosomes in Melandrium album (XY-type)

k. Nucleus

Discovered by Robert Brown (1831) in orchid root cells. It is the largest, usually spherical organelle and the control centre of the cell. Parts:

• Nuclear envelope: double membrane; outer membrane is continuous with ER and bears ribosomes; the gap between membranes is the perinuclear space (10–50 nm).
• Nuclear pores: octagonal openings with a nuclear pore complex of proteins (nucleoporins); regulate exchange of mRNA, ribosomal subunits and proteins between nucleus and cytoplasm.
• Nucleoplasm (karyolymph): gel-like matrix with enzymes, nucleotides and ions.
• Chromatin: DNA + histone and non-histone proteins. Euchromatin is lightly stained, loosely coiled and genetically active; heterochromatin is darkly stained, condensed and mostly inactive.
• Nucleolus: dense, non-membranous body formed at the nucleolar organiser region (NOR); site of rRNA synthesis and ribosome subunit assembly. Disappears in prophase and reappears in telophase.

Functions: stores genetic information (DNA); controls cell activities through RNA and protein synthesis; DNA replication before division; forms ribosomes; responsible for heredity and variation.

5. Morphology of typical eukaryotic chromosome

The term chromosome was given by Waldeyer (1888). Chromosomes are best studied at metaphase, when most condensed. A typical metaphase chromosome shows:

• Chromatids: two identical sister chromatids joined at the centromere; each has a coiled thread, the chromonema, with bead-like chromomeres.
• Centromere (primary constriction): joins the chromatids; bears disc-like kinetochores to which spindle fibres attach.
• Secondary constriction: a non-centromeric constriction; often the NOR forming nucleolus.
• Satellite: small terminal knob beyond the secondary constriction; such a chromosome is a SAT-chromosome.
• Telomeres: special ends that seal the chromosome and prevent end-to-end fusion; made of repeated sequences (TTTAGGG in plants).
• Arms: short arm (p) and long arm (q).

Types based on position of centromere:

6. Karyotype and idiogram

Karyotype: The complete set of chromosomes of a species as seen at metaphase, described by their number, size, shape, centromere position, secondary constrictions and satellites. It is constant for a species, e.g. onion (Allium cepa) 2n = 16, maize (Zea mays) 2n = 20, garden pea (Pisum sativum) 2n = 14, human 2n = 46.

Idiogram: A diagrammatic representation of the karyotype in which homologous chromosome pairs are drawn and arranged in order of decreasing length, with centromeres aligned in a line.

• Symmetric karyotype: chromosomes of similar size with median centromeres (considered primitive). Asymmetric karyotype: large differences in size and centromere position (advanced).

Uses: taxonomy and phylogeny (cytotaxonomy); detection of chromosomal aberrations and polyploidy; study of evolution of species; sex determination.

7. Molecular organization of chromosome – Nucleosome model

The nucleosome model was proposed by R.D. Kornberg (1974); the bead-like units were seen as "ν (nu) bodies" by Olins and Olins (1974) and named nucleosomes by Oudet et al. (1975).

1. Histone octamer: core of 8 histone molecules – two each of H2A, H2B, H3 and H4.
2. Core DNA: about 146 bp of DNA wrapped around the octamer in about 1¾ left-handed turns, forming the nucleosome core particle (about 11 nm diameter).
3. Linker DNA: DNA connecting adjacent nucleosomes (usually about 50–60 bp, varies with species).
4. H1 histone: binds the DNA where it enters and leaves the core, sealing the two turns (forming the chromatosome, about 166 bp). One nucleosome repeat is about 200 bp.

Higher order packing: "beads-on-a-string" (11 nm fibre) → coiled into the solenoid (30 nm fibre, about 6 nucleosomes per turn, H1 helps) → looped domains attached to a non-histone protein scaffold (300 nm) → further coiling (700 nm) → metaphase chromosome (about 1400 nm wide). This packing compresses DNA about 10,000 times.

8. Sex chromosomes in Melandrium album (XY-type)

Melandrium album (= Silene latifolia, white campion, family Caryophyllaceae) is a dioecious plant. Its sex is determined by the XY mechanism, studied by Westergaard (1940s) and Warmke.

• 2n = 24: 22 autosomes + 2 sex chromosomes. Female = 22A + XX (homogametic); male = 22A + XY (heterogametic).
• Unusually, the Y chromosome is larger than the X and is the largest chromosome of the set.
• The Y is strongly male-determining: any plant with a Y is male, irrespective of the number of X chromosomes (e.g. polyploids 4A+XXXY are male; 4A+XXXX are female). This differs from Drosophila, where sex depends on the X : autosome ratio.

Regions of the Y chromosome (Westergaard): Using plants with broken Y chromosomes, he identified on the differential segment of Y:

1. Region I – female suppressor: suppresses development of the gynoecium; its loss gives bisexual (hermaphrodite) flowers.
2. Region II – male promoting (initiating): starts anther development; its loss gives female plants.
3. Region III – male fertility (completing): needed for pollen formation; its loss gives male-sterile flowers with abortive anthers.
4. Region IV – pairing (homologous) segment: pairs with the homologous part of X during meiosis.

The X chromosome has a homologous pairing segment and a differential segment carrying female-tendency genes. Cross: XX female × XY male → gametes X (egg) and X, Y (pollen) → 1 XX female : 1 XY male, giving a 1:1 sex ratio.

5 likely exam questions

Long answer

1. Describe the morphology of a typical eukaryotic chromosome. Add a note on the types of chromosomes based on the position of centromere.
2. Explain the nucleosome model of chromosome organisation with a labelled diagram.

Short answer / short notes

3. Describe the ultra-structure of the nucleus.
4. Differentiate between karyotype and idiogram.
5. Write a note on sex chromosomes in Melandrium album.

Syllabus topics (official RTMNU)

• 9. Cell cycle
• 10. Mitosis in plants
• 11. Meiosis in plants
• 12. Significance of Mitosis and Meiosis
• 13. Mendelism: Monohybrid and dihybrid cross, Laws of inheritance – Law of segregation and Law of independent assortment

9. Cell cycle

The cell cycle is the sequence of events from the formation of a cell to its own division into two daughter cells (Howard and Pelc, 1953). It has two main phases:

• Interphase (preparatory phase, about 90–95% of the cycle):
  • G₁ phase (Gap 1): cell grows, synthesises RNA, proteins and organelles; decides whether to divide. Cells that stop dividing enter G₀ (quiescent) phase.
  • S phase (Synthesis): DNA replication and histone synthesis; DNA content doubles (2C → 4C) but chromosome number stays the same.
  • G₂ phase (Gap 2): synthesis of proteins for spindle (tubulin), further growth; preparation for mitosis.
• M phase (division phase): karyokinesis (nuclear division) followed by cytokinesis (cytoplasmic division).

The cycle is controlled at checkpoints (G₁/S, G₂/M, metaphase checkpoint) by cyclins and cyclin-dependent kinases (CDKs).

10. Mitosis in plants

Mitosis (term by Flemming, 1882) is equational division in which a cell divides into two daughter cells with the same chromosome number as the parent (2n → 2n). In plants it occurs in meristems (root tip, shoot tip, cambium). Best material: onion root tip.

1. Prophase: chromatin condenses into visible chromosomes, each of two chromatids joined at centromere; nucleolus and nuclear envelope disappear; spindle starts forming (in plants without centrioles – anastral spindle).
2. Metaphase: spindle complete; chromosomes arrange at the equator forming the metaphase (equatorial) plate; spindle fibres attach to kinetochores. Chromosomes are most condensed – best stage for counting.
3. Anaphase: centromeres split; sister chromatids (now daughter chromosomes) move to opposite poles, appearing V, J or I shaped.
4. Telophase: chromosomes reach poles and uncoil; nuclear envelope and nucleolus reappear; two daughter nuclei form.

Cytokinesis in plants: by cell plate method – Golgi vesicles collect at the equator in the phragmoplast, fuse to form the cell plate, which grows centrifugally (centre to periphery) and becomes the middle lamella. (Animal cells divide by furrowing.)

11. Meiosis in plants

Meiosis (term by Farmer and Moore, 1905) is reductional division in which a diploid cell forms four haploid cells (2n → n). In flowering plants it occurs in spore mother cells – microspore mother cells in anthers and megaspore mother cell in ovule. It has two divisions:

Meiosis I (reductional):

• Prophase I – long and divided into five sub-stages:
  1. Leptotene: chromosomes become visible as long thin threads with chromomeres.
  2. Zygotene: homologous chromosomes pair (synapsis) through the synaptonemal complex; pairs are called bivalents.
  3. Pachytene: each bivalent shows four chromatids (tetrad); crossing over takes place between non-sister chromatids.
  4. Diplotene: homologues begin to separate but remain joined at X-shaped chiasmata.
  5. Diakinesis: maximum condensation; terminalisation of chiasmata; nucleolus and nuclear envelope disappear.
• Metaphase I: bivalents arrange at the equator.
• Anaphase I: homologous chromosomes (each with two chromatids) separate to opposite poles; centromeres do not split – this is the reduction step.
• Telophase I: two haploid nuclei form; followed by a short interkinesis (no DNA replication).

Meiosis II (equational): like mitosis – prophase II, metaphase II, anaphase II (centromeres split, chromatids separate), telophase II – giving four haploid nuclei. Cytokinesis may be successive (isobilateral tetrad, common in monocots) or simultaneous (tetrahedral tetrad, common in dicots).

12. Significance of Mitosis and Meiosis

13. Mendelism: Monohybrid and dihybrid cross, Laws of inheritance – Law of segregation and Law of independent assortment

Gregor Johann Mendel (1822–1884), the Father of Genetics, worked on garden pea (Pisum sativum) and published in 1866. His work was rediscovered in 1900 by de Vries, Correns and von Tschermak. Pea was suitable because it is self-pollinating, has short life cycle, many seeds and seven pairs of clear contrasting characters.

Monohybrid cross (one pair of contrasting characters): Tall (TT) × Dwarf (tt) → F₁ all Tall (Tt). Selfing F₁ (Tt × Tt) gives F₂: TT, Tt, Tt, tt.

• Phenotypic ratio 3 Tall : 1 Dwarf; genotypic ratio 1 TT : 2 Tt : 1 tt.

Dihybrid cross (two pairs): Round Yellow (RRYY) × Wrinkled Green (rryy) → F₁ all Round Yellow (RrYy). F₁ forms four types of gametes in equal number: RY, Ry, rY, ry. A 4 × 4 Punnett square gives 16 combinations in F₂:

• 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green (phenotypic ratio 9:3:3:1).
• Genotypic ratio 1:2:1:2:4:2:1:2:1 (9 genotypes).

Law of dominance: of two contrasting factors, only one (dominant) expresses in F₁; the other (recessive) stays hidden.

Law of segregation (Law of purity of gametes): the two alleles of a gene remain together in the hybrid without mixing and separate (segregate) during gamete formation, so each gamete gets only one allele and is "pure". Proof: reappearance of dwarf plants in F₂ and the 1:1 ratio of the test cross (Tt × tt).

Law of independent assortment: when two or more pairs of characters are inherited together, the alleles of one pair segregate independently of the other pair during gamete formation. Proof: 9:3:3:1 ratio, and new combinations (round green, wrinkled yellow) in F₂; dihybrid test cross gives 1:1:1:1. This law holds only for genes on different chromosomes (or far apart); linked genes are the exception.

5 likely exam questions

Long answer

1. Describe the process of meiosis in plants with labelled diagrams. Add a note on its significance.
2. Explain the dihybrid cross with the help of a checkerboard and state Mendel's Law of independent assortment.

Short answer / short notes

3. Describe the phases of the cell cycle.
4. Describe the stages of mitosis in plants.
5. Explain Mendel's Law of segregation with a monohybrid cross.