Biology Grade 12 First Semester Comprehensive Study Guide

Unit 1: Chemistry of Life

Bioorganic Compounds

• Fundamental Composition of Living Organisms:

  • Living organisms consist of vital elements including Hydrogen ($\text{H}$), Carbon ($\text{C}$), Oxygen ($\text{O}$), Nitrogen ($\text{N}$), Calcium ($\text{Ca}$), and Phosphorus ($\text{P}$).
  • Carbon is the central structural element present in all organic compounds due to its ability to form four covalent bonds with other carbon atoms or different elements.

• Definition of Bioorganic Compounds:

  • Chemical compounds found in living organisms that primarily contain carbon and hydrogen atoms bound together covalently, often incorporating additional elements such as nitrogen and oxygen.
  • Divided into four primary macromolecular classes: Carbohydrates, Proteins, Lipids, and Nucleic Acids.

• Carbohydrates:

  • Composed of Carbon, Hydrogen, and Oxygen atoms.
  • Classified according to the number of monomeric units into Monosaccharides, Disaccharides, and Polysaccharides.
  • Monosaccharides:
  • Simplest form of carbohydrates; hydrophilic and readily soluble in water.
  • General chemical formula: $\left(\text{CH}_2\text{O}\right)_n$, where $n$ ranges from $3$ to $7$ carbon atoms.
  • Can exist in open-chain or cyclic ring structures.
  • Serve as building blocks for complex carbohydrates.
  • Examples:
    • Glucose ($\text{C}_6\text{H}_{12}\text{O}_6$): Primary monomeric building block for major polysaccharides.
    • Fructose ($\text{C}_6\text{H}_{12}\text{O}_6$): Fruit sugar.
    • Galactose ($\text{C}_6\text{H}_{12}\text{O}_6$): Milk sugar monomer.
    • Ribose ($\text{C}_5\text{H}_{10}\text{O}_5$): Pentose sugar in RNA.
  • Disaccharides:
  • Consist of two monosaccharides joined together by a covalent glycosidic bond formed via a dehydration reaction (loss of one water molecule, $\text{H}_2\text{O}$).
  • Examples:
    • Maltose (Malt sugar): $\text{Glucose} + \text{Glucose}$
    • Sucrose (Table sugar): $\text{Glucose} + \text{Fructose}$
    • Lactose (Milk sugar): $\text{Glucose} + \text{Galactose}$
  • Polysaccharides:
  • Polymers composed of numerous monosaccharide units (or derivatives) linked by glycosidic bonds.
  • Examples:
    • Starch: Plant storage carbohydrate consisting of two forms:
    • Amylose: Unbranched helical chain of glucose monomers.
    • Amylopectin: Branched chain of glucose monomers.
    • Plant starch composition: $20\% - 30\%$ amylose and $70\% - 80\%$ amylopectin.
    • Glycogen: Highly branched polymer of glucose stored in animal liver and muscle tissues.
    • Cellulose: Structural polysaccharide in plant cell walls consisting of unbranched, parallel chains of glucose linked by glycosidic bonds, held together laterally by hydrogen bonds to form rigid microfibrils.

• Proteins:

  • Polymers formed from fundamental monomeric units called amino acids, linked together by covalent peptide bonds.
  • General Chemical Structure of an Amino Acid:
  • Central Carbon atom ($\alpha$\text{-carbon}) bonded to:
    1. Amino group ($-\text{NH}_2$)
    2. Carboxyl group ($-\text{COOH}$)
    3. Hydrogen atom ($-\text{H}$)
    4. Variable Side Chain ($-\text{R}$ group), which confers unique chemical properties to each amino acid.
  • Examples of Side Chains ($-\text{R}$):
    • Glycine: $\text{R} = -\text{H}$
    • Serine: $\text{R} = -\text{CH}_2\text{OH}$
    • Cysteine: $\text{R} = -\text{CH}_2\text{SH}$
  • Amino Acid Diversity:
  • $20$ different standard amino acids enter protein synthesis.
  • The human body can synthesize $11$ amino acids, while the remaining $9$ are essential amino acids that must be acquired through diet.
  • Classified based on side-chain polarity into hydrophilic and hydrophobic amino acids.
  • Biological Functions of Proteins:
  • Catalyzing metabolic reactions (Enzymes).
  • Immune defense (Antibodies).
  • Gas transport in blood (Hemoglobin).
  • Cellular communication and signaling (Hormone receptors).
  • Structural support (Collagen).
  • Glycoproteins and Human Blood Typing:
  • Glycoproteins: Proteins covalently conjugated to carbohydrates, functioning as cell-surface markers (Antigens).
  • ABO Blood Group System:
    • Type A: Red blood cells (RBCs) express Antigen A; plasma contains Anti-B antibodies.
    • Type B: RBCs express Antigen B; plasma contains Anti-A antibodies.
    • Type AB: RBCs express both Antigen A and Antigen B; plasma contains no ABO antibodies (universal recipient for RBCs).
    • Type O: RBCs express neither antigen; plasma contains both Anti-A and Anti-B antibodies (universal donor for RBCs).
  • Rhesus (Rh) Factor System:
    • $Rh^+$: RBCs express Antigen D.
    • $Rh^-$: RBCs lack Antigen D; produces Anti-D antibodies upon exposure to $Rh^+$ blood.
  • Transfusion Rules and Complications:
    • RBC transfusion compatibility requires matching donor antigens against recipient antibodies.
    • Incompatible blood transfusions cause antibody-antigen agglutination and hemolysis, resulting in fever, chills, acute renal failure, or death.
  • Levels of Protein Structure:
  • Primary Structure ($1^\circ$): Linear sequence of amino acids in a polypeptide chain linked by peptide bonds, extending from the N-terminus (free $-\text{NH}_2$) to the C-terminus (free $-\text{COOH}$). Non-functional in isolation.
  • Secondary Structure ($2^\circ$): Coiling or folding formed by hydrogen bonds between backbone carboxyl oxygen and amino hydrogen atoms. Common motifs:
    • $\alpha$-Helix: Spiral coil with hydrogen bonds every 4th amino acid.
    • $\beta$-Pleated Sheet: Parallel or antiparallel polypeptide segments arranged in a zig-zag sheet.
  • Tertiary Structure ($3^\circ$): Three-dimensional folding of secondary structural elements driven by R-group interactions (Hydrogen bonds, Disulfide bridges $-\text{S}-\text{S}-$, Ionic bonds, and hydrophobic/Van der Waals interactions). Example: Myoglobin.
  • Quaternary Structure ($4^\circ$): Complex formed by two or more polypeptide chains (subunits). Examples: Hemoglobin (composed of two $\alpha$-globin and two $\beta$-globin chains) and Collagen (triple-stranded helix).

• Lipids:

  • Hydrophobic, non-polar molecules insoluble in water.
  • Functions: Thermal insulation, cell membrane framework, steroid hormones, fat-soluble vitamin storage ($\text{Vitamins A, D, E, K}$), and concentrated energy storage.
  • Major Lipid Classes:
  • Fatty Acids: Hydrocarbon chain attached to a terminal carboxyl group ($-\text{COOH}$).
    • Saturated Fatty Acids: Contain only single carbon-carbon bonds ($\text{C}-\text{C}$). Example: Palmitic acid (palm oil component).
    • Unsaturated Fatty Acids: Contain one or more double carbon-carbon bonds ($\text{C}=\text{C}$). Example: Oleic acid (olive oil component).
  • Triglycerides: One glycerol molecule esterified to three fatty acid molecules via covalent ester bonds, releasing 3 $\text{H}_2\text{O}$ molecules.
    • Saturated triglycerides are solid at room temperature (fats, butter).
    • Unsaturated triglycerides are liquid at room temperature (oils).
  • Phospholipids: Consist of one glycerol molecule linked to two fatty acid tails (hydrophobic) and one negatively charged phosphate group (hydrophilic head).
    • Form the amphipathic lipid bilayer of biological membranes, with hydrophilic heads facing aqueous environments and hydrophobic tails sequestered inside.
  • Steroids: Lipids composed of four fused carbon rings (three 6-membered rings and one 5-membered ring) attached to functional groups.
    • Example: Cholesterol, synthesized in the liver and ingested via animal products; essential for membrane fluidity and precursory synthesis of steroid hormones such as Aldosterone.

• Nucleic Acids:

  • Biological polymers that store and transmit genetic information.
  • Monomeric unit: Nucleotide, comprising:
  1. Nitrogenous Base
  2. Pentose Sugar (5-carbon sugar)
  3. Phosphate Group ($\text{PO}_4^{3-}$)
  • Nitrogenous Bases:
  • Purines (Double-ring structure): Adenine ($\text{A}$), Guanine ($\text{G}$).
  • Pyrimidines (Single-ring structure): Cytosine ($\text{C}$), Thymine ($\text{T}$ in DNA), Uracil ($\text{U}$ in RNA).
  • Deoxyribonucleic Acid (DNA):
  • Double-stranded helical molecule storing genetic instructions.
  • Nucleotides along a single strand are linked by covalent phosphodiester bonds ($5'$ phosphate to $3'$ hydroxyl).
  • Complementary strands run antiparallel ($5' \rightarrow 3'$ opposite $3' \rightarrow 5'$) held together by hydrogen bonds:
    • Adenine pairs with Thymine via $2$ hydrogen bonds ($\text{A}=\text{T}$).
    • Guanine pairs with Cytosine via $3$ hydrogen bonds ($\text{G}\equiv\text{C}$).
  • Chargaff's Rules: In double-stranded DNA, $\%\text{A} = \%\text{T}$ and $\%\text{G} = \%\text{C}$.
  • Ribonucleic Acid (RNA):
  • Single-stranded nucleic acid containing ribose sugar and Uracil ($\text{U}$) instead of Thymine ($\text{T}$).
  • Types: Messenger RNA (mRNA), Transfer RNA (tRNA), Ribosomal RNA (rRNA).

Enzymes and Energy Storage Molecules

• Enzymes:

  • Biological catalysts, predominantly globular proteins, that accelerate chemical reactions without being consumed.
  • Activation Energy ($E_a$): The minimum energy required to initiate a chemical reaction. Enzymes increase reaction rates by lowering $E_a$.
  • Active Site: Specific 3D pocket or groove on the enzyme formed by specific amino acid residues where the substrate binds to form the Enzyme-Substrate Complex.
  • Hypotheses of Enzyme-Substrate Binding:
  • Lock-and-Key Hypothesis: Rigid complementation where the substrate shape perfectly fits the static active site.
  • Induced-Fit Hypothesis: Dynamic binding where active site undergoes a flexible conformational change upon substrate binding to optimize catalytic contact.
  • Factors Influencing Enzyme Activity:
  • Temperature: Reaction velocity increases with temperature up to an optimal temperature (typically $35^\circ\text{C} - 40^\circ\text{C}$ in humans, centered around $37^\circ\text{C}$). Exceeding optimal temperature disrupts non-covalent bonds, leading to thermal denaturation and permanent loss of active site conformation. Lowering temperature reduces kinetic collisions without denaturing.
  • pH: Activity depends on hydrogen ion concentration. Most human enzymes operate optimally between $\text{pH } 6.0 - 8.0$. Exceptions: Pepsin in stomach operates optimally at $\text{pH } 1.5 - 2.0$; Trypsin in small intestine operates at $\text{pH } 8.0$
  • Enzyme Concentration: Direct linear proportional relationship to reaction rate, provided substrate supply is unlimited.
  • Substrate Concentration: Reaction rate increases progressively until all enzyme active sites are fully saturated (reaching maximum velocity, $V_{\text{max}}$). Further substrate addition causes no additional rate increase.
  • Cofactors and Coenzymes:
  • Cofactors: Non-protein chemical helper components required for catalytic activity.
  • Coenzymes: Organic cofactors (e.g., $\text{NAD}^+$, $\text{FAD}$, $\text{NADP}^+$) that act as transient electron carriers in redox reactions:       $\text{NAD}^+ + 2\text{H}^+ + 2e^- \rightleftharpoons \text{NADH} + \text{H}^+$$\text{FAD} + 2\text{H}^+ + 2e^- \rightleftharpoons \text{FADH}_2$

• Adenosine Triphosphate (ATP):

  • Primary cellular energy currency.
  • Structure: Nitrogenous base Adenine + Ribose Sugar + Three Phosphate groups ($\text{P}_i$
  • Synthesis and Hydrolysis Cycle:
  • Synthesis via Phosphorylation (catalyzed by ATP Synthase):       $\text{ADP} + \text{P}_i + \text{Energy} \rightarrow \text{ATP}$
  • Hydrolysis (catalyzed by ATPase):       $\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$$\text{ADP} \rightarrow \text{AMP} + \text{P}_i + \text{Energy}$

Cellular Chemical Reactions

• Metabolism:

  • Sum total of chemical processes within an organism.
  • Anabolism: Metabolic pathways that construct complex molecules from simpler ones, requiring energy input (e.g., Photosynthesis).
  • Catabolism: Metabolic pathways that break down complex organic molecules into simpler compounds, releasing stored chemical energy (e.g., Cellular Respiration).

• Cellular Respiration:

  • Process of oxidative breakdown of glucose to generate ATP.
  • General Summary Equation:     $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy (ATP + Heat)}$
  • Stage 1: Glycolysis:
  • Occurs in the cytosol under anaerobic conditions.
  • Cleaves one 6-carbon Glucose molecule into two 3-carbon Pyruvate molecules.
  • Net Yield per glucose: $2 \text{ ATP}$ (via substrate-level phosphorylation) and $2 \text{ NADH}$.
  • Stage 2: Aerobic Respiration (Inside Mitochondria):
  • 1. Pyruvate Oxidation:
    • Pyruvate transported into mitochondrial matrix.
    • Each pyruvate is decarboxylated releasing $\text{CO}_2$, oxidized ($\text{NAD}^+ \rightarrow \text{NADH}$), and combined with Coenzyme A (CoA) to yield Acetyl-CoA ($2\text{C}$).
    • Yield per glucose: $2 \text{ Acetyl-CoA} + 2 \text{ CO}_2 + 2 \text{ NADH}$.
  • 2. Krebs Cycle (Citric Acid Cycle):
    • Occurs in mitochondrial matrix.
    • Acetyl-CoA ($2\text{C}$) condenses with Oxaloacetate ($4\text{C}$) to form Citrate ($6\text{C}$).
    • Two turns of the cycle per glucose molecule yield:         $4 \text{ CO}_2 + 2 \text{ ATP} + 6 \text{ NADH} + 2 \text{ FADH}_2$
  • 3. Oxidative Phosphorylation (Electron Transport Chain & Chemiosmosis):
    • Occurs across the inner mitochondrial membrane (cristae).
    • High-energy electrons from $\text{NADH}$ and $\text{FADH}_2$ pass along membrane-bound electron transport complexes to final electron acceptor Oxygen ($\text{O}_2$), combining with protons to form $\text{H}_2\text{O}$.
    • Proton pumping creates an electrochemical gradient ($\text{H}^+$ accumulation in intermembrane space).
    • Protons flow back into matrix through ATP Synthase (Chemiosmosis), driving ATP synthesis.
    • ATP Equivalents:
    • $1 \text{ NADH} \rightarrow 3 \text{ ATP}$ (or $2.5 \text{ ATP}$ theoretical standard).
    • $1 \text{ FADH}_2 \rightarrow 2 \text{ ATP}$ (or $1.5 \text{ ATP}$ theoretical standard).
    • Total ATP yield per completely oxidized glucose = $38 \text{ ATP}$ (or $36-38 \text{ ATP}$).

• Anaerobic Pathways:

  • Anaerobic Respiration: Uses Electron Transport Chain with an inorganic terminal electron acceptor other than $O_2$ (e.g., Sulfate-reducing bacteria reducing $\text{SO}_4^{2-}$ to $\text{H}_2\text{S}$).
  • Fermentation: Cytosolic process operating without oxygen or ETC to regenerate $\text{NAD}^+$ for continued glycolysis.
  • Lactic Acid Fermentation: Pyruvate directly reduced by $\text{NADH}$ to Lactic Acid (Lactate), yielding net $2 \text{ ATP}$. Occurs in strenuous muscle contraction and lactic acid bacteria (dairy processing).
  • Alcoholic Fermentation: Pyruvate converted to Acetaldehyde with $\text{CO}_2$ release, then reduced by $\text{NADH}$ to Ethanol, yielding net $2 \text{ ATP}$. Used by yeast in bread dough expansion and brewing.

Unit 2: Cell Cycle and Protein Synthesis

The Cell Cycle

• Definition and Overview:

  • An ordered sequence of events extending from the origin of a cell by division of a parent cell until its own division into two daughter cells.
  • Cycle duration varies by cell type and environmental conditions (e.g., plant root tip apical meristem cells divide every $\sim 20$ hours; human intestinal epithelial cells divide every $10 - 12$ hours).

• Phases of the Cell Cycle:

  • 1. Interphase:
  • Accounts for $\sim 90\%$ of total cycle duration. Active growth and preparation for division.
  • $G_1$ Phase (First Gap): Rapid cell growth, high metabolic activity, organelle duplication.
  • $S$ Phase (Synthesis): Replication of nuclear DNA resulting in duplication of genetic material.
  • $G_2$ Phase (Second Gap): Final growth phase, synthesis of mitotic spindle proteins (microtubules), centrosome duplication in animal cells.
  • $G_0$ Phase (Resting/Quiescent State):
  • Non-dividing state entered from $G_1$ phase upon absence of proliferative signals.
  • Terminally differentiated cells (neurons, skeletal muscle cells) remain permanently in $G_0$; others (liver hepatocytes) can re-enter $G_1$ when stimulated.
  • 2. M Phase (Mitotic Phase):
  • Division of nuclear material (Karyokinesis) followed by Cytokinesis.

• Regulation of Cell Cycle:

  • Cellular Signals:
  • Internal signals (e.g., DNA damage detection, nutrient levels) vs. External signals (e.g., growth factors).
  • Signal functional types: Go-ahead signals (promote cycle progression), Stop signals (halt cycle), Apoptosis signals (trigger programmed cell death via enzymatic cell destruction).
  • Checkpoints:
  • $G_1$ Checkpoint (Restriction Point): Determines whether cell proceeds to $S$ phase or exits into $G_0$.
  • $G_2$ Checkpoint: Verifies fidelity of DNA replication and ensures no DNA damage before entering mitotic phase.
  • M Checkpoint (Spindle Assembly Checkpoint): Occurs between metaphase and anaphase to verify correct attachment of spindle fibers to kinetochores of all sister chromatids.
  • Molecular Control Mechanism:
  • Cyclins: Regulatory proteins whose concentrations fluctuate predictably throughout cycle phases.
  • Cyclin-Dependent Kinases (CDKs): Catalytic enzymes that, when activated by binding specific cyclins, phosphorylate target proteins to drive cycle progression. Cyclins degrade rapidly after checkpoint passage.

Cell Division and Its Importance

• Mitosis:

  • Nuclear division resulting in two genetically identical daughter nuclei, maintaining diploid ($2n$) chromosome number.
  • Phases of Mitosis:
  1. Prophase: Chromatin condenses into visible discrete chromosomes (each comprising two sister chromatids joined at a centromere). Nucleolus disappears, nuclear envelope disintegrates, centrosomes move toward opposite cell poles as mitotic spindle forms.
  2. Metaphase: Spindle fibers align chromosomes along the equatorial plane (metaphase plate).
  3. Anaphase: Sister chromatids are separated by shortening kinetochore spindle fibers and pulled toward opposite poles, becoming individual daughter chromosomes ($V$\text{-shaped appearance}).
  4. Telophase: Nuclear envelopes reassemble around daughter chromosome clusters, nucleoli reappear, chromosomes decondense back into chromatin.
  • Cytokinesis:
  • Animal Cells: Cleavage furrow forms via contraction of a microfilament ring composed of actin and myosin proteins.
  • Plant Cells: Cell plate forms along equator via fusion of Golgi-derived vesicles, forming new plasma membrane and cell wall layers.
  • Biological Significance: Organismal growth, tissue repair and regeneration, asexual reproduction.

• Meiosis:

  • Specialized nuclear division in germ cells yielding four haploid ($1n$) gametes, halving chromosome content.
  • Meiosis I (Reductional Division):
  • Prophase I: Synapsis pairs homologous chromosomes into tetrads. Crossing Over occurs at chiasmata, exchanging non-sister chromatid segments to generate recombinant chromatids.
  • Metaphase I: Homologous chromosome pairs align randomly at metaphase plate (Independent Assortment).
  • Anaphase I: Homologous chromosomes separate to opposite poles; sister chromatids remain attached.
  • Telophase I & Cytokinesis: Yields two haploid ($1n$) intermediate cells.
  • Meiosis II (Equational Division):
  • Proceeds without intervening DNA replication.
  • Prophase II, Metaphase II, Anaphase II (sister chromatids separate), Telophase II & Cytokinesis yield four genetically distinct haploid ($1n$) gametes.
  • Biological Significance: Production of gametes for sexual reproduction, maintenance of species chromosome number across generations, generation of genetic diversity.

DNA Replication and Gene Expression

• DNA Replication Mechanism:

  • Semi-Conservative Replication Model: Each newly synthesized DNA double helix comprises one intact original parental template strand and one newly synthesized complementary strand.
  • Key Enzymes and Proteins:
  • Helicase: Unwinds double helix and breaks hydrogen bonds between nitrogenous base pairs at ATP expense.
  • Single-Strand Binding Proteins (SSBPs): Stabilize single-stranded DNA and prevent template re-annealing.
  • RNA Primase: Synthesizes short RNA primers ($5 - 10$ nucleotides) providing a free $3'\text{-OH}$ group.
  • DNA Polymerase: Catalyzes elongation by adding complementary DNA nucleotides exclusively in the $5' \rightarrow 3'$ direction.
  • Leading Strand: Synthesized continuously toward the replication fork ($5' \rightarrow 3'$).
  • Lagging Strand: Synthesized discontinuously away from fork as short Okazaki fragments ($5' \rightarrow 3'$).
  • DNA Ligase: Covalently seals gaps between Okazaki fragments via phosphodiester bonds.

• DNA Repair Mechanisms:

  • Proofreading: DNA Polymerase detects mismatched bases during polymerization, excising and correcting them instantly.
  • Mismatch Repair: Post-replication enzyme complexes correct mispaired bases escaping proofreading.
  • Nucleotide Excision Repair: Nuclease excises damaged DNA segments (e.g., UV-induced pyrimidine dimers), DNA Polymerase fills gap, DNA Ligase seals backbone.

• Protein Synthesis (Gene Expression):

  • Flow of genetic information: $\text{DNA} \rightarrow \text{mRNA} \rightarrow \text{Protein}$.
  • 1. Transcription:
  • Synthesis of RNA from DNA template in nucleus.
  • Initiation: Transcription factors recognize promoter motifs (e.g., TATA box, CAAT box) upstream of start site, recruiting RNA Polymerase to unwound DNA.
  • Elongation: RNA Polymerase reads template strand in $3' \rightarrow 5'$ direction, synthesizing pre-mRNA in $5' \rightarrow 3'$ direction (replacing Thymine with Uracil).
  • Termination: RNA Polymerase encounters termination signal and releases pre-mRNA.
  • 2. RNA Processing:
  • Pre-mRNA converted to mature mRNA inside nucleus.
  • Splicing: Non-coding sequences (Introns) are excised, and coding sequences (Exons) are ligated together.
  • Mature mRNA exported to cytoplasm via nuclear pores.
  • 3. Translation:
  • Decodes mRNA sequence into amino acids at ribosomes.
  • Genetic Code: Triplet nucleotide sequence on mRNA (Codons). Start codon: AUG (Met); Stop codons: UAA, UAG, UGA.
  • Transfer RNA (tRNA): Translates codons; carries specific amino acid at $3'$ acceptor stem and complementary Anticodon triplet at loop end.
  • Ribosome Structure: Composed of rRNA and proteins arranged in Small and Large Subunits containing three functional sites: A Site (Aminoacyl), P Site (Peptidyl), E Site (Exit).
  • Translation Stages:
    1. Initiation: Small ribosomal subunit binds mRNA at start codon (AUG); initiator tRNA ($\text{Met}$, anticodon UAC) binds; large subunit attaches using GTP energy.
    2. Elongation: Codon recognition at A site $\rightarrow$ peptide bond formation by peptidyl transferase $\rightarrow$ translocation of ribosome one codon along $5' \rightarrow 3'$ direction at GTP expense.
    3. Termination: Release factor binds stop codon at A site, hydrolyzing polypeptide chain from P-site tRNA and disassembling ribosomal complex.

• Gene Regulation and Cell Differentiation:

  • Gene Expression: Process where genetic information directs synthesis of functional protein or RNA products.
  • Cell Differentiation: Unspecialized stem cells turn into specialized cell types with unique structures/functions through selective expression of specific gene subsets while silencing others.

Unit 3: Genetics

Mendelian Inheritance

• Chromosomal Foundation:

  • Humans carry $46$ chromosomes ($23$ pairs) per somatic cell: $22$ autosome pairs and $1$ sex chromosome pair ($\text{XX}$ female, $\text{XY}$ male).
  • Gamete Diversity Mechanisms:
  • Independent Assortment: Random alignment of maternal and paternal homologous chromosomes at Metaphase I ($2^n$ possible combinations, where $n=23$ in humans).
  • Crossing Over: Reciprocal exchange of homologous non-sister chromatid genetic material during Prophase I.

• Mendel's Laws of Inheritance:

  • Principle of Complete Dominance: Dominant allele masks expression of recessive allele in heterozygous genotype.
  • Law of Segregation (First Law): Two alleles for a trait segregate during gamete formation, so each gamete carries only one allele per gene.
  • Law of Independent Assortment (Second Law): Alleles of unlinked gene pairs segregate independently during gamete formation.
  • Phenotypic and Genotypic Ratios:
  • Monohybrid Cross ($Tt \times Tt$): Genotypic ratio $1\text{TT} : 2\text{Tt} : 1\text{tt}$; Phenotypic ratio $3\text{ Dominant} : 1\text{ Recessive}$.
  • Dihybrid Cross ($YyRr \times YyRr$): Phenotypic ratio $9\text{ Both Dominant} : 3\text{ Dom/Rec} : 3\text{ Rec/Dom} : 1\text{ Both Recessive}$.

Non-Mendelian Inheritance

• Incomplete Dominance:

  • Heterozygote expresses an intermediate, blended phenotype between two homozygous phenotypes.
  • Example: Snapdragon (Antirrhinum majus) flower color: Red ($C^R C^R$) $\times$ White ($C^W C^W$) $\rightarrow$ Pink ($C^R C^W$). Cross of Pink $\times$ Pink yields $1\text{ Red} : 2\text{ Pink} : 1\text{ White}$.

• Codominance:

  • Heterozygote fully and simultaneously expresses both allele phenotypes distinctly without blending.
  • Examples:
  • Camellia flower color: Red allele ($C^R$) and White allele ($C^W$) yield red-and-white spotted flowers ($C^R C^W$).
  • MN Blood Group System: Alleles $L^M$ and $L^N$ produce M and N surface antigens; genotype $L^M L^N$ expresses both antigens.

• Multiple Alleles:

  • Genes governed by more than two alternative allelic forms in a population.
  • Example: ABO Blood Group System (governed by $I^A$, $I^B$, and $i$ alleles):
  • $I^A$ produces Antigen A; $I^B$ produces Antigen B; $i$ produces no antigen.
  • $I^A$ and $I^B$ are codominant to each other and both completely dominant over recessive $i$

• Polygenic Inheritance:

  • Single phenotypic trait controlled by additive effects of multiple independent genes, displaying continuous variation (bell-shaped distribution curve).
  • Examples: Human skin pigmentation, wheat kernel color.

• Sex-Linked Traits:

  • Traits governed by genes located on sex chromosomes (primarily X-linked).
  • Human X-Linked Recessive Diseases: Red-Green Color Blindness, Hemophilia, Duchenne Muscular Dystrophy (DMD).
  • Males ($\text{X}^Y\text{Y}$) are hemizygous; a single recessive X-linked allele results in disease expression, making X-linked recessive disorders significantly more frequent in males.

• Sex-Influenced Traits:

  • Autosomal traits whose phenotypic expression differs between sexes due to systemic sex hormone levels (e.g., Male pattern baldness governed by allele $Z$: genotype $HZ$ causes baldness in males due to high testosterone, but non-baldness in females).

• Gene Linkage and Genetic Mapping:

  • Linked Genes: Genes residing close together on the same chromosome tend to be inherited together as a unit, deviating from independent assortment.
  • Recombination Frequency:$\text{Recombination Frequency (\%)} = \frac{\text{Number of Recombinant Offspring}}{\text{Total Number of Offspring}} \times 100\%$
  • $1\%$ Recombination Frequency $= 1\text{ Map Unit (cM)}$ distance on a chromosome map.

• Epigenetics:

  • Heritable modifications in gene expression occurring without changes to underlying DNA nucleotide sequences.
  • Mechanisms:
  • Histone Acetylation: Relaxes chromatin structure, promoting transcription (active gene).
  • DNA Methylation: Addition of methyl groups to cytosine bases condenses chromatin, inhibiting transcription (gene silencing).

Mutations and Genetic Disorders

• Mutations:

  • Alterations in genetic material caused by physical mutagens (UV light, X-rays) or chemical mutagens (tobacco toxins, fungal aflatoxins).

• Gene Mutations:

  • Substitution Mutations: Replacement of one base pair by another.
  • Silent Mutation: Codon change encodes same amino acid (no effect).
  • Missense Mutation: Codon change alters single amino acid in polypeptide.
  • Nonsense Mutation: Codon changes to premature STOP codon, truncating polypeptide.
  • Frameshift Mutations: Insertion or deletion of base pairs in numbers not divisible by 3, altering downstream reading frame.

• Chromosomal Mutations:

  • Numerical Changes (Aneuploidy & Polyploidy):
  • Caused by Nondisjunction (failure of chromosomes/chromatids to separate during Meiosis I or II).
  • Aneuploidy Examples:
    • Down Syndrome: Trisomy 21 ($47, +21$).
    • Turner Syndrome: Monosomy X ($45, \text{XO}$) in females; causes short stature, sterility, cardiovascular issues.
    • Klinefelter Syndrome: XXY ($47, \text{XXY}$) in males; causes underdeveloped testes, learning difficulties.
    • Triple X Female: ($47, \text{XXX}$).
  • Polyploidy: Multiplication of entire chromosome sets ($3n$, $4n$); common in plants (e.g., Hibiscus rosa-sinensis).
  • Structural Changes: Deletion, Duplication, Inversion, Translocation.

• Specific Human Genetic Disorders:

  • Huntington's Disease: Autosomal dominant disorder caused by mutation in HTT gene on chromosome 4, leading to mutant huntingtin protein accumulation in brain neurons (causes motor and cognitive decline, onset at age 30-40).
  • Cystic Fibrosis: Autosomal recessive disorder caused by mutation in CFTR gene on chromosome 7, disrupting chloride ion membrane transport and leading to thick mucus accumulation in lungs and pancreas.

• Prenatal Genetic Diagnosis:

  • Non-invasive maternal blood sampling (analyzing cell-free fetal DNA after 10th week of pregnancy).
  • Amniocentesis (sampling amniotic fluid) and Chorionic Villus Sampling (CVS).

Unit 4: Biotechnology

Tools of Biotechnology

• Enzymes Used in Recombinant DNA Technology:

  • Restriction Endonucleases: Bacterial enzymes that cleave double-stranded DNA at specific palindromic recognition sequences.
  • Cleavage yields Sticky ends (overhanging single-stranded ends) or Blunt ends (double-stranded flush ends).
  • Examples: EcoRI, BamHI, HindIII, PstI.
  • DNA Ligase: Joins DNA fragments by reforming phosphodiester bonds.
  • Taq DNA Polymerase: Thermostable DNA polymerase isolated from Thermus aquaticus, active at high temperatures during PCR.
  • Reverse Transcriptase: Retroviral enzyme synthesizing complementary DNA (cDNA) from mRNA template.

• Gene Vectors:

  • Vehicles used to transfer foreign genetic material into target host cells.
  • Plasmids: Small, circular, self-replicating extrachromosomal bacterial DNA carrying:
  1. Origin of Replication (ORI)
  2. Multiple Cloning Site (MCS) with restriction sites
  3. Promoter region
  4. Selectable Marker Genes (e.g., antibiotic resistance, Green Fluorescent Protein - GFP)
  • Bacteriophages: Viruses used as vectors for larger DNA inserts.
  • Liposomes: Phospholipid vesicles used to deliver genes or drugs in gene therapy.

• Polymerase Chain Reaction (PCR):

  • Automated technique to exponentially amplify target DNA sequences in vitro using a Thermal Cycler.
  • Reaction Components: Template DNA, Primers, Taq Polymerase, dNTPs ($\text{dATP, dCTP, dGTP, dTTP}$).
  • Thermal Cycle Stages:
  1. Denaturation ($94^\circ\text{C} - 96^\circ\text{C}$): Heat breaks hydrogen bonds, separating DNA strands.
  2. Annealing ($55^\circ\text{C} - 65^\circ\text{C}$): Primers bind complementarily to template strands.
  3. Extension ($70^\circ\text{C} - 75^\circ\text{C}$): Taq Polymerase synthesizes new complementary strands.
  • Yield after $n$ cycles = $2^n$ DNA copies.

• Gel Electrophoresis:

  • Method to separate DNA fragments based on molecular size and charge.
  • DNA fragments carry negative charges (phosphate backbone) and migrate toward positive anode through agarose gel matrix.
  • Shorter fragments migrate faster and farther than longer fragments.
  • Visualized under UV light using fluorescent DNA dyes.

Applications of Biotechnology

• DNA Fingerprinting:

  • Profiling technique analyzing Variable Number Tandem Repeats (VNTRs) separated via gel electrophoresis.
  • Used in forensic investigations, paternity testing, and victim identification in disasters.

• Genetic Engineering Steps:

  1. Isolation: Target gene isolated using restriction enzymes, cDNA synthesis, or PCR amplification.
  2. Ligation: Target gene spliced into vector using DNA Ligase to form Recombinant DNA.
  3. Transformation & Selection: Recombinant vector introduced into host cell (E. coli). Transformed cells selected via marker genes (e.g., GFP fluorescence or antibiotic resistance).
  4. Reproduction & Harvesting: Transformed cells cultured in bioreactors to produce commercial recombinant proteins (e.g., Human Insulin, Human Growth Hormone, Factor VIII, Erythropoietin).

• Gene Therapy:

  • Insertion of functional genes into patient cells to correct genetic defects (e.g., treating Cystic Fibrosis or SCID-ADA).

• Cloning:

  • Plant Cloning: Micropropagation tissue culture (e.g., Steward's carrot experiment, orchids).
  • Animal Cloning: Somatic Cell Nuclear Transfer (SCNT), where a diploid nucleus from a somatic cell is fused with an enucleated egg cell, implanted into a surrogate mother (e.g., Dolly the Sheep, 1997).

• Human Genome Project (HGP):

  • Completed in 2003; sequenced all $3+$ billion base pairs of human genome (~99.9% sequence identity across all humans).

• Bioinformatics and Proteomics:

  • Bioinformatics: Application of computer science and software tools to store, search, and analyze biological sequence data (Databases: BLAST, COSMIC, NCBI).
  • Proteomics: Large-scale study of full protein complements (Proteome), structures, and interactions (Human Protein Reference Database - HPRD).

• Ethical Issues in Biotechnology:

  • Biosafety concerns, unintended ecological disruptions, gene editing ethics, biological weapons misuse, and equity of access.