Chapter 14 · Class 11-12 & NEET
Evolution
37 topics in 13 parts
Key ideas
| Biological Parameter | Primary / Normal Condition | Alternative / Specialized Variant |
|---|---|---|
| Morphological Feature | Conserved anatomical architecture | Adapted/Modified structure for specialized function |
| Energetic / Metabolic State | Basal physiological activity | Induced / Stress-responsive activation |
| Regulatory Control | Endocrine & enzymatic feedback | Autonomous / Environmental signaling |
| Evolutionary Fitness Impact | Essential for organismal survival | Enhances reproductive efficiency & niche adaptation |
Conceptual Foundation & Core Theory
Genetics and Molecular Inheritance form the bedrock of modern biology, establishing how chemical information stored in polymeric deoxyribonucleic acid (DNA) is faithfully replicated, transmitted across generations, and expressed as cellular proteins via the Central Dogma.
1. The Molecular Blueprint: DNA Structure & Replication
According to the Watson-Crick double helix model (1953):
- DNA consists of two antiparallel polynucleotide chains ($5' \to 3'$ and $3' \to 5'$) with a sugar-phosphate backbone outside and purine-pyrimidine base pairs projected inward.
- Adenine pairs with Thymine via two hydrogen bonds ($A=T$); Guanine pairs with Cytosine via three hydrogen bonds ($G\equiv C$). Hence, purines equal pyrimidines: $\frac{A+G}{T+C} = 1$ (Chargaff's Rule).
- Semiconservative Replication (Meselson & Stahl, 1958): Each daughter duplex retains one conserved parental strand and one newly synthesized strand. Catalyzed by DNA-dependent DNA polymerase which adds dNTPs exclusively in the $5' \to 3'$ direction. Consequently, the leading strand is synthesized continuously towards the replication fork, whereas the lagging strand is synthesized discontinuously as Okazaki fragments, later spliced by DNA ligase.
2. Mendelian Genetics & Allelic Interactions
Gregor Mendel deduced fundamental laws of transmission genetics through garden pea (Pisum sativum) hybridization experiments:
- Law of Segregation (Purity of Gametes): Alleles of a gene separate during meiosis such that each gamete receives only one allele. This law has no biological exceptions in sexual organisms.
- Law of Independent Assortment: Alleles of different unlinked gene pairs assort independently during gamete formation, generating an $F_2$ phenotypic ratio of $9:3:3:1$ in a dihybrid cross. (Note: Genes located close together on the same chromosome exhibit linkage, discovered by T.H. Morgan in Drosophila, violating independent assortment).
- Non-Mendelian Ratios: Incomplete dominance (e.g. Mirabilis jalapa, $1:2:1$ red, pink, white) and Codominance (e.g. ABO blood group $I^A, I^B, i$).
Key Scientific Principles, Pathways & Diagnostic Points
// Molecular & Genetic Ratios & Governing Laws
1. Chargaff's Equivalence Rule (dsDNA):
%A = %T and %G = %C => (A + G) = (T + C)
If DNA has 20% Cytosine → G = 20%, so G + C = 40% → A + T = 60% → A = 30%, T = 30%
2. Mendelian F2 Dihybrid Cross (RrYy x RrYy):
Phenotypic Ratio : 9 (Round Yellow) : 3 (Round Green) : 3 (Wrinkled Yellow) : 1 (Wrinkled Green)
Genotypic Ratio : 1:2:1:2:4:2:1:2:1 (9 distinct genotypes)
3. Lac Operon Regulatory Dynamics:
- Absence of Inducer (Lactose) : Lac Repressor (i gene) binds Lac Operator (O) → Blocks RNA Polymerase
- Presence of Inducer (Allolactose): Binds Repressor protein → Inactivates it → RNA Pol transcribes:
* lacZ (β-Galactosidase)
* lacY (Permease)
* lacA (Transacetylase)Core Genetic Processes: Replication vs Transcription vs Translation
| Feature / Parameter | Replication | Transcription | Translation |
|---|---|---|---|
| Template Used | Both strands of genomic dsDNA | Single antisense (3' to 5') template strand | Mature mRNA transcript (reading 5' to 3') |
| Key Catalytic Enzyme | DNA Polymerase III / I + Primase | DNA-dependent RNA Polymerase | Ribosome (23S/28S rRNA peptidyl transferase) |
| Substrates Consumed | Deoxyribonucleoside triphosphates (dNTPs) | Ribonucleoside triphosphates (rNTPs: ATP, UTP, GTP, CTP) | Aminoacyl-tRNA complexes + GTP/ATP |
| Polymerization Direction | Strictly 5' → 3' | Strictly 5' → 3' | N-terminus to C-terminus of polypeptide chain |
NEET Problem: Molecular Chargaff Calculation
NEET Problem: A sample of double-stranded DNA contains 340 Adenine nucleotides, representing 34% of the total base pairs. Calculate the number of Cytosine bases present in this DNA molecule.
Step-by-step Solution:
1. By Chargaff’s Rule in dsDNA: $%A = %T$. Therefore, $%T = 34\%$.
2. Total percentage of Adenine + Thymine: $34\% + 34\% = 68\%$.
3. Remaining percentage belonging to Guanine + Cytosine: $100\% - 68\% = 32\%$.
4. Because $%G = \%C$, the proportion of Cytosine is: $\frac{32\%}{2} = 16\%$.
5. If $34\%$ equals 340 bases, the total number of bases is: $\frac{340}{0.34} = 1000$ nucleotides.
6. Cytosine count: $16\% \text{ of } 1000 = 160$ Cytosine bases.
Answer: 160 Cytosine bases.
Checkpoint
1. During DNA replication in E. coli, which enzyme unwinds the double helix at the replication fork by breaking hydrogen bonds between complementary bases?
All 37 topics in Evolution
Human Evolution
- Pre-Historic Man & Evolutionary Stages
- Homo Species & Modern Human Origin
- Primate Phylogeny & Chromosomal Homology
Natural Selection
- Stabilising, Directional & Disruptive Selection
- Industrial Melanism & Anthropogenic Selection
- Antibiotic & Herbicide Resistance
Adaptive Radiation
- Darwin's Finches & Galapagos Islands
- Australian Marsupials & Placental Mammals
- Biogeographical Distribution & Island Fauna
Convergent Evolution
- Analogous Organs & Functional Adaptations
- Parallel Adaptation in Marsupials & Placentals
Hardy-Weinberg Law
- Equilibrium Equation & Allele Frequencies
- Factors Affecting Equilibrium
- Gene Pool & Gene Frequency
Theories of Evolution
- Lamarckism & Inheritance of Acquired Characters
- Mutation Theory & Saltation
- Modern Synthetic Theory & Speciation
- Embryological Support & Disproof by von Baer
Origin of Life
- Big Bang Theory & Earth Formation
- Primitive Earth & Reducing Atmosphere
- Chemical Evolution & Oparin-Haldane Hypothesis
- Spontaneous Generation & Biogenesis Experiments
Geological Time Scale
- Eras, Periods & Dominant Life Forms
- Vertebrate & Reptilian Diversification
- Mass Extinctions & Dinosaur Record
Homologous Organs
- Divergent Evolution & Structural Homology
- Comparative Anatomy & Forelimb Modification
- Vestigial Organs & Atavism
Genetic Drift
- Founder Effect & Population Bottleneck
- Random Fluctuations in Allele Frequencies
Miller-Urey Experiment
- Spark Discharge Apparatus & Gas Mixture
- Amino Acid Synthesis & Organic Molecules
Darwinism
- Voyage of HMS Beagle & Field Observations
- Malthusian Essay on Population & Overproduction
- Branching Descent & Survival of Fittest
Fossil Evidence
- Paleontological Records & Coelacanth
- Living Fossils & Connecting Links