Class 12 Notes

Class 12 Biology Notes Chapter 6 (Molecular basis of inheritance) – Biology Book

Philoid

Philoid

— 12 min read

Biology
Alright class, let's delve deep into the Molecular Basis of Inheritance. This chapter is fundamental, not just for your Class 12 understanding, but it forms the bedrock for many questions in competitive government exams where Biology is a component. Pay close attention to the details, the experiments, and the key molecules involved.

Chapter 6: Molecular Basis of Inheritance - Detailed Notes

1. The Search for Genetic Material

  • Early beliefs pointed towards proteins due to their diversity and complexity. However, experiments gradually established DNA as the genetic material.
  • Griffith's Experiment (1928) - Transformation:
    • Used Streptococcus pneumoniae (bacteria causing pneumonia) with two strains:
      • S-strain (Smooth): Virulent, polysaccharide mucus coat.
      • R-strain (Rough): Non-virulent, no coat.
    • Observations:
      • Living S-strain → Mice die.
      • Living R-strain → Mice live.
      • Heat-killed S-strain → Mice live.
      • Heat-killed S-strain + Living R-strain → Mice die (Living S-strain recovered).
    • Conclusion: Some 'transforming principle' transferred from heat-killed S-strain enabled R-strain to synthesize a smooth coat and become virulent. The biochemical nature wasn't identified yet.
  • Avery, MacLeod, and McCarty Experiment (1933-44) - Biochemical Characterization:
    • Purified biochemicals (proteins, DNA, RNA) from heat-killed S-cells.
    • Tested which one could transform live R-cells into S-cells.
    • Found that digestion with proteases and RNases did not affect transformation.
    • Digestion with DNase did inhibit transformation.
    • Conclusion: DNA is the hereditary material (though not universally accepted immediately).
  • Hershey and Chase Experiment (1952) - Unequivocal Proof:
    • Used bacteriophages (viruses that infect bacteria, T2 phage in this case).
    • Grew phages on two media:
      • One with radioactive phosphorus (³²P) - incorporated into DNA (as P is part of DNA backbone, not protein).
      • One with radioactive sulfur (³⁵S) - incorporated into protein coat (as S is in amino acids methionine/cysteine, not DNA).
    • Allowed these labelled phages to infect E. coli.
    • Steps: Infection → Blending (to remove virus particles attached outside bacteria) → Centrifugation (to separate bacteria from virus particles).
    • Observations:
      • Bacteria infected with ³²P-labelled phages were radioactive (DNA entered the cell).
      • Bacteria infected with ³⁵S-labelled phages were not radioactive; radioactivity was found in the supernatant (protein coat remained outside).
    • Conclusion: DNA is the genetic material that is passed from virus to bacteria.

2. Properties of Genetic Material (DNA vs RNA)

A molecule must fulfill the following criteria to act as genetic material:

  • Replication: Able to generate its own copies. (Both DNA & RNA can)

  • Stability: Chemically and structurally stable. (DNA is more stable due to double helix and deoxyribose sugar - less reactive 2'-OH group compared to ribose in RNA).

  • Mutation: Scope for slow changes (mutations) required for evolution. (Both can mutate, RNA mutates faster).

  • Expression: Able to express itself in the form of 'Mendelian Characters'. (RNA can directly code for protein synthesis, DNA depends on RNA).

  • Why DNA is preferred: Greater stability is crucial for storing genetic information reliably across generations.

  • RNA World Hypothesis: RNA was likely the first genetic material. It can act as genetic material (e.g., some viruses) and as a catalyst (ribozymes). DNA evolved later for better stability.

3. DNA Structure

  • Polynucleotide Chain: DNA is a long polymer of deoxyribonucleotides.
  • Nucleotide Components:
    • Deoxyribose Sugar: A pentose sugar.
    • Nitrogenous Base: Purines (Adenine - A, Guanine - G) and Pyrimidines (Cytosine - C, Thymine - T).
    • Phosphate Group: Makes DNA acidic.
  • Linkages:
    • N-glycosidic linkage: Between sugar and nitrogenous base (forms nucleoside).
    • Phosphoester linkage: Between phosphate and 5'-OH of a nucleoside (forms nucleotide).
    • Phosphodiester linkage: Between 3'-OH of one sugar and 5'-OH of the next sugar via a phosphate group, forming the polynucleotide backbone.
  • Watson and Crick Model (1953) - Double Helix: Based on X-ray diffraction data (Wilkins & Franklin) and Chargaff's rules.
    • Two polynucleotide chains: Backbone of sugar-phosphate, bases project inwards.
    • Antiparallel polarity: One chain runs 5' → 3', the other runs 3' → 5'.
    • Base Pairing (Complementary):
      • A always pairs with T via 2 hydrogen bonds (A=T).
      • G always pairs with C via 3 hydrogen bonds (G≡C).
      • This ensures uniform distance between the two strands.
    • Right-handed helix: The chains coil in a right-handed fashion.
    • Dimensions: Pitch = 3.4 nm (34 Å), ~10 base pairs (bp) per turn. Distance between adjacent bp ≈ 0.34 nm (3.4 Å). Diameter ≈ 2 nm (20 Å).
  • Chargaff's Rules: For double-stranded DNA:
    • Ratio of A to T is 1 (A=T).
    • Ratio of G to C is 1 (G=C).
    • Ratio of Purines (A+G) to Pyrimidines (C+T) is 1. (A+G = C+T).
  • Central Dogma of Molecular Biology (Francis Crick): DNA → (transcription) → RNA → (translation) → Protein. (Reverse transcription, RNA → DNA, occurs in some viruses).

4. Packaging of DNA Helix

  • Length of human DNA (diploid) ≈ 2.2 meters. Nucleus size ≈ 10⁻⁶ meters. Requires extensive packaging.
  • Prokaryotes (e.g., E. coli):
    • No defined nucleus.
    • DNA is circular, held with some (non-histone) proteins in a region called the nucleoid. DNA is organised in large loops.
  • Eukaryotes:
    • More complex organisation.
    • Histones: Set of positively charged (rich in basic amino acids - lysines and arginines), basic proteins. Organised into an octamer (two copies each of H2A, H2B, H3, H4).
    • Nucleosome: Negatively charged DNA wraps around the positively charged histone octamer. A typical nucleosome contains ~200 bp of DNA helix. H1 histone acts as a linker outside the octamer.
    • Chromatin: Repeating units of nucleosomes form a "beads-on-string" structure in the nucleus.
    • Further Condensation: Chromatin is further coiled and condensed to form chromatin fibers, then chromatids, and finally chromosomes (visible during cell division).
    • Non-Histone Chromosomal (NHC) Proteins: Required for higher-level packaging.
    • Euchromatin: Loosely packed, transcriptionally active regions of chromatin (stains light).
    • Heterochromatin: Densely packed, transcriptionally inactive regions (stains dark).

5. DNA Replication

  • Semi-Conservative Replication (Watson & Crick proposal; Meselson & Stahl proof): Each new DNA molecule consists of one parental (conserved) strand and one newly synthesized strand.
  • Meselson and Stahl Experiment (1958):
    • Grew E. coli in ¹⁵N medium (heavy isotope) for many generations. DNA becomes heavy.
    • Transferred bacteria to ¹⁴N medium (normal, light isotope).
    • Took samples after successive generations (20 min, 40 min etc.).
    • Extracted DNA and separated based on density using Cesium Chloride (CsCl) density gradient centrifugation.
    • Results:
      • Generation 0 (¹⁵N): Heavy band only.
      • Generation 1 (20 min): Hybrid band (¹⁵N/¹⁴N) only.
      • Generation 2 (40 min): Hybrid band and Light band (¹⁴N/¹⁴N) in equal amounts.
    • Conclusion: Proved the semi-conservative model. (Similar experiment by Taylor et al. on Vicia faba using radioactive thymidine).
  • Mechanism of Replication:
    • Origin of Replication (ori): Replication begins at specific sites. Prokaryotes usually have one ori; Eukaryotes have multiple.
    • Enzymes Involved:
      • Helicase: Unwinds the DNA double helix by breaking H-bonds.
      • Topoisomerase: Relieves tension/supercoiling ahead of the replication fork.
      • Primase: Synthesizes short RNA primers, providing a free 3'-OH group for DNA polymerase.
      • DNA-dependent DNA Polymerase (Main enzyme): Adds deoxyribonucleotides to the 3'-OH end of the growing chain, using the parental DNA strand as a template. Polymerises only in the 5' → 3' direction. Highly accurate (proofreading ability). E. coli uses DNA Pol III primarily; Eukaryotes have several types.
      • DNA Ligase: Joins DNA fragments (specifically Okazaki fragments) by forming phosphodiester bonds.
    • Replication Fork: Y-shaped structure where unwinding and synthesis occur.
    • Continuous and Discontinuous Synthesis:
      • Leading Strand: Synthesized continuously in the 5' → 3' direction towards the replication fork (on the 3' → 5' template strand).
      • Lagging Strand: Synthesized discontinuously in small fragments (Okazaki fragments) away from the replication fork (on the 5' → 3' template strand). Each fragment requires a primer. Fragments are later joined by DNA ligase.
    • Energy: Provided by deoxyribonucleoside triphosphates (dATP, dGTP, dCTP, dTTP) - terminal phosphates are hydrolysed.

6. Transcription

  • Process of copying genetic information from one strand of DNA into RNA.
  • Principle of complementarity governs the process (except A pairs with U instead of T).
  • Only a segment of DNA and only one of the strands is transcribed.
  • Why both strands are not copied:
    • Would produce two different RNA molecules, coding for different proteins - complicates information transfer.
    • Would produce double-stranded RNA, preventing translation.
  • Transcription Unit: A segment of DNA defined by:
    • Promoter: DNA sequence where RNA polymerase binds (usually towards 5'-end/upstream of structural gene). Defines template strand and coding strand.
    • Structural Gene: Sequence that codes for the RNA molecule (mRNA, tRNA, rRNA).
    • Terminator: DNA sequence where transcription stops (usually towards 3'-end/downstream of structural gene).
  • Template Strand: The DNA strand with polarity 3' → 5' that acts as a template for RNA synthesis.
  • Coding Strand: The DNA strand with polarity 5' → 3' (sequence same as RNA, except T instead of U). Reference point for defining promoter/terminator.
  • Enzyme: DNA-dependent RNA Polymerase: Catalyses polymerization in 5' → 3' direction.
    • Prokaryotes: Single RNA polymerase synthesizes all types of RNA (mRNA, tRNA, rRNA). Needs sigma (σ) factor for initiation. Rho (ρ) factor needed for termination in some cases.
    • Eukaryotes: Three types of RNA polymerases:
      • RNA Pol I: Transcribes rRNAs (28S, 18S, 5.8S).
      • RNA Pol II: Transcribes precursor of mRNA (hnRNA - heterogeneous nuclear RNA) and some snRNAs.
      • RNA Pol III: Transcribes tRNA, 5S rRNA, and snRNAs (small nuclear RNAs).
  • Process of Transcription (Prokaryotes):
    • Initiation: RNA polymerase binds to promoter (facilitated by σ factor). Unwinds DNA locally.
    • Elongation: RNA polymerase moves along the DNA, adding ribonucleotides complementary to the template strand. Sigma factor dissociates.
    • Termination: RNA polymerase reaches the terminator sequence. Nascent RNA and polymerase fall off. May involve Rho factor (Rho-dependent) or occur due to terminator sequence itself (Rho-independent).
  • Process of Transcription (Eukaryotes) - More Complex:
    • Occurs in the nucleus.
    • RNA Pol II transcribes hnRNA.
    • Post-transcriptional Processing: hnRNA undergoes modifications to become functional mRNA before moving to cytoplasm:
      • Splicing: Removal of introns (non-coding intervening sequences) and joining of exons (coding sequences) in a defined order. Catalysed by spliceosome (complex of snRNA and proteins).
      • Capping: Addition of an unusual nucleotide (methyl guanosine triphosphate) to the 5'-end. Protects mRNA, helps in ribosome binding.
      • Tailing: Addition of adenylate residues (200-300) at the 3'-end (poly-A tail). Increases stability, aids transport.
    • Fully processed hnRNA is called mRNA.

7. Genetic Code

  • The relationship between the sequence of nucleotides in mRNA and the sequence of amino acids in a polypeptide.
  • Codon: A sequence of 3 nucleotides in mRNA that specifies a particular amino acid or stop signal.
  • Discovery: Contributions from George Gamow (triplet code idea), Har Gobind Khorana (synthesizing RNA with defined sequences), Marshall Nirenberg (cell-free system for protein synthesis), Severo Ochoa (polynucleotide phosphorylase for RNA synthesis).
  • Salient Features:
    • Triplet Code: 64 possible codons (4³). 61 code for amino acids, 3 are stop codons.
    • Unambiguous: One codon specifies only one amino acid.
    • Degenerate: Some amino acids are coded by more than one codon.
    • Comma-less: Read continuously without punctuation.
    • Nearly Universal: Same code used by most organisms (few exceptions in mitochondria, protozoa).
    • Start Codon: AUG (codes for Methionine - Met). Also acts as the initiator codon.
    • Stop Codons (Nonsense codons): UAA, UAG, UGA. Do not code for any amino acid; signal termination of translation.
  • Mutations and Genetic Code:
    • Point Mutation: Change in a single base pair (e.g., Sickle cell anemia - GAG to GUG in beta-globin gene). Can be silent, missense, or nonsense.
    • Frameshift Mutation: Insertion or deletion of one or two bases changes the reading frame from the point of mutation onwards, often resulting in a non-functional protein. Insertion/deletion of three bases (or multiples) adds/removes amino acids but doesn't shift the frame.

8. Translation

  • Process of polymerization of amino acids to form a polypeptide, based on the sequence of codons in mRNA.
  • Occurs in the cytoplasm on ribosomes.
  • Requirements:
    • mRNA: Template carrying the genetic code.
    • Ribosomes: Cellular machinery for protein synthesis (composed of rRNA and proteins). Consists of large and small subunits. Acts as a catalyst (peptidyl transferase activity resides in rRNA - 23S in prokaryotes, 28S in eukaryotes). Has sites for mRNA and tRNA binding (P-site, A-site, E-site).
    • tRNA (transfer RNA or adapter molecule): Reads the code on mRNA and brings the specific amino acid. Has an anticodon loop (complementary to mRNA codon) and an amino acid acceptor end (3'-end, CCA sequence) where the amino acid binds. Specific tRNAs for each amino acid.
    • Aminoacyl-tRNA Synthetases: Enzymes that link specific amino acids to their corresponding tRNAs ("charging" or "aminoacylation" of tRNA). Requires ATP.
    • Energy (ATP & GTP): For charging tRNA and for ribosome movement.
  • Steps in Translation:
    • Initiation:
      • Small ribosomal subunit binds to mRNA at the start codon (AUG).
      • Initiator tRNA (carrying Met in eukaryotes, fMet in prokaryotes) binds to the start codon in the P-site.
      • Large ribosomal subunit binds, forming the initiation complex. Requires initiation factors and GTP.
    • Elongation: (Cycle repeats)
      • Codon Recognition: Aminoacyl-tRNA corresponding to the next codon binds to the A-site. Requires elongation factors and GTP.
      • Peptide Bond Formation: Peptide bond forms between the amino acid in the P-site and the amino acid in the A-site. Catalysed by peptidyl transferase activity of the ribosome. The polypeptide chain is transferred to the tRNA in the A-site.
      • Translocation: Ribosome moves one codon ahead along the mRNA (5' → 3'). The tRNA carrying the polypeptide chain moves from A-site to P-site. The uncharged tRNA moves from P-site to E-site (exit site) and is released. Requires elongation factors and GTP.
    • Termination:
      • Ribosome reaches a stop codon (UAA, UAG, UGA) in the A-site.
      • Release factors bind to the stop codon.
      • Hydrolysis of the bond between the polypeptide and the tRNA in the P-site, releasing the completed polypeptide.
      • Ribosomal subunits, mRNA, and tRNA dissociate.
  • UTR (Untranslated Regions): Sequences on mRNA before the start codon (5' UTR) and after the stop codon (3' UTR). Required for efficient translation initiation and termination/stability.

9. Regulation of Gene Expression

  • Control of the rate/timing of transcription and translation. Crucial for development, differentiation, and response to environmental changes.
  • Levels of Regulation (Eukaryotes):
    • Transcriptional level (formation of primary transcript) - Primary level.
    • Processing level (regulation of splicing).
    • Transport of mRNA from nucleus to cytoplasm.
    • Translational level.
  • Regulation in Prokaryotes (e.g., Lac Operon): Primarily at the transcriptional initiation level.
    • Operon Concept (Jacob and Monod): A unit of prokaryotic gene expression including structural genes, operator site, promoter site, and regulator gene.
    • Lac Operon (Inducible system in E. coli): Controls lactose metabolism.
      • Components:
        • Regulator Gene (i): Codes for a repressor protein (constitutively expressed).
        • Promoter (p): Binding site for RNA polymerase.
        • Operator (o): Binding site for the repressor protein. Overlaps with promoter.
        • Structural Genes:
          • lacZ: Codes for β-galactosidase (hydrolyses lactose into glucose and galactose).
          • lacY: Codes for permease (increases cell permeability to lactose).
          • lacA: Codes for transacetylase (minor role).
      • Mechanism:
        • In Absence of Inducer (Lactose): Repressor protein binds to the operator region. Prevents RNA polymerase from binding to the promoter and transcribing the structural genes. Operon is 'switched off'. (Negative regulation).
        • In Presence of Inducer (Lactose/Allolactose): Inducer binds to the repressor protein. Causes conformational change in the repressor, making it unable to bind to the operator. Operator site is free. RNA polymerase binds to the promoter and transcribes the structural genes (lacZ, lacY, lacA). Enzymes for lactose metabolism are produced. Operon is 'switched on'.
    • Also subject to positive regulation by Catabolite Activator Protein (CAP) and cyclic AMP (cAMP), which senses glucose levels.

10. Human Genome Project (HGP)

  • Launched in 1990, completed in 2003. Mega project to sequence the entire human genome.
  • Goals:
    • Identify all human genes (approx. 20,000-25,000).
    • Determine the sequence of the 3 billion chemical base pairs.
    • Store this information in databases.
    • Improve tools for data analysis.
    • Transfer related technologies to other sectors.
    • Address the Ethical, Legal, and Social Issues (ELSI) arising from the project.
  • Methodologies:
    • Expressed Sequence Tags (ESTs): Identifying all genes expressed as RNA.
    • Sequence Annotation: Sequencing the whole genome (coding and non-coding) and assigning functions to different regions.
    • Procedure: DNA isolation → Fragmentation → Cloning into vectors (BACs - Bacterial Artificial Chromosomes, YACs - Yeast Artificial Chromosomes) → Amplification → Sequencing using automated sequencers (based on Sanger method - dideoxy chain termination) → Assembly of sequences using computer programs.
  • Salient Features of Human Genome:
    • Genome contains 3164.7 million nucleotide bases.
    • Average gene size: 3000 bases (largest: Dystrophin - 2.4 million bases).
    • Total number of genes estimated at ~20,500 (much lower than previous estimates).
    • Functions unknown for over 50% of discovered genes.
    • Less than 2% of the genome codes for proteins.
    • Large portion consists of repetitive sequences (sequences repeated many times).
    • Chromosome 1 has the most genes (2968), Y has the fewest (231).
    • ~1.4 million locations identified with single-base DNA differences (SNPs - Single Nucleotide Polymorphisms) - useful in finding disease-associated sequences and human history tracing.
  • Applications: Diagnostics, treatment development, understanding human biology and evolution.

11. DNA Fingerprinting

  • Technique to identify differences in specific regions of DNA sequence called repetitive DNA. Developed by Alec Jeffreys.
  • Principle: DNA sequence varies between individuals (DNA polymorphism). Repetitive DNA (satellite DNA) shows high degree of polymorphism.
    • Satellite DNA: Classified based on base composition, length of segment, and number of repetitive units (e.g., micro-satellites, mini-satellites).
    • Variable Number of Tandem Repeats (VNTRs): Mini-satellites used in fingerprinting. The number of repeats is highly variable from person to person, forming a unique pattern. Inherited from parents.
  • Technique (Southern Blotting Hybridization):
    • 1. Isolation of DNA: From cells (blood, hair follicle, skin, saliva etc.).
    • 2. Digestion of DNA by Restriction Endonucleases: Cut DNA at specific sites, generating fragments.
    • 3. Separation of DNA fragments by Gel Electrophoresis: Separates fragments based on size.
    • 4. Transferring (Blotting): Separated DNA fragments transferred from gel to a synthetic membrane (nitrocellulose or nylon). This is Southern Blotting.
    • 5. Hybridization: Membrane exposed to labelled (radioactive or fluorescent) VNTR probes (short DNA sequences complementary to the VNTRs). Probes bind to specific VNTR sequences on the membrane.
    • 6. Detection: Autoradiography (if radioactive probe used) detects the hybridised fragments, revealing a pattern of bands unique to the individual.
  • Applications:
    • Forensic science (crime investigation - matching suspect DNA with crime scene samples).
    • Paternity/Maternity testing.
    • Diagnosis of genetic disorders.
    • Conservation of wildlife.
    • Studying population genetics and evolution.

Multiple Choice Questions (MCQs)

  1. The experiment that provided unequivocal proof that DNA is the genetic material used:
    (A) Streptococcus pneumoniae and mice
    (B) Bacteriophages and radioactive isotopes (³²P and ³⁵S)
    (C) Purified DNA, RNA, and proteins with DNase, RNase, Protease
    (D) Vicia faba and radioactive thymidine

  2. During DNA replication, Okazaki fragments are synthesized on the:
    (A) Leading strand towards the replication fork
    (B) Lagging strand away from the replication fork
    (C) Leading strand away from the replication fork
    (D) Lagging strand towards the replication fork

  3. In eukaryotes, the process of removing introns and joining exons in a primary transcript is called:
    (A) Capping
    (B) Tailing
    (C) Splicing
    (D) Termination

  4. The feature of the genetic code where an amino acid can be specified by more than one codon is known as:
    (A) Unambiguity
    (B) Degeneracy
    (C) Universality
    (D) Non-overlapping

  5. Which molecule acts as an 'adapter' during protein synthesis, carrying a specific amino acid and reading the codon on mRNA?
    (A) rRNA
    (B) hnRNA
    (C) tRNA
    (D) snRNA

  6. In the Lac operon of E. coli, lactose (or allolactose) acts as the:
    (A) Repressor
    (B) Inducer
    (C) Operator
    (D) Promoter

  7. According to Chargaff's rules for double-stranded DNA:
    (A) A + T = G + C
    (B) A/G = C/T
    (C) (A + G) / (C + T) = 1
    (D) The ratio A+T / G+C is constant for all species

  8. A key finding of the Human Genome Project was that:
    (A) The human genome contains over 100,000 genes.
    (B) More than 50% of the genome codes for proteins.
    (C) The vast majority of the genome consists of coding sequences.
    (D) Less than 2% of the genome codes for proteins.

  9. DNA fingerprinting relies on the polymorphism observed in:
    (A) Exons
    (B) Introns
    (C) Satellite DNA (like VNTRs)
    (D) Promoter regions

  10. The basic repeating structural unit of chromatin, consisting of DNA wrapped around a histone octamer, is called a:
    (A) Nucleoid
    (B) Chromosome
    (C) Nucleosome
    (D) Solenoid

Answer Key:

  1. (B)
  2. (B)
  3. (C)
  4. (B)
  5. (C)
  6. (B)
  7. (C)
  8. (D)
  9. (C)
  10. (C)

Make sure you understand the concepts behind each point and MCQ. This chapter requires careful study of processes and the roles of different molecules. Good luck with your preparation!

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