Class 12 Notes

Class 12 Biology Notes Chapter 6 (Molecular Basis of Inheritance) – Biology

Priyanka Rawat

Priyanka Rawat

— 11 min read

Biology

Okay, here are detailed notes for Chapter 6: Molecular Basis of Inheritance from the NCERT Class 12 Biology textbook, suitable for government exam preparation.

Chapter 6: Molecular Basis of Inheritance

1. The DNA (Deoxyribonucleic Acid)

  • Structure:
    • DNA is a long polymer of deoxyribonucleotides.
    • Length is defined by the number of nucleotide pairs (base pairs - bp).
    • Nucleotide Components:
      • A Pentose Sugar (Deoxyribose)
      • A Nitrogenous Base (Purines: Adenine (A), Guanine (G); Pyrimidines: Cytosine (C), Thymine (T))
      • A Phosphate Group
    • Nucleoside: Nitrogenous base + Sugar (e.g., Adenosine, Deoxyadenosine)
    • Nucleotide: Nucleoside + Phosphate group (e.g., Deoxyadenylic acid)
    • Polynucleotide Chain Formation: Two nucleotides are linked by a 3'-5' phosphodiester bond. The phosphate group links the 5'-OH of one sugar to the 3'-OH of the next sugar. This forms a backbone of sugar and phosphate. The nitrogenous bases project inwards from the backbone.
  • Watson and Crick Model (B-DNA):
    • Proposed a double helix structure based on X-ray diffraction data (Wilkins & Franklin) and Chargaff's rules.
    • Salient Features:
      • Two polynucleotide chains coiled in a right-handed fashion.
      • Backbone: Sugar-phosphate; Bases project inwards.
      • Antiparallel Polarity: One chain runs 5' → 3', the other runs 3' → 5'.
      • Base Pairing: A pairs with T (2 hydrogen bonds); G pairs with C (3 hydrogen bonds). This is complementary base pairing. Purine always pairs with a Pyrimidine, maintaining uniform distance between strands.
      • Chargaff's Rules: For double-stranded DNA, the ratios A/T and G/C are equal to 1 (A+G = T+C).
      • Dimensions: Pitch of helix = 3.4 nm (34 Å); ~10 bp per turn. Distance between adjacent bp = 0.34 nm (3.4 Å). Diameter = 2 nm (20 Å).
      • Stability provided by H-bonds between bases and stacking interactions between base pairs.
  • Packaging of DNA Helix:
    • Prokaryotes (e.g., E. coli): DNA is not scattered; held with proteins (non-histone, positively charged) in a region called the nucleoid. DNA is organised in large loops.
    • Eukaryotes: More complex packaging.
      • Histones: Set of positively charged, basic proteins (rich in lysine and arginine). Organised into an octamer (two molecules 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 links nucleosomes.
      • Chromatin: Repeating units of nucleosomes form a structure resembling 'beads-on-string'.
      • Further coiling and condensation form chromatin fibers, then chromatids, and finally chromosomes (visible during cell division).
      • Euchromatin: Loosely packed, transcriptionally active regions. Stains light.
      • Heterochromatin: Densely packed, transcriptionally inactive regions. Stains dark.

2. The Search for Genetic Material

  • Griffith's Experiment (1928) - Transforming Principle:
    • Used Streptococcus pneumoniae (bacteria causing pneumonia).
    • S-strain (smooth, virulent, polysaccharide coat) & R-strain (rough, non-virulent, no coat).
    • Observations:
      • S-strain injected into mice → Mice die.
      • R-strain injected into mice → Mice live.
      • Heat-killed S-strain injected → Mice live.
      • Heat-killed S-strain + Live R-strain injected → Mice die. (Live 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 was not defined.
  • 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.
    • Observations:
      • Digestion with proteases (protein-digesting enzymes) did not affect transformation.
      • Digestion with RNases (RNA-digesting enzymes) did not affect transformation.
      • Digestion with DNases (DNA-digesting enzymes) inhibited transformation.
    • Conclusion: DNA is the hereditary material (transforming principle). Not universally accepted by biologists at the time.
  • Hershey and Chase Experiment (1952) - Unequivocal Proof:
    • Used bacteriophages (viruses that infect bacteria, e.g., T2 phage infecting E. coli).
    • Phages grown in two media:
      • One with radioactive phosphorus (³²P) - labels DNA (phosphate backbone).
      • One with radioactive sulfur (³⁵S) - labels protein coat (contains sulfur).
    • Steps:
      • Infection: Allowed phages to infect E. coli.
      • Blending: Agitated in a blender to separate phage coats from bacteria.
      • Centrifugation: Spun to separate bacteria (pellet) from lighter phage particles (supernatant).
    • Observations:
      • Bacteria infected with ³²P-labeled phages were radioactive (DNA entered). Radioactivity was low in the supernatant.
      • Bacteria infected with ³⁵S-labeled phages were not radioactive (protein coat stayed outside). Radioactivity was high in the supernatant.
    • Conclusion: DNA, not protein, entered the bacteria from the virus. Therefore, DNA is the genetic material.

3. 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 replicate).
    • Stability: Chemically and structurally stable. (DNA is more stable than RNA).
      • DNA: Double-stranded, presence of Thymine (more stable than Uracil), Deoxyribose sugar (less reactive -OH group at 2').
      • RNA: Single-stranded (usually), presence of Uracil, Ribose sugar (reactive -OH group at 2'), making it labile and easily degradable. RNA is also catalytic, hence reactive.
    • Mutation: Scope for slow changes (mutations) required for evolution. (Both DNA & RNA can mutate). RNA mutates faster.
    • Expression: Able to express itself in the form of 'Mendelian Characters'. (RNA directly codes for proteins; DNA depends on RNA for protein synthesis).

  • Conclusion: DNA, being more stable, is preferred for storage of genetic information. RNA is better for transmission of genetic information and catalysis.

4. RNA World

  • RNA was likely the first genetic material.
  • Essential life processes (metabolism, splicing, translation) evolved around RNA.
  • RNA acted as both genetic material and catalyst (e.g., Ribozymes).
  • DNA evolved later from RNA for better stability, taking on the role of primary genetic storage.

5. Replication

  • Semiconservative Replication: Proposed by Watson & Crick. Each new DNA molecule consists of one parental (conserved) strand and one newly synthesized strand.
  • Meselson and Stahl's Experiment (1958) - Experimental Proof:
    • Used E. coli and heavy isotope of nitrogen (¹⁵N) and normal (¹⁴N).
    • Grew E. coli in ¹⁵N medium for many generations (DNA becomes ¹⁵N-¹⁵N).
    • Transferred to ¹⁴N medium and took samples after successive generations (20 min, 40 min).
    • Used Cesium Chloride (CsCl) density gradient centrifugation to separate DNA based on density.
    • Observations:
      • Generation 0 (¹⁵N): Heavy band.
      • Generation 1 (20 min): Hybrid band (¹⁵N-¹⁴N), intermediate density.
      • Generation 2 (40 min): One light band (¹⁴N-¹⁴N) and one hybrid band (¹⁵N-¹⁴N), equal amounts.
    • Conclusion: Confirmed the semiconservative mode of DNA replication. (Similar experiments by Taylor et al. on Vicia faba using radioactive thymidine).
  • The Machinery and Enzymes:
    • DNA-dependent DNA Polymerase: Main enzyme. Catalyzes polymerization only in 5' → 3' direction. Requires a template and a primer. Very accurate. High processivity. (e.g., DNA Pol III in prokaryotes). Also has proofreading capabilities (3'→5' exonuclease activity). DNA Pol I removes RNA primers and fills gaps.
    • Helicase: Unwinds the DNA helix, separating the two strands using ATP energy.
    • Topoisomerase: Relieves tension/supercoiling created by unwinding.
    • Primase: Synthesizes short RNA primers, providing a free 3'-OH end for DNA polymerase to start synthesis.
    • DNA Ligase: Joins DNA fragments (Okazaki fragments) by forming phosphodiester bonds.
    • Single-Strand Binding Proteins (SSBPs): Stabilize the separated single strands and prevent re-annealing.
  • Process of Replication:
    • Origin of Replication (ori): Replication begins at specific sites. Prokaryotes usually have one ori; Eukaryotes have multiple ori.
    • Replication Fork: Y-shaped structure formed when helicase unwinds DNA.
    • Activation of Deoxyribonucleotides: dAMP, dGMP, dCMP, dTMP are converted to active triphosphates (dATP, dGTP, dCTP, dTTP) which provide energy and act as substrates.
    • Primer Synthesis: Primase synthesizes RNA primers.
    • Elongation: DNA polymerase adds deoxyribonucleotides complementary to the template strand in the 5' → 3' direction.
      • Leading Strand: Synthesized continuously towards the replication fork (template polarity 3' → 5').
      • Lagging Strand: Synthesized discontinuously away from the fork in short fragments called Okazaki fragments (template polarity 5' → 3'). Requires multiple primers.
    • Primer Removal & Gap Filling: RNA primers are removed (by DNA Pol I in prokaryotes) and replaced with DNA.
    • Joining: DNA ligase joins the Okazaki fragments.
    • Replication in eukaryotes occurs during the S-phase of the cell cycle.

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 usually only one strand is transcribed.
  • Transcription Unit: A segment of DNA defined by three regions:
    • Promoter: DNA sequence where RNA polymerase binds (upstream, 5'-end of structural gene). Defines template strand and transcription start site.
    • Structural Gene: DNA sequence that is transcribed into RNA. The strand with polarity 3'→5' acts as the template strand. The other strand (5'→3') is the coding strand (sequence same as RNA, except T instead of U).
    • Terminator: DNA sequence where transcription stops (downstream, 3'-end of structural gene).
  • Enzyme: DNA-dependent RNA Polymerase. Catalyzes polymerization in 5' → 3' direction. Does not require a primer.
  • Transcription in Prokaryotes:
    • Single RNA polymerase synthesizes all types of RNA (mRNA, tRNA, rRNA).
    • Initiation: RNA polymerase binds to promoter, aided by sigma (σ) factor. Unwinds DNA locally.
    • Elongation: RNA polymerase moves along the DNA, adding ribonucleotides using the template strand. Sigma factor dissociates.
    • Termination: Reaches terminator sequence. RNA polymerase separates from DNA and the nascent RNA is released. Termination requires Rho (ρ) factor (Rho-dependent) or occurs due to specific sequences (Rho-independent).
    • mRNA does not require processing; transcription and translation can be coupled.
  • Transcription in Eukaryotes: More complex.
    • Three RNA Polymerases:
      • RNA Pol I: Transcribes rRNAs (28S, 18S, 5.8S).
      • RNA Pol II: Transcribes precursor of mRNA (hnRNA) and snRNAs.
      • RNA Pol III: Transcribes tRNA, 5S rRNA, and snRNAs.
    • Processing of primary transcript (hnRNA): Occurs in the nucleus.
      • Splicing: Removal of non-coding sequences (introns) and joining of coding sequences (exons) in a defined order. Carried out by spliceosome (complex of snRNA and proteins).
      • Capping: Addition of an unusual nucleotide (methyl guanosine triphosphate) to the 5'-end.
      • Tailing: Addition of polyadenylate residues (poly-A tail, 200-300) at the 3'-end.
    • Fully processed hnRNA is now called mRNA and is transported out of the nucleus for translation.

7. Genetic Code

  • The sequence of nucleotides in mRNA that specifies the sequence of amino acids in a protein.
  • Codon: A sequence of 3 nucleotides that codes for a specific amino acid.
  • Properties of Genetic Code:
    • Triplet: Each codon consists of 3 bases (64 possible codons: 4³).
    • Universal: A specific codon codes for the same amino acid in almost all organisms (some exceptions in mitochondria, protozoa).
    • Degenerate (Redundant): Some amino acids are coded by more than one codon (except AUG - Met, UGG - Trp).
    • Non-overlapping: Codons are read sequentially without overlapping.
    • Unambiguous: One codon specifies only one particular amino acid.
    • Comma-less: Read continuously without punctuation.
    • Start Codon: AUG (codes for Methionine, also acts as initiator).
    • Stop Codons (Nonsense codons): UAA, UAG, UGA (do not code for any amino acid, signal termination).
  • Mutations and Genetic Code:
    • Point Mutation: Change in a single base pair (e.g., Sickle cell anemia - GAG to GUG in beta-globin chain, Glu to Val).
    • Frameshift Mutation: Insertion or deletion of one or two bases, changes the reading frame from the point of mutation onwards, resulting in a completely different protein. Insertion/deletion of three bases (or multiples) adds/removes amino acids but doesn't shift the reading frame.

8. Translation

  • Process of polymerization of amino acids to form a polypeptide, based on the sequence of codons in mRNA.
  • Occurs in the ribosome.
  • Requirements: mRNA (template), Ribosomes (structural & catalytic role), tRNA (adapter molecule), Amino acids, Enzymes (aminoacyl-tRNA synthetases), Energy (ATP, GTP).
  • tRNA (Transfer RNA) - The Adapter Molecule:
    • Reads the genetic code on mRNA and brings the specific amino acid.
    • Structure: Clover-leaf shape (2D), inverted L-shape (3D).
    • Has an Anticodon loop with bases complementary to the mRNA codon.
    • Has an Amino acid acceptor end (3'-end, CCA sequence) where the amino acid binds.
    • Specific tRNAs for each amino acid.
    • Aminoacylation (Charging) of tRNA: Activation of amino acid (using ATP) and linking it to its cognate tRNA, catalyzed by aminoacyl-tRNA synthetase.
  • Ribosomes: Cellular factories for protein synthesis. Composed of rRNA and proteins. Consist of large and small subunits.
    • Small subunit binds mRNA.
    • Large subunit has binding sites for tRNA (A-site: aminoacyl-tRNA, P-site: peptidyl-tRNA, E-site: exit) and catalyzes peptide bond formation (enzyme: peptidyl transferase, actually a function of 23S rRNA in bacteria / 28S rRNA in eukaryotes - ribozyme).
  • Process of Translation:
    • Initiation:
      • Small ribosomal subunit binds to mRNA at the start codon (AUG).
      • Initiator tRNA (carrying Methionine/fMet in prokaryotes) binds to the start codon.
      • Large ribosomal subunit binds, forming the initiation complex. Initiator tRNA is in the P-site. Requires GTP.
    • Elongation:
      • Charged tRNA with anticodon complementary to the next codon enters the A-site.
      • Peptide bond forms between the amino acid on the tRNA in the P-site and the amino acid on the tRNA in the A-site (catalyzed by peptidyl transferase).
      • Ribosome translocates one codon along the mRNA (towards 3'-end). Requires GTP.
      • The tRNA carrying the growing polypeptide chain moves from A-site to P-site.
      • Uncharged tRNA moves from P-site to E-site and exits.
      • Cycle repeats, adding amino acids one by one.
    • Termination:
      • Ribosome reaches a stop codon (UAA, UAG, UGA) in the A-site.
      • No tRNA binds to stop codons. Release factors bind to the stop codon.
      • Polypeptide chain is released from the tRNA.
      • Ribosomal subunits and mRNA dissociate.
  • Untranslated Regions (UTRs): Sequences on mRNA before the start codon (5' UTR) and after the stop codon (3' UTR). They are required for efficient translation initiation and termination/stability.

9. Regulation of Gene Expression

  • Gene expression results in the formation of a polypeptide; it is a highly regulated process.
  • Regulation can occur at multiple levels, especially in eukaryotes (transcriptional, processing, transport, translational).
  • Regulation allows cells to produce specific proteins only when needed, conserving energy and resources. It's crucial for development, differentiation, and response to environmental changes.
  • Lac Operon (Jacob and Monod, 1961) - Example in Prokaryotes (E. coli):
    • An inducible operon system regulating lactose metabolism.
    • Components:
      • Regulator gene (i): Codes for a repressor protein. Constitutively expressed.
      • Operator (o): Sequence where the repressor protein binds. Located adjacent to the promoter.
      • Promoter (p): Binding site for RNA polymerase.
      • Structural genes:
        • lacZ: Codes for β-galactosidase (hydrolyzes lactose into glucose and galactose).
        • lacY: Codes for permease (increases cell permeability to lactose).
        • lacA: Codes for transacetylase.
    • Mechanism:
      • In Absence of Inducer (Lactose): The i gene produces repressor protein → Repressor binds to the operator region (o) → Prevents RNA polymerase from binding to the promoter (p) and transcribing the structural genes → Operon is 'switched off'.
      • In Presence of Inducer (Lactose/Allolactose): Lactose (or its isomer allolactose) acts as the inducer → Inducer binds to the repressor protein → Causes conformational change in the repressor, making it unable to bind to the operator → Operator 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'.
    • Regulation of lac operon is also under positive control by Catabolite Activator Protein (CAP) and cyclic AMP (cAMP), which is related to glucose levels (low glucose -> high cAMP -> CAP activation -> enhanced transcription).

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 sequences of the 3 billion chemical base pairs.
    • Store information in databases.
    • Improve tools for data analysis.
    • Transfer technologies to other sectors.
    • Address ethical, legal, and social issues (ELSI).
  • 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. (Used BAC - Bacterial Artificial Chromosomes and YAC - Yeast Artificial Chromosomes for cloning, followed by sequencing using automated sequencers - Sanger method).
  • 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 ~30,000 (much lower than previous estimates). Now revised to ~20,000-25,000.
    • Almost all (99.9%) nucleotide bases are exactly the same in all people.
    • 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 for finding chromosomal locations for disease-associated sequences and human history tracing.
  • Applications and Future Challenges: Diagnosis, treatment, prevention of genetic disorders; understanding human biology, evolution; deriving new biotechnological applications. Requires bioinformatics, high-speed computation.

11. DNA Fingerprinting

  • Technique to identify differences in specific regions of DNA sequence called repetitive DNA. These regions show high degree of polymorphism (variation at genetic level).
  • Principle: Based on variations in Variable Number of Tandem Repeats (VNTRs) - short nucleotide sequences repeated tandemly many times. The number of repeats is highly variable between individuals, forming the basis of identification. VNTRs are a type of minisatellite.
  • Technique (Developed by Alec Jeffreys): Involves Southern Blotting Hybridisation using radiolabelled VNTR probes.
    • Isolation of DNA.
    • Digestion of DNA by Restriction Endonucleases.
    • Separation of DNA fragments by Gel Electrophoresis.
    • Transferring (Blotting) separated DNA fragments to synthetic membranes (nitrocellulose or nylon).
    • Hybridisation using labelled VNTR probe.
    • Detection of hybridised DNA fragments by autoradiography.
  • The pattern of bands obtained (autoradiogram) is unique to an individual (except identical twins).
  • PCR (Polymerase Chain Reaction) can be used to amplify DNA if the sample size is small.
  • Applications: Forensic science (crime investigation), Paternity testing, Determining population diversity, Studying genetic evolution.

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