Molecular Basis of Inheritance – Comprehensive Study Notes

DNA: Structure, Nucleotides & Base Pairing

• Nucleic acids store genetic information; two main types: DNA and RNA.
• Building blocks: nucleotides = sugar + phosphate group + nitrogenous base.
  • Nucleotide structure: 3 components
  • Pentose sugar (deoxyribose in DNA; ribose in RNA)
  • Phosphate group
  • Nitrogenous base
  • N-glycosidic bond links base to the sugar forming a nucleoside.
• Nitrogenous bases:
  • Purines: Adenine (A), Guanine (G)
  • Pyrimidines: Cytosine (C), Thymine (T) in DNA; Uracil (U) in RNA
  • Thymine vs Uracil: DNA has thymine; RNA has uracil (thymine not in RNA)
  • Note: Thymine is 5-methyl uracil (5-methyluracil).
• DNA structure basics:
  • DNA is a polymer of nucleotides (polynucleotide chain).
  • Backbone made of sugar–phosphate linkages; nitrogenous bases project inward.
  • Two polynucleotide chains form a double helix; antiparallel orientation: one runs 5′→3′, the other 3′→5′.
• Base pairing:
  • A pairs with T via 2 hydrogen bonds; G pairs with C via 3 hydrogen bonds.
  • Base pairing maintains uniform distance between strands.
• Polarity and direction:
  • DNA synthesis occurs 5′ to 3′; nucleotides added to the 3′-OH end.
• DNA: dimensional measurements (typical values):
  • Distance between adjacent base pairs: $d_{bp} = 0.34\text{ nm}$
  • One full helical turn contains 10 base pairs; pitch per turn: $P = 3.4\text{ nm}$
  • Length per bp: 0.34 nm; per turn: 10 bp × 0.34 nm = 3.4 nm
• Nucleosides and Nucleotides:
  • Nucleoside = base + sugar (without phosphate)
  • Nucleotide = nucleoside + one or more phosphate groups
  • RNA nucleosides include ribose; DNA nucleosides include deoxyribose.
• DNA replication and transcription prerequisites (overview):
  • Replication copies DNA; transcription copies DNA into RNA.
  • Central dogma (Crick): DNA → RNA → Protein; reverse transcription in some viruses (e.g., HIV).

Structure of the Polynucleotide Chain

• Polynucleotides are polymers of nucleotides; DNA and RNA are polynucleotides.
• Components of a nucleotide:
  • Pentose sugar (5-membered ring)
  • Phosphate group
  • Nitrogenous base
• In DNA, the sugar is deoxyribose; in RNA, it is ribose.
• A nucleotide is formed by linking a phosphate group to the 5′ carbon of the sugar and a base to the 1′ carbon via N-glycosidic bond.
• Nucleosides vs nucleotides: nucleoside = base + sugar; nucleotide = nucleoside + phosphate(s).

DNA Structure: Double Helix & Packaging

• DNA is composed of two polynucleotide chains coiled into a right-handed double helix.
• Backbones are sugar–phosphate; bases project inward where they form base pairs.
• Anti-parallel polarity: one strand 5′→3′; the other 3′→5′.
• Base pairs form hydrogen bonds: A=T (2 H-bonds), G≡C (3 H-bonds).
• Measurements:
  • Distance between base pairs: $0.34\text{ nm}$
  • Diameter of helix: not explicitly stated here
  • Length per turn: 10 bp, corresponding to $3.4\text{ nm}$ per turn
  • Overall helical pitch: $3.4\text{ nm}$ per turn; base pair spacing $0.34\text{ nm}$
• Chargaff’s rule (later section): in DNA, [A] ≈ [T] and [G] ≈ [C].
• DNA major and minor grooves are formed by the helical geometry; bases are inside the helix; backbone on the outside.
• In DNA, the sugar–phosphate backbone is negatively charged; histones and other proteins help with packaging in eukaryotes.

DNA Packaging and Chromatin (Eukaryotes vs Prokaryotes)

• Prokaryotes:
  • DNA is not free-floating; forms a nucleoid with associated basic/positively charged proteins.
  • Plasmid DNA exists in some bacteria; chromosomal DNA is the main genome.
  • Size example: plasmid DNA shown in TEM image; DNA is highly compacted inside a small cell.
• Eukaryotes:
  • DNA wraps around histone proteins to form nucleosomes.
  • Histones are rich in positively charged residues (lysine, arginine).
  • Nucleosome: core of histone octamer (H2A, H2B, H3, H4) with DNA wrapped around it; H1 stabilizes the structure.
  • A typical nucleosome contains ~200 bp of DNA.
  • Chromatin fibers: beads-on-a-string form (11 nm fiber); further condensed to 30 nm chromatin fiber; higher-order packaging leads to metaphase chromosome (~700 nm).
  • Euchromatin vs Heterochromatin:
  • Euchromatin: loosely packed, transcriptionally active (stains lighter).
  • Heterochromatin: densely packed, transcriptionally inactive (stains darker).
  • Telomeres and centromeres are key structural elements; heterochromatin is often enriched at these regions.
• Quantitative notes:
  • Human genome has about 6.6 × 10^9 bp per diploid genome; nucleosome repeat unit ~200 bp; thus ~3.3 × 10^7 nucleosomes in a haploid such genome, or ~6.6 × 10^7 per diploid
  • The 11 nm bead-on-a-string to 30 nm chromatin fiber describes the progression of packaging levels.

The Search for Genetic Material

• Griffith’s Transforming Principle (1928):
  • S (smooth, virulent) and R (rough, non-virulent) strains of Streptococcus pneumoniae.
  • Heat-killed S strain plus live R strain transferred a transforming principle to R, converting it to S (virulent) phenotype; implies transfer of genetic material.
• Biochemical characterization (Avery–MacLeod–McCarty, 1944):
  • Enzymatic digestion experiments showed that DNase inhibited transformation, while protease and RNase did not; DNA was the transforming principle.
• Hershey–Chase experiment (Blender experiment, 1952):
  • Used bacteriophages labeled with radioactive P-32 (DNA) and S-35 (protein).
  • After infection and blending, only the DNA entered bacterial cells and directed viral replication; protein did not enter cells.
  • Conclusion: DNA is the genetic material.

Properties of Genetic Material

• Essential properties:
  • Ability to replicate (replication)
  • Chemical and structural stability
  • Capacity to mutate (allow evolution)
  • Ability to express information as Mendelian traits (transcription/translation)
• Stability vs mutability:
  • DNA is stable due to double-stranded structure and lack of 2′-OH on the sugar; RNA is more reactive due to 2′-OH and single-stranded nature, leading to higher mutability.
  • RNA viruses mutate faster due to low stability of RNA.
• Central Dogma (Crick): DNA → RNA → Protein; reverse transcription in some viruses (e.g., retroviruses like HIV).
• RNA world concept (in many notes): RNA could have been the original genetic material and catalyst; DNA evolved later for stability; protein-based catalysis emerged later.

DNA Replication: Semiconservative Model & Machinery

• Semiconservative replication (Watson–Crick proposal; later proven by Meselson–Stahl, 1958):
  • Each daughter DNA molecule consists of one parental strand and one newly synthesized strand.
• The Meselson–Stahl experiment:
  • Two E. coli cultures: one grown in heavy nitrogen (^15N) and then shifted to light nitrogen (^14N).
  • First generation after shift yielded DNA of intermediate density (hybrid: 15N/14N).
  • Second generation yielded both hybrid and light DNA, supporting semiconservative replication.
• Length and replication rate in bacteria:
  • E. coli replicates ~2,000 bp/s; completes in ~18 minutes.
• Origin of replication (ori):
  • Replication begins at specific sites; replicates bidirectionally forming replication forks.
  • A replicon is a unit with one origin.
• Enzymes and players:
  • Helicase: unwinds the double helix at the replication fork.
  • Primase: lays down RNA primers to start synthesis.
  • DNA polymerase III: main replicative enzyme; extends new DNA strands by adding nucleotides in the 5′→3′ direction.
  • DNA polymerase I: removes RNA primers and fills in with DNA.
  • DNA ligase: joins Okazaki fragments on the lagging strand and seals nicks in the backbone.
  • Single-stranded DNA-binding proteins stabilize the separated strands.
• Leading vs lagging strands:
  • Leading strand synthesized continuously toward the replication fork in the 5′→3′ direction.
  • Lagging strand synthesized discontinuously as Okazaki fragments away from the fork; later joined by ligase.
• Okazaki fragments:
  • Short segments on the lagging strand; later connected to form a complete strand.
• Directionality and proofreading:
  • DNA polymerase adds nucleotides to the 3′ end; proofreading capability reduces misincorporation.
• Overall replication process summary:
  • Initiation at origin; helicase opens; primase lays primers; DNA pol extend; RNA primers removed; gaps filled; ligase seals.

Transcription: From DNA to RNA

• Transcription unit:
  • Promoter: binding site for RNA polymerase; upstream (5′ side).
  • Structural gene: region between promoter and terminator where transcription occurs.
  • Terminator: transcription termination site downstream (3′ end).
• Template vs coding strand:
  • Template strand (3′→5′) used to synthesize RNA.
  • Coding strand (5′→3′) has identical sequence to the RNA (except T/U).
• In prokaryotes:
  • A single RNA polymerase catalyzes synthesis of all RNA types.
  • Transcription is coupled with translation (no nucleus).
• In eukaryotes:
  • Three RNA polymerases exist: RNA Pol I (rRNA), RNA Pol II (hnRNA/mRNA precursor), RNA Pol III (tRNA, 5S rRNA, snRNA).
  • Primary transcripts (hnRNA) contain exons and introns; introns are removed during processing.
  • 5′ cap and 3′ poly(A) tail are added; splicing removes introns via spliceosome.
• Processing of hnRNA → mRNA:
  • Capping: add 5′ cap (m7GpppN).
  • Splicing: introns removed; exons joined.
  • Tailing: poly-A tail added (200–300 adenylates).
• Transcriptional start and termination signals:
  • Promoter and terminator demarcate transcription boundaries.
• Genetic code: bridge to translation (review below).

Translation: From mRNA to Protein

• Key players:
  • mRNA carries codons (three-nucleotide units).
  • tRNA molecules serve as adapters; each tRNA carries a specific amino acid and contains an anticodon that base-pairs with the codon on mRNA.
  • Ribosome: molecular machine with A, P, and E sites; catalyzes peptide bond formation via peptidyl transferase (ribozyme activity in rRNA).
• tRNA structure:
  • Cloverleaf 2-D structure; L-shaped 3-D structure.
  • Amino acid attachment site at 3′ end; anticodon loop recognizes codons.
  • Aminoacyl-tRNA synthetase charges tRNAs with correct amino acids (tRNA charging).
• Translation stages:
  • Initiation: ribosomal subunits assemble on mRNA; initiator tRNA (Met) binds to the start codon (AUG) at the P site; ribosome assembles with large subunit to form initiation complex.
  • Elongation: successive tRNAs bring amino acids; peptide bonds form; ribosome translocates along the mRNA (A → P → E sites); growing polypeptide exits via P site.
  • Termination: stop codon recognized by a release factor; ribosome releases polypeptide and dissociates.
• The genetic code:
  • Triplet codons; 64 possible codons; 61 sense codons code for amino acids; 3 stop codons (UAA, UAG, UGA).
  • AUG is Start codon and codes for Methionine (Met) in eukaryotes; in prokaryotes, formyl-Met is used.
  • The code is universal (with known exceptions in mitochondria and some protozoa).
  • Degenerate (redundant): most amino acids are encoded by more than one codon; methionine (AUG) has a unique codon; tryptophan (UGG) is uniquely encoded as well.
• Polyribosomes:
  • Multiple ribosomes can translate a single mRNA simultaneously, forming a polyribosome complex.
• Translation in action (conceptual steps):
  • Initiation: ribosome assembles at 5′ cap and start codon; Met-tRNA binds in the P site.
  • Elongation: new tRNA binds to A site; peptide bond forms; translocation shifts tRNAs to next positions.
  • Termination: release factor binds stop codon; polypeptide released; ribosomal subunits dissociate.

The Genetic Code: Details & Implications

• Codon properties:
  • Triplet code; non-overlapping; comma-less reading frame.
  • Unambiguous: each codon specifies a single amino acid.
  • Universal: largely the same across organisms, with some mitochondrial exceptions.
  • Initiator AUG also codes for Methionine; not all codons encode Methionine.
• Examples and codon table features:
  • Methionine (AUG) initiates translation, but may also code for methionine within a protein sequence.
  • Stop codons: UAA, UAG, UGA terminate translation.
• Mutations and the genetic code:
  • Reading frame is crucial; insertions/deletions not in multiples of three cause frameshifts, dramatically altering the protein sequence.
  • Silent mutations: nucleotide changes that do not alter the encoded amino acid due to degeneracy.
  • Nonsense mutations create a premature stop codon, truncating the protein.
  • Missense mutations change one amino acid to another; effects vary by position and chemical differences.
• Mutation details:
  • Point mutations: single-base changes; transitions (purine↔purine or pyrimidine↔pyrimidine) and transversions (purine↔pyrimidine).
  • Frameshift mutations result from insertions/deletions not in multiples of three.

Regulation of Gene Expression (Prokaryotes & Eukaryotes)

• Gene regulation overview:
  • Operon concept in bacteria: a cluster of functionally related genes regulated together.
  • Lac operon as a classic model: lacZ (β-galactosidase), lacY (permease), lacA (transacetylase).
  • Regulatory gene produces a repressor protein; operator is the binding site; promoter is where RNA polymerase binds.
• Lac operon regulation:
  • In the absence of lactose (inducer): repressor binds operator; transcription is OFF (negative regulation).
  • In the presence of lactose: lactose (inducer) binds the repressor, preventing it from binding the operator; RNA polymerase can initiate transcription (ON).
  • Glucose levels influence operon activity; lactose alone does not activate transcription if glucose is high.
• Eukaryotic regulation:
  • More complex with 3 RNA polymerases (I, II, III).
  • Primary transcripts (hnRNA) contain introns that must be removed; processing includes capping, splicing, and tailing.
  • Gene regulation occurs at multiple levels: transcriptional, RNA processing, mRNA transport, and translational control.
• Regulation beyond operons:
  • Gene expression responds to metabolic, physiological, and environmental conditions (e.g., beta-galactosidase in E. coli: lactose metabolism).
  • Regulatory networks control development, differentiation, and responses to stimuli.

The Human Genome Project (HGP) & Genomic Era

• Goals and scope:
  • Identify all estimated genes in the human genome.
  • Sequence ~3 × 10^9 base pairs; store information in databases; improve analytical tools; address ethical, legal, and social issues (ELSI).
• Key statistics:
  • Human genome contains about 3 × 10^9 nucleotide bases; roughly 30,000 genes.
  • Average gene length ~3,000 bases; largest gene (dystrophin) ~2.4 million bases.
  • 99.9% of bases are identical among humans; 0.1% (about 3 × 10^7 bases) account for individual variation.
  • Only ~2% of the genome codes for proteins; the rest includes regulatory and non-coding sequences.
  • Over 1.4 million locations have single-nucleotide polymorphisms (SNPs).
• Genome architecture:
  • Genome packaged in 23 human chromosomes (22 autosomes + X/Y).
  • Repeats and repetitive DNA constitute large portions of the genome; VNTRs and STRs are common non-coding repeats used in DNA fingerprinting.
• Methodologies:
  • Expressed Sequence Tags (ESTs) to identify expressed genes.
  • Sequence annotation to map coding and non-coding regions and assign functions.
  • Techniques: cloning in BACs/YACs, Sanger sequencing, automated sequencers, overlapping sequence assembly, and bioinformatics for analysis.
• Outcomes and implications:
  • Creation of public databases (GenBank) for gene sequences and proteins.
  • Advancement in fields from molecular medicine to evolutionary biology; enabling disease gene discovery and forensic applications.
• Challenges and future directions:
  • Determining exact gene numbers, regulation, chromosomal structure, non-coding DNA functions, and complex trait genetics.

DNA Fingerprinting (DNA Profiling)

• Basis:
  • Most humans differ in non-coding repetitive DNA sequences (satellite DNA).
  • VNTRs and STRs vary in copy number between individuals, producing unique DNA fingerprints.
• Workflow (Southern blot approach; later PCR-enabled methods):
  1) Isolate DNA from sample (blood, hair, tissue, etc.).
  2) Digest DNA with restriction enzymes.
  3) Separate fragments by gel electrophoresis.
  4) Transfer (blot) onto a membrane (nitrocellulose/nylon).
  5) Hybridize with radioactive VNTR probe to detect repeats.
  6) Detect via autoradiography to produce an image (DNA fingerprint).
• PCR enhancement:
  • PCR allows amplification of tiny DNA samples (even single cells) to obtain a fingerprint.
• Applications:
  • Forensics (paternity, crime investigations), disease diagnosis, phylogenetics, population genetics, and biodiversity studies.

Additional Notes: RNA Types & Translation Details

• Types of RNA:
  • mRNA: messenger RNA; template for protein synthesis.
  • rRNA: ribosomal RNA; structural and catalytic role in translation (e.g., bacterial 23S rRNA acts as a ribozyme).
  • tRNA: transfer RNA; brings amino acids and reads the genetic code via anticodons.
• tRNA structure:
  • Anticodon loop recognizes codon on mRNA; amino acid attachment site at 3′ end; ribosome-binding region and enzyme-binding region.
• Initiation, elongation, and termination in translation:
  • Initiation: small ribosomal subunit binds to mRNA at start codon; initiator tRNA (Met) binds in P site; large subunit joins to form the complete ribosome.
  • Elongation: successive tRNAs add amino acids; peptide bond formation via peptidyl transferase; translocation moves ribosome along mRNA.
  • Termination: release factor binds stop codon; polypeptide released; ribosome dissociates.
• Polyribosomes (polysomes): multiple ribosomes translating a single mRNA simultaneously.

Key Equations & Numerical References (LaTeX)

• DNA length calculation: For a haploid genome of $N{bp}$ base pairs with average base-pair rise $d{bp}$, the length is
  $L = N<em>{bp} imes d</em>{bp}.$
  Example: Human haploid genome with $N<em>{bp} \,=\, 6.6\times 10^{9}$ and $d</em>{bp} = 0.34\text{ nm}$ gives
  $L = 6.6\times 10^{9} \times 0.34\times 10^{-9}\text{ m} \approx 2.24\text{ m}.$
• DNA pitch per turn and base pair spacing:
  • Base pair spacing: $d_{bp} = 0.34\text{ nm}.$
  • Pitch per turn: $P = 10\,\text{bp} \times d_{bp} = 3.4\text{ nm}.$
• Semi-conservative replication (conceptual): each daughter DNA contains one parental strand and one newly synthesized strand.
• Codon usage (genetic code) examples:
  • Start codon: AUG (Methionine, Met)
  • Stop codons: UAA, UAG, UGA
  • A set of 61 sense codons code for amino acids; 3 stop codons terminate translation.
• Charged tRNA formation (aminoacylation):
  • Amino acid + ATP → aminoacyl-AMP + PPi; amino acid is then transferred to tRNA by aminoacyl-tRNA synthetase, forming charged tRNA.

Connections to Foundational Principles & Real-World Relevance

• Central dogma underpins all molecular biology: information flow from DNA to RNA to protein; exceptions (reverse transcription) explain retroviruses like HIV.
• Structure–function relationship: DNA’s stable double helix enables reliable genetic storage; RNA’s versatility supports transcription and translation, though its instability drives mutation rates.
• Chromatin architecture links molecular biology to cellular function: packaging controls gene expression; euchromatin vs heterochromatin correlates with transcriptional activity.
• The Human Genome Project (HGP) advanced biotechnology and personalized medicine, enabling gene discovery, genetic screening, and forensic applications, while raising ethical considerations (privacy, data sharing).
• DNA fingerprinting combines molecular biology with forensic science, enabling identity verification, paternity testing, and crime solving.
• Practical implications include: genetic testing, gene therapy prospects, evolutionary studies, and pharmaceutical development based on gene regulation and expression.

Quick Reference: Major Concepts (Summary)

• DNA structure: double helix, antiparallel strands, A–T and G–C pairing, 0.34 nm per bp, 10 bp per turn (3.4 nm per turn).
• Nucleotides: sugar (deoxyribose in DNA), phosphate, nitrogenous base; N-glycosidic bond forms nucleoside; polynucleotides are DNA/RNA chains.
• Packaging: nucleosome core (histone octamer) with DNA, 11 nm fiber; 30 nm fiber; chromatin and chromosome organization; euchromatin vs heterochromatin.
• Replication: semi-conservative model; replication fork; leading vs lagging strands; Okazaki fragments; enzymes (helicase, primase, DNA polymerase I/III, ligase); proofreading.
• Transcription (prokaryotes vs eukaryotes): RNA polymerase(s), promoters, terminators, processing (capping, splicing, poly-A tail) in eukaryotes; coupling of transcription and translation in prokaryotes.
• Translation: ribosome structure (A, P, E sites); tRNA structure and charging; initiation, elongation, termination; codon table; universal genetic code with start/stop codons.
• Mutations: point mutations (transitions/transversions), insertions/deletions, frameshift, silent and nonsense mutations; reading frame importance.
• Regulation: operons (lac operon) in bacteria; promoter, operator, repressor, inducer; negative/positive control; complexity in eukaryotic regulation.
• Human Genome Project: sequencing 3×10^9 bp; approx. 30k genes; SNPs and VNTRs; ESTs and annotation; ethical, legal, social implications (ELSI).
• DNA fingerprinting: VNTRs/STRs; Southern blot and PCR-based methods; forensic and diagnostic applications.

Note on Sources & Nomenclature

• The content above compiles key points from the transcript slides across sections on DNA structure, replication, transcription, translation, regulation, human genome project, and DNA fingerprinting. Equations and numerical values are presented in standard LaTeX formatting when appropriate.