Molecular Basis of Inheritance

Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA) are linear polymers of repeating monomeric units termed nucleotides. Each nucleotide consists of three distinct chemical constituents:

• Pentose Sugar: A 5-carbon aldopentose; $eta$-D-2-deoxyribose in DNA (lacking an oxygen atom at the $C2'$ position) and $eta$-D-ribose in RNA (possessing a reactive $-OH$ group at $C2'$, making RNA chemically labile and reactive).
• Nitrogenous Bases: Heterocyclic planar aromatic rings:
  • Purines (Double-ringed dicyclic): Adenine ($A$) and Guanine ($G$), shared by both DNA and RNA. Linked to the sugar via an $N9-C1'$ $eta$-N-glycosidic bond.
  • Pyrimidines (Single-ringed monocyclic): Cytosine ($C$), Thymine ($T$ in DNA only), and Uracil ($U$ in RNA only). Linked to the sugar via an $N1-C1'$ $eta$-N-glycosidic bond. Thymine is chemically 5-methyluracil, whose methyl group confers thermodynamic stability against photochemical degradation.
• Phosphate Group: Phosphoric acid ($H_3PO_4$) esterified to the $5'-OH$ of the nucleoside via a phosphoester bond. Successive nucleotides are covalently joined by $3' o 5'$ phosphodiester bonds, forming the alternating sugar-phosphate backbone with free $5'$-phosphate and $3'$-hydroxyl termini.

Using paper chromatography and UV spectrophotometry on double-stranded DNA across diverse organisms, Erwin Chargaff established four fundamental stoichiometric rules (strictly applicable ONLY to double-stranded DNA):

• Equimolar Base Pairs: The molar amount of Adenine equals Thymine ($[A] = [T]$), and the molar amount of Guanine equals Cytosine ($[G] = [C]$).
• Purines Equal Pyrimidines: Total purine content equals total pyrimidine content: $[A] + [G] = [T] + [C]$, meaning $rac{[A] + [G]}{[T] + [C]} = 1.0$.
• Species-Specific Base Ratio: The ratio $rac{[A] + [T]}{[G] + [C]}$ is constant for a given biological species, but varies widely between species (in human DNA, $rac{A+T}{G+C} pprox 1.52$, representing AT-rich DNA; in *E. coli*, it is $pprox 0.93$, representing GC-rich DNA).

Synthesizing the X-ray crystallographic diffraction patterns of Photo 51 obtained by Rosalind Franklin and Maurice Wilkins with Chargaff's chemical data, James Watson and Francis Crick (1953) proposed the secondary structure of double-helical B-DNA:

British physician Frederick Griffith conducted experiments on Streptococcus pneumoniae (pneumococcus bacteria causing pneumonia in mice), working with two strains:

• S-Strain (Smooth / Virulent): Produces a smooth mucous polysaccharide capsule that protects it from mouse immune phagocytosis; kills mice by causing fatal pneumonia.
• R-Strain (Rough / Avirulent): Lacks the polysaccharide capsule (rough colony morphology); non-pathogenic, mice survive upon injection.
• Experimental Protocol:
  • Series 1: Live S-strain injected $ o$ Mice develop pneumonia and Die.
  • Series 2: Live R-strain injected $ o$ Mice Survive.
  • Series 3: Heat-killed S-strain injected $ o$ Mice Survive.
  • Series 4: Heat-killed S-strain + Live R-strain injected $ o$ Mice develop pneumonia and Die. Living virulent S-strain bacteria were recovered from the dead mice!
• Conclusion: Griffith deduced that some cell-free "Transforming Principle" had transferred from the dead S-strain to living R-strain bacteria, enabling them to synthesize a protective polysaccharide capsule and transform into virulent S-type permanently.

Oswald Avery, Colin MacLeod, and Maclyn McCarty fractionated cell-free extracts of heat-killed S-strain into purified biomacromolecules (proteins, RNA, and DNA) and tested their transforming capability using specific hydrolytic enzymes:

• Extract treated with Proteases (trypsin, chymotrypsin) $ o$ Transformation of R to S was unaffected.
• Extract treated with Ribonuclease (RNase) $ o$ Transformation of R to S was unaffected.
• Extract treated with Deoxyribonuclease (DNase) $ o$ Completely abolished transformation!
• Conclusion: DNA is the chemical transforming substance, proving that genes are made of DNA, not proteins.

Alfred Hershey and Martha Chase provided unequivocal, universally accepted proof that DNA is the genetic material using the bacteriophage T2 infecting Escherichia coli:

In a human diploid cell ($2n$), the total genome comprises $6.6 imes 10^9 ext{ base pairs}$. Since adjacent base pairs are separated by $0.34 imes 10^{-9} ext{ meters}$, the total physical length of DNA per cell is calculated as:

$ ext{Total Length} = 6.6 imes 10^9 imes 0.34 imes 10^{-9} ext{ m} pprox 2.24 ext{ meters}$

Packaging a $2.24 ext{ meter}$ macromolecule inside a microscopic eukaryotic nucleus measuring only $pprox 10^{-6} ext{ meters}$ in diameter requires a staggering 10,000-fold compaction achieved by multi-level hierarchical folding mediated by specialized basic proteins called histones.

Histones are small, basic proteins carrying a strong positive net charge due to an abundance of basic amino acid residues with positively charged side chains, predominantly Lysine and Arginine:

• Histone Octamer: Eight histone molecules associate to form a disc-shaped core complex: two molecules each of $H_2A$, $H_2B$, $H_3$, and $H_4$ organized as $(H_3-H_4)_2$ tetramer and two $(H_2A-H_2B)$ dimers.
• DNA Wrapping: The negatively charged polyanionic DNA (due to phosphate groups) winds tightly around the positively charged histone octamer, completing 1.65 left-handed superhelical turns comprising 146 base pairs.
• Linker DNA & $H_1$ Histone: Successive nucleosomes are interconnected by a stretch of Linker DNA (~54 bp). A fifth histone protein, Histone $H_1$ (Linker histone), binds to the entry and exit points of DNA on the octamer surface, sealing the nucleosome into a complete 200-bp repeating chromatosome.
• Beads-on-a-String Fiber: Under transmission electron microscopy, chromatin appears as a flexible $10 ext{ nm}$ fiber resembling "beads on a string".

The $10 ext{ nm}$ nucleosomal array coils helically into a $30 ext{ nm}$ solenoid fiber (6 nucleosomes per turn), which further loops around a proteinaceous scaffold formed by Non-Histone Chromosomal (NHC) proteins into $300 ext{ nm}$ looped domains, condensed into $700 ext{ nm}$ chromatids, and ultimately reaching $1400 ext{ nm}$ at metaphase.

Matthew Meselson and Franklin Stahl demonstrated the semiconservative replication of DNA using E. coli cultured in medium containing heavy isotope $^{15} ext{NH}_4 ext{Cl}$, followed by equilibrium density gradient centrifugation in cesium chloride ($ ext{CsCl}$):

• Generation 0: Bacteria grown for multiple generations in $^{15} ext{N}$ contained uniformly heavy DNA ($^{15} ext{N}/^{15} ext{N}$), banding at the highest density near the tube bottom.
• Generation 1 (after 20 min in $^{14} ext{N}$): Exactly one replication cycle yielded DNA displaying a single band of intermediate density ($^{15} ext{N}/^{14} ext{N}$ hybrid). This decisively ruled out conservative replication (which would have yielded 1 heavy and 1 light band).
• Generation 2 (after 40 min in $^{14} ext{N}$): Two equal bands were resolved: $50\%$ intermediate hybrid ($^{15} ext{N}/^{14} ext{N}$) and $50\%$ light DNA ($^{14} ext{N}/^{14} ext{N}$), confirming that each parent strand acts as a template for a new strand.
• Herbert Taylor's Validation (1958): Proved semiconservative replication in eukaryotic chromosomes using broad bean (*Vicia faba*) root tip cells treated with radioactive tritiated thymidine ($^3 ext{H}$-thymidine) and autoradiography.

DNA replication initiates at specific nucleotide sequences designated the origin of replication (ori). Because unwinding the entire chromosome simultaneously requires unsustainable thermodynamic energy, replication proceeds bidirectionally within a Y-shaped replication fork coordinated by multi-enzyme complexes:

• DNA Helicase: Uses ATP hydrolysis to unwind and sever the hydrogen bonds between complementary strands at the replication fork.
• Single-Stranded DNA-Binding Proteins (SSB): Bind cooperatively to exposed single-stranded DNA to prevent re-annealing and protect single strands from nuclease degradation.
• Topoisomerase (DNA Gyrase in prokaryotes): Introduces transient single- or double-strand breaks to release positive supercoiling torsional strain ahead of the advancing fork.
• RNA Primase (DnaG): Synthesizes a short (~10 nucleotide) RNA primer providing a free $3'-OH$ group required by DNA polymerases to initiate synthesis.
• DNA Polymerase III (Main Replicating Enzyme): Catalyzes $5' o 3'$ polymerization with astonishing speed (~2000 bp/sec in *E. coli*) and high fidelity ensured by its intrinsic $3' o 5'$ exonuclease proofreading activity.
• DNA Polymerase I (Kornberg Enzyme): Possesses unique $5' o 3'$ exonuclease activity that excises RNA primers and fills the gaps with complementary deoxyribonucleotides.
• DNA Ligase: Catalyzes phosphodiester bond formation between adjacent $3'-OH$ and $5'$-phosphate ends, sealing nicks between Okazaki fragments.

Because DNA polymerases synthesize strictly in the $5' o 3'$ direction, the two antiparallel template strands are replicated via distinctly asymmetric mechanisms:

Proposed by Francis Crick (1958), the Central Dogma of Molecular Biology states that genetic information flows directionally from DNA to RNA to functional protein: $ ext{DNA} o ext{mRNA} o ext{Protein}$. In retroviruses, Howard Temin and David Baltimore discovered Reverse Transcription ($ ext{RNA} o ext{cDNA}$).

A transcription unit in DNA is delineated by three essential structural regions:

• Promoter: Binding site for RNA polymerase located upstream ($5'$-end of coding strand); contains conserved consensus sequences (e.g., TATAAT / Pribnow box at $-10$ in bacteria; TATAAA / Hogness-Goldberg box at $-25$ in eukaryotes).
• Structural Gene: The DNA segment flanked by promoter and terminator. The strand with $3' o 5'$ polarity acts as the Template / Antisense strand, while the strand with $5' o 3'$ polarity is the Coding / Sense strand (whose sequence is identical to the mRNA except Uracil replaces Thymine).
• Terminator: Downstream region signaling transcript termination (e.g., GC-rich hairpin loops or rho factor binding sites).

Unlike bacteria which employ a single RNA polymerase to transcribe all RNA classes, eukaryotes utilize three distinct nuclear RNA polymerases:

• RNA Polymerase I: Transcribes 28S, 18S, and 5.8S ribosomal RNAs (rRNAs).
• RNA Polymerase II: Transcribes precursor mRNA (heterogeneous nuclear RNA or hnRNA).
• RNA Polymerase III: Transcribes transfer RNA (tRNA), 5S rRNA, and small nuclear RNAs (snRNAs).

Eukaryotic structural genes are split genes containing coding sequences (exons) interrupted by non-coding intervening sequences (introns). The primary transcript (hnRNA) undergoes three essential processing events:

• Capping at 5'-end: Addition of an unusual nucleotide, 7-methylguanosine triphosphate ($m^7 ext{Gppp}$), protecting mRNA from $5'$ exonucleases and serving as the ribosome recognition signal during translation initiation.
• Tailing at 3'-end: Template-independent addition of a Poly-A tail (200–300 adenylate residues) by poly-A polymerase, enhancing cytoplasmic stability and nuclear export.
• Splicing: Precise excision of non-coding introns and covalent ligation of exons catalyzed by the Spliceosome, a macromolecular ribonucleoprotein complex composed of five small nuclear ribonucleoproteins (snRNPs: U1, U2, U4, U5, U6).

Elucidated through pioneering biochemical experiments by Marshall Nirenberg, Har Gobind Khorana, and Severo Ochoa, the genetic code translates 64 triplet nucleotide combinations into 20 standard amino acids:

Translation is the polymerisation of amino acids into polypeptides guided by the sequence of codons on mRNA, executing in four sequential steps:

• Activation & Aminoacylation of tRNA (Charging): Amino acids are activated by ATP to form aminoacyl-AMP, then transferred to the $3'-CCA$ terminus of their cognate tRNA by aminoacyl-tRNA synthetase.
• Initiation: The small ribosomal subunit (30S/40S) binds mRNA at the ribosome-binding site (Shine-Dalgarno in bacteria; $5'$ cap in eukaryotes). Initiator $tRNA^{Met}$ pairs with AUG at the P-site (Peptidyl site), followed by binding of the large subunit (50S/60S).
• Elongation: Successive aminoacyl-tRNAs enter the A-site (Aminoacyl site). Peptide bond formation is catalyzed by Peptidyl Transferase, an intrinsic ribozyme component of the large ribosomal subunit (23S rRNA in bacteria; 28S rRNA in eukaryotes). The ribosome translocates $5' o 3'$ along the mRNA using EF-G / eEF-2 and GTP hydrolysis.
• Termination: Upon encountering a stop codon (UAA, UAG, or UGA) in the A-site, Release Factors (RFs) bind, hydrolyzing the peptidyl-tRNA ester bond and releasing the completed polypeptide.

French geneticists François Jacob and Jacques Monod elucidated the transcriptional control of lactose metabolism in E. coli through the classic Lac Operon, a coordinated cluster of polycistronic genes:

Human Genome Project (1990–2003): A 13-year international mega-project that mapped and sequenced the complete human genome ($3 imes 10^9 ext{ bp}$):

• The human genome contains $3164.7 ext{ million nucleotide base pairs}$ and an estimated 20,000–25,000 genes.
• The average gene consists of 3000 bases; the largest known human gene is Dystrophin on the X chromosome spanning $2.4 ext{ million bases}$ ($2.4 ext{ Mb}$).
• Chromosome 1 has the most genes (2,968 genes); the Y chromosome has the fewest (231 genes).
• Less than $2\%$ of the entire genome encodes functional proteins; repetitive non-coding sequences make up a substantial portion. About 1.4 million locations possess single-base differences (Single Nucleotide Polymorphisms or SNPs).

DNA Fingerprinting (Sir Alec Jeffreys, 1984; Lalji Singh in India): A forensic technique relying on satellite DNA polymorphism in Variable Number of Tandem Repeats (VNTRs / Minisatellites):

• Step 1: High-molecular-weight DNA isolation from blood, semen, hair roots, or tissue.
• Step 2: Cleavage into fragments using Restriction Endonucleases.
• Step 3: Size fractionation of restriction fragments by Agarose Gel Electrophoresis.
• Step 4: Alkaline denaturation and capillary transfer (Southern Blotting) onto a nitrocellulose or nylon membrane.
• Step 5: Hybridization with radioactively labeled $^{32} ext{P}$-VNTR probes.
• Step 6: Detection of hybridized bands via Autoradiography using X-ray film, producing unique bar-code band profiles establishing individual identity with astronomical statistical certainty.