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GENETICS · FINE-STRUCTURE ANALYSISClassical vs. MolecularConcepts of the GeneCistron · Recon · Muton — Fine-Structure Analysis of the rII Locus in Bacteriophage T4 A Postgraduate Seminar in Molecular GeneticsPK
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OVERVIEWWhat We Will Cover Classical Gene ConceptThe “beads on a string” model of Morgan and Sturtevant Cracks in the ModelGarrod, Beadle & Tatum, and the rise of the biochemical gene Benzer's rII SystemAn ultrasensitive phage assay for detecting rare recombinants Fine-Structure & Deletion MappingOrdering thousands of mutations to the nucleotide Cistron, Recon, MutonFunction, recombination, and mutation — redefined at the molecular level Classical vs. Molecular GeneA side-by-side synthesis of the two concepts LegacyThe triplet code, colinearity, and today's more flexible gene conceptClassical vs. Molecular Concepts of the Gene1PK
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THE CLASSICAL VIEW · PRE-1950SThe Gene as a “Bead on a String”Mendel (1866) treated the gene as an abstract “unit factor” controlling one trait, transmitted intact from parent to offspring.Morgan, Sturtevant & Muller (1910s–1930s) localized genes to fixed positions (“loci”) on chromosomes, ordered like beads strung along a wire.A gene was treated as a single, indivisible unit that simultaneously served as the unit of structure, function, mutation, AND recombination. Three Load-Bearing Assumptions1Structural indivisibilityRecombination occurs between genes — never within one.2Mutational unityA gene changes wholesale from one allelic state to another.3Functional atomicityA gene is wholly functional or wholly non-functional; it has no “parts.”Each “bead” = one indivisible gene locus on the chromosome (red = a mutant gene)Classical vs. Molecular Concepts of the Gene2PK
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Classical genetics (Mendel through Morgan/Sturtevant/Muller) treated the gene as the smallest unit of heredity: indivisible by recombination or mutation, and equivalent to the unit of function. Source: Morgan, T.H. (1910) Science 32:120-122; standard treatment in Griffiths et al., Introduction to Genetic Analysis; Snustad & Simmons, Principles of Genetics.3PK
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TOWARD A BIOCHEMICAL GENE · 1900S–1940SFrom Trait to Enzyme: the Biochemical Gene Garrod (1902–1909)Studied “inborn errors of metabolism” such as alkaptonuria and proposed that one mutant gene blocks one specific step in a metabolic pathway. Beadle & Tatum (1941)X-ray-induced auxotrophic mutants of Neurospora crassa each lost exactly one enzyme activity — the “one gene–one enzyme” hypothesis. Awarded the Nobel Prize in 1958. Later refinementMany enzymes are built from more than one polypeptide chain, each separately encoded — sharpened to “one gene–one polypeptide” (e.g. tryptophan synthetase, haemoglobin).Genes were now understood to act through discrete molecular products — but the internal architecture of the gene itself remained unknown. Answering that required a new experimental system: bacteriophage.Classical vs. Molecular Concepts of the Gene3PK
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Garrod's 'inborn errors of metabolism' (alkaptonuria, etc.) anticipated Beadle & Tatum's 1941 one-gene-one-enzyme hypothesis from Neurospora crassa mutants (Beadle & Tatum, 1941, PNAS 27:499-506), which won the 1958 Nobel Prize. Multi-subunit proteins later required the 'one gene-one polypeptide' refinement.4PK
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THE EXPERIMENTAL SYSTEM · SEYMOUR BENZER, 1950S–60SA Phage System of Extraordinary PowerSeymour Benzer, trained as a physicist, joined the Cold Spring Harbor phage course and worked with André Lwoff at the Pasteur Institute.He chose rII mutants of bacteriophage T4 — easily scored by their “rapid lysis” plaque morphology on E. coli B.The decisive trick: rII⁻ mutants cannot form plaques on E. coli K12(λ), while rII⁺ (wild type) can — a “restrictive” host that acts as a molecular sieve. A single rII⁺ revertant among 10⁸ mutant phage produces a visible plaque — a resolving power far beyond classical recombination mapping in eukaryotes.Classical vs. Molecular Concepts of the Gene4PK
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Benzer's system exploited the rII locus of T4: rII mutants lyse E. coli B rapidly (visible as large plaques) but cannot grow on E. coli K12 lysogenic for phage lambda [K12(λ)], while wild-type phage can grow on both. Source: Benzer, S. (1955) PNAS 41:344-354; historical account in Resonance 13:898-908 (Indian Academy of Sciences).5PK
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