Presentation on theme: "FINE STRUCTURE OF GENE Seymour Benzer ( )"— Presentation transcript:
1 FINE STRUCTURE OF GENE Seymour Benzer (1950-60) Bead Theory: The gene is the fundamental unit ofStructureChange &FunctionBenzer: if a gene were a sequence of bases it could be divisible at many different locations
2 Benzer studied of the rII region of T4: (rII mutant causing rapid lysis of E. coli than wild type)Two types of traits:plaque morphologyHost range propertyPermissive host E. coli B; all (rII- & rII+phages grow.Restrictive host E. coli K12; rII+ recombinants grow.
4 Recombination frequency Gene order is determined by frequency of recombinants.If recombination rate is high, genes are far apart.If recombination rate is low, genes are close together.
5 Intragenic Recombination Recombination between two mutants to give a wild type (non-mutant) form of the gene.OXMutant 1Mutant 2Wild typeDouble Mutant
6 Summary of fine structure mapping of rII region in T4
7 Seymour Benzer’s conclusion: Benzer studied 3000 rII mutants showing nucleotide deletions at different levels of subdivision & determined that the rII region is sub-divisible into >300 mutable sites by series of nested analyses (ANOVA) and comparisons.hypothesis #1: gene is fundamental unit of change.. ? is wrong..Nucleotide pair is the fundamental unit of change/mutation (muton)
8 Seymour Benzer’s conclusion: Benzer’s recombination frequency between some pairs of these was as low as 0.02.The T4 genome has 160,000 base pairs of DNA extending over ~1,600 centimorgans (cM).So 1 cM ≅ 100 base pairsSo 0.02 cM represents a pair of adjacent nucleotides.hypothesis #2: gene is fundamental unit of structure…indivisible by crossing over ? is wrong..Nucleotide pair is the fundamental unit of structure (recombination: recon)
9 ComplementationThe production of the wild-type phenotype by a cell that contains two mutant genes. The deficiency of one homologue is supplied by the normal allele of the other
10 complementation or recombination ? protein level, mixing of gene productsno change in genotypes relative to parentsrequires 1 generation of crosses to investigaterecombination...DNA levelresults in new genotypes relative to parentsrequires 2 generations of crosses to investigate
11 GENE = UNIT OF FUNCTION?hypothesis #3: gene is fundamental unit of function... parts of genes cannot function in complementation tests ?
12 GENE = UNIT OF FUNCTION?hypothesis #3: gene is fundamental unit of function... parts of genes cannot function in complementation tests ?... Correcta unit of DNA sequence that codes for a single polypeptide or proteinCistron: Term coined by Benzer for the smallest genetic unit that does NOT show genetic complementation when two different mutations are in trans position; but shows wild-type phenotype when the same mutations are in cis.
13 (human genes ~30,000 & proteins ~1,20,000 ???) GENE = UNIT OF FUNCTION?Alternative Splicing ? ? ?(human genes ~30,000 & proteins ~1,20,000 ???)One Gene / One EnzymeOne Gene / One Polypeptide“One Gene / One set of connected transcriptsThe definition of a gene by Gerstein et al.  as‘a union of genomic sequences encoding a coherent set of potentially overlapping functional products’allows genes to have an overlapping sequence, to be alternatively spliced and to exert functions other than protein coding...
14 GENE = UNIT OF FUNCTION? Gene definition: After ENCODE (Encyclopedia of DNA Elements) PROJECTGene definition:Definition 1860s–1900s: Gene as a discrete unit of heredityDefinition 1910s: Gene as a distinct locusDefinition 1940s: Gene as a blueprint for a proteinDefinition 1950s: Gene as a physical moleculeDefinition 1960s: Gene as transcribed codeDefinition 1970s–1980s: Gene as open reading frame (ORF)Definition 1990s–2000s: Annotated genomic entity,Definition (Gerstein et al. 2007) The gene is a union of genomic sequences encoding a coherent set of potentially overlapping functional products.TIME LINE OF GENE DEFINITION
15 FINE STRUCTURE OF GENE Seymour Benzer (1950-60) Revised Bead Theory:The nucleotide pair isthe fundamental unit ofStructureChange &The gene is the fundamental unit of FunctionNew Terms: cistron (unit of function), recon (unit of recombination) and muton (unit of mutation).