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FINE STRUCTURE OF GENE Seymour Benzer (1950-60) Bead Theory: The gene is the fundamental unit of Structure Change & Function Benzer: if a gene were a sequence.

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Presentation on theme: "FINE STRUCTURE OF GENE Seymour Benzer (1950-60) Bead Theory: The gene is the fundamental unit of Structure Change & Function Benzer: if a gene were a sequence."— Presentation transcript:

1 FINE STRUCTURE OF GENE Seymour Benzer (1950-60) Bead Theory: The gene is the fundamental unit of Structure Change & Function Benzer: 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 morphology Host range property 1.Permissive host E. coli B; all (rII - & rII + phages grow. 2.Restrictive host E. coli K12; rII+ recombinants grow.

3 Recombinants of two rII mutants of T4.

4 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. Recombination frequency

5 Intragenic Recombination Recombination between two mutants to give a wild type (non-mutant) form of the gene. O O X Mutant 1 Mutant 2 OO Wild type Double Mutant

6 Summary of fine structure mapping of rII region in T4

7 Seymour Benzers 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 Benzers conclusion: Benzers 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 pairs So 0.02 cM represents a pair of adjacent nucleotides. 1. hypothesis #2 : gene is fundamental unit of structure…indivisible by crossing over ? is wrong.. 2.Nucleotide pair is the fundamental unit of structure (recombination: recon)

9 Complementation The 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 F complementation... F protein level, mixing of gene products F no change in genotypes relative to parents F requires 1 generation of crosses to investigate F recombination... F DNA level F results in new genotypes relative to parents F requires 2 generations of crosses to investigate complementation or recombination ?

11 F hypothesis #3: gene is fundamental unit of function... parts of genes cannot function in complementation tests ? GENE = UNIT OF FUNCTION?

12 hypothesis #3: gene is fundamental unit of function... parts of genes cannot function in complementation tests ?... Correct Fa unit of DNA sequence that codes for a single polypeptide or protein FCistron: 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. GENE = UNIT OF FUNCTION?

13 Alternative Splicing ? ? ? (human genes ~30,000 & proteins ~1,20,000 ???) One Gene / One Enzyme One Gene / One Polypeptide One Gene / One set of connected transcripts The definition of a gene by Gerstein et al. [2007] as Fa 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... GENE = UNIT OF FUNCTION?

14 Gene definition: Definition 1860s–1900s: Gene as a discrete unit of heredity Definition 1910s: Gene as a distinct locus Definition 1940s: Gene as a blueprint for a protein Definition 1950s: Gene as a physical molecule Definition 1960s: Gene as transcribed code Definition 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 GENE = UNIT OF FUNCTION? After ENCODE (Encyclopedia of DNA Elements) PROJECT

15 FINE STRUCTURE OF GENE Seymour Benzer (1950-60) Revised Bead Theory: The nucleotide pair is the fundamental unit of Structure Change & The gene is the fundamental unit of Function New Terms: cistron (unit of function), recon (unit of recombination) and muton (unit of mutation).


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