FINE STRUCTURE OF GENE Seymour Benzer 1950 60

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

Benzer studied of the r. II region of T 4: (r. II 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 (r. II- & r. II+phages grow. 2. Restrictive host E. coli K 12; r. II+ recombinants grow.

Recombinants of two r. II mutants of T 4.

Recombination frequency ü Gene order is determined by frequency of recombinants. ü If recombination rate is high, genes are far apart. ü If recombination rate is low, genes are close together.

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

Summary of fine structure mapping of r. II region in T 4

Seymour Benzer’s conclusion: Benzer studied 3000 r. II mutants showing nucleotide deletions at different levels of subdivision & determined that the r. II 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)

Seymour Benzer’s conclusion: F Smallest unit of recombination detectable? • recombination frequency (RF) = 1 x 10– 9 • smallest distance = 1 x 10– 7 c. M (centi. Morgon) • if 1 c. M 5 x 105 bp (on average, in flies) • bp are separated by 2 x 10– 6 c. M 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)

complementation or recombination ? 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

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

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

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).