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MICROBIAL GENETICS

DNA

AS

THE

GENETIC MATERIAL

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Presented by M.Banupriya

Assistant Professor

Department of Microbiology

Dr. R.A.N.M Arts and Science College, Erode.

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DNA as the Genetic Material

  • Introduction
  • DNA as the Genetic Material
  • Direct Evidences on Experiments
  • Griffith’s Bacterial Transformation Experiment
  • Avery etal Experiment
  • Bacteriophage Infection by Hershey & Chase

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DNA, deoxyribonucleic acid, is the genetic material in your cells. It was passed on to you from your parents and determines your characteristics.

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INTRODUCTION

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DNA AS THE GENETIC MATERIAL

  • It has been accepted that DNA is the genetic material and the DNA forms genes which control all the activities of microorganisms.
  • The DNA carries the genetic information from generation to generation.
  • This concept may be proved by a number of Experiments.

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Direct Evidences on Experiments

  • Griffith’s Bacterial Transformation
  • Avery etal Experiment
  • Hershey and Chase Experiment

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Griffith’s Bacterial Transformation

  • In 1928, Frederick Griffith experimented with S.pneumoniae bacteria in mice. He used two strains of S.pneumoniae (Smooth & Rough)
  • When the avirulent strains of S.pneumoniae injected into mice, there is no effect and the mice does not cause disease.
  • When the virulent strains of S.pneumoniae injected into mice, it has died of Pneumonia.
  • When heat killed virulent strain of S.pneumoniae injected into mice, there is no effect and the mice does not cause disease.
  • When the mice is injected with a mixture of avirulent and heat killed virulent strain, mice died due to pneumonia.

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Griffith’s Bacterial Transformation

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Griffith’s Bacterial Transformation

  • Griffith called this change of non-virulent bacteria into virulent pathogen as Transformation.

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Avery etal Experiment

  • Oswald Avery, MacLeod and Mcarthy shown that only DNA extracts from smooth cells caused transformation of Rough Cells to Smooth Cells.

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Avery etal Experiment

  • Cell free extract of heat killed virulent strain was treated with DNAase, the transforming property was lost.
  • The same extract was treated with protease, the transforming ability has retained.
  • The Publication of these studies by Avery etal provided the first evidence that Griffith’s transforming principle was DNA and therefore that DNA carried genetic information.

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Hershey & Chase Experiment

  • In 1952, Alfred Hershey & Martha Chase performed several experiments that indicating DNA was the genetic material in a bacteriophage by using radioactive markers .
  • A group of T2 phages was labelled with radioactive markers as P32 and S35.
  • They infected the E.coli with radio actively labelled T2 phages.

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Bacteriophage Infection by Hershey & Chase

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Hershey & Chase Experiment

  • When E.coli was infected with T2 phage containing 35S protein, most of the radioactivity remained outside the host cell.
  • When a T2 Phage containing 32P DNA was mixed with host bacterium, the radio active DNA was injected into the cell and phages were produced.
  • Thus DNA was carrying the Virus’s genetic information.