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History of DNA - The Genetic Material. Scientific History . The march to understanding that DNA is the genetic material T.H. Morgan (1908) Frederick Griffith (1928) Avery, McCarty & MacLeod (1944) Hershey & Chase (1952) Watson & Crick (1953) Meselson & Stahl (1958). 1908 | 1933.

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History of DNA -

The Genetic Material


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Scientific History

  • The march to understanding that DNA is the genetic material

    • T.H. Morgan (1908)

    • Frederick Griffith (1928)

    • Avery, McCarty & MacLeod (1944)

    • Hershey & Chase (1952)

    • Watson & Crick (1953)

    • Meselson & Stahl (1958)


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1908 | 1933

Genes are on chromosomes

  • T.H. Morgan

    • working with Drosophila (fruit flies)

    • genes are on chromosomes

    • but is it the protein or the DNA of the chromosomes that are the genes?

      • through 1940 proteins were thought to be genetic material… Why?

What’s so impressiveabout proteins?!


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1928

The “Transforming Factor”

  • Frederick Griffith

    • Streptococcus pneumonia bacteria

      • was working to find cure for pneumonia

    • harmless live bacteria mixed with heat-killed infectious bacteria causes disease in mice

    • substance passed from dead bacteria to live bacteria = “Transforming Factor”


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The “Transforming Factor”

mix heat-killed pathogenic & non-pathogenic

bacteria

live pathogenic

strain of bacteria

live non-pathogenic

strain of bacteria

heat-killed pathogenicbacteria

A.

B.

D.

C.

mice die

mice live

mice live

mice die

Transformation?

something in heat-killed bacteria could still transmit disease-causing properties


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1944

DNA is the “Transforming Factor”

  • Avery, McCarty & MacLeod

    • purified both DNA & proteins from Streptococcus pneumonia bacteria

      • which will transform non-pathogenic bacteria?

    • injected protein into bacteria

      • no effect

    • injected DNA into bacteria

      • transformed harmless bacteria into virulent bacteria

What’s the

conclusion?


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1944

Avery, McCarty & MacLeod

Oswald Avery

Colin MacLeod

Maclyn McCarty


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1952 | 1969

Hershey

Confirmation of DNA

  • Hershey & Chase

    • classic “blender” experiment

    • worked with bacteriophage

      • viruses that infect bacteria

    • grew phage viruses in 2 media, radioactively labeled with either

      • 35S in their proteins

      • 32P in their DNA

    • infected bacteria with labeled phages

Why useSulfurvs.Phosphorus?


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Protein coat labeled

with 35S

DNA labeled with 32P

Hershey & Chase

T2 bacteriophages

are labeled with

radioactive isotopes

S vs. P

bacteriophages infect

bacterial cells

bacterial cells are agitated

to remove viral protein coats

Which radioactive marker is found inside the cell?

Which molecule carries viral genetic info?

32P radioactivity foundin the bacterial cells

35S radioactivity

found in the medium


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Blender experiment

  • Radioactive phage & bacteria in blender

    • 35S phage

      • radioactive proteins stayed in supernatant

      • therefore protein did NOT enter bacteria

    • 32P phage

      • radioactive DNA stayed in pellet

      • therefore DNA did enter bacteria

    • Confirmed DNA is “transforming factor”

Taaa-Daaa!


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1952 | 1969

Hershey

Hershey & Chase

Martha Chase

Alfred Hershey


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1947

Chargaff

  • DNA composition: “Chargaff’s rules”

    • varies from species to species

    • all 4 bases not in equal quantity

    • bases present in characteristic ratio

      • humans:

        A = 30.9%

        T = 29.4%

        G = 19.9%

        C = 19.8%

That’s interesting!What do you notice?


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1953 | 1962

Structure of DNA

  • Watson & Crick

    • developed double helix model of DNA

      • other scientists working on question:

        • Rosalind Franklin

        • Maurice Wilkins

        • Linus Pauling

Wilkins

Pauling

Franklin


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1953 article in Nature

Watson and Crick

Watson

Crick



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Copying DNA

  • Replication of DNA

    • base pairing allows each strand to serve as a template for a new strand


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Can you designa nifty experimentto verify?

Models of DNA Replication

  • Alternative models

    • so how is DNA copied?

conservative

semiconservative

dispersive


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1958

Semi-conservative replication

  • Meselson & Stahl

    • label “parent” nucleotides in DNA strands with heavy nitrogen =15N

    • label new nucleotides with lighter isotope = 14N

“The Most Beautiful Experiment in Biology”

parent

replication

Make predictions…


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1958

Semi-conservative replication

  • Make predictions…

    • 15Nstrands replicated in 14N medium

    • 1st round of replication?

    • 2nd round?

where should the bands be?


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Meselson & Stahl

Matthew Meselson

Franklin Stahl

Franklin Stahl

Matthew Meselson


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The “Central Dogma”

  • Flow of genetic information in a cell

transcription

translation

DNA

RNA

protein

replication


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Science …. Fun

Party Time!

Any Questions??