Regents Biology

12.1 – History of the Discovery of DNA

Regents Biology

Objectives §  Describe the experiments that identified DNA

(rather than protein) as the genetic material. §  Describe the overall structure of DNA.

Regents Biology

Scientific History §  T.H. Morgan (1908) §  Frederick Griffith

(1928) §  Avery, McCarty &

MacLeod (1944) §  Erwin Chargaff (1950) §  Hershey & Chase

(1952) §  Franklin & Wilkins

(1952) §  Watson & Crick (1953)

Regents Biology

T. H. Morgan §  working with Drosophila (fruit

flies) §  Genes are on chromosomes. §  But what exactly are they?

Protein or DNA? §  Through 1940, proteins

were thought to be genetic material.

§  Why?

What’s so impressive about proteins?!

Regents Biology

Frederick Griffith

§  Something in heat-killed bacteria could still transmit disease-causing properties

§  Transforming factor?

live pathogenic strain of bacteria

live non-pathogenic strain of bacteria

mice die mice live

heat-killed pathogenic bacteria

mix heat-killed pathogenic & non-pathogenic bacteria

mice live mice die

A. B. C. D.

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Avery, McCarty & MacLeod §  Tried to determined whether protein or DNA

(from pathogenic bacteria) 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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Hershey & Chase

Hershey & Chase Blender Experiment Animation http://bcs.whfreeman.com/hillis1e/

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§  classic “blender” experiment §  worked with bacteriophage

(viruses that infect bacteria) §  grew bacteriophages in 2

media, radioactively labeled with either -  35S in their proteins -  32P in their DNA

§  infected bacteria with labeled bacteriophages

Regents Biology

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Blender experiment §  Radioactive phage and bacteria in

blender §  35S phages -  radioactive proteins stayed in

supernatant -  therefore, protein did NOT enter

bacteria §  32P phages -  radioactive DNA stayed in pellet -  therefore, DNA did enter bacteria

§  Confirmed DNA is “transforming factor”

Taaa-Daaa!

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DNA (from strawberries)

Regents Biology

Nucleotides in DNA §  DNA is made up of

nucleotides. §  A nucleotide is a

monomer of nucleic acids made up of: -  a five-carbon

sugar called deoxyribose

-  a phosphate group

-  a base

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Types of Bases §  Purines -  double-ring -  adenine (A) -  guanine (G)

§  Pyrimidines -  single-ring -  cytosine (C) -  thymine (T) - uracil (U) in RNA

Purine = AG Pure silver!

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Erwin Chargaff §  noticed that DNA

composition - varies from species

to species - bases present in

characteristic ratio A = 30.9% T = 29.4% G = 19.9% C = 19.8%

§  Chargaff’s Rule: A = T C = G

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Rosalind Franklin

§  X-ray crystallography reveals helical structure for DNA

Regents Biology

Watson and Crick

§  used information from Chargaff, Franklin and Wilkins

§  deduced the double helical structure of DNA

§  Nobel Prize in Medicine in 1962

Nobel Prize Ceremony http://www.nobelprize.org/

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