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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.
Regents Biology
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?
Regents Biology
Hershey & Chase
Hershey & Chase Blender Experiment Animation http://bcs.whfreeman.com/hillis1e/
#667501__674143__
§ 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
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!
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
Regents Biology
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!
Regents Biology
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