Transcript
CH 12.1 DNAThe Genetic Material
Learning Objectives: 1. Summarize the experiments leading
to the discovery of DNA as the genetic material.
2. Diagram and Label the basic structure of DNA
Scientific History • The march to understanding that DNA is
the genetic material– T.H. Morgan (1908)– Frederick Griffith (1928)– Avery, McCarty & MacLeod (1944)– Erwin Chargaff (1947)– Hershey & Chase (1952)– Watson & Crick (1953)– Meselson & Stahl (1958)
Chromosomes related to phenotype • T.H. Morgan
– working with Drosophila • fruit flies
– associated phenotype with specific chromosome
• white-eyed male had specific X chromosome
1908 | 1933
Genes are on chromosomes • Morgan’s conclusions
– genes are on chromosomes– but is it the protein or the DNA
of the chromosomes that are the genes?
• initially proteins were thought to be genetic material… Why?
1908 | 1933
What’s so impressiveabout proteins?!
The “Transforming Principle” 1928• Frederick Griffith
– Streptococcus pneumonia bacteria• was working to find cure for pneumonia
– harmless live bacteria (“rough”) mixed with heat-killed pathogenic bacteria (“smooth”) causes fatal disease in mice
– a substance passed from dead bacteria to live bacteria to change their phenotype
• “Transforming Principle”
The “Transforming Principle”
Transformation = change in phenotypesomething in heat-killed bacteria could still transmit disease-causing properties
live pathogenicstrain of bacteria
live non-pathogenicstrain of bacteria
mice die mice live
heat-killed pathogenic bacteria
mix heat-killed pathogenic & non-pathogenicbacteria
mice live mice die
A. B. C. D.
DNA is the “Transforming Principle”
• Avery, McCarty & MacLeod– purified both DNA & proteins separately 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
1944
What’s theconclusion?
mice die
Oswald Avery Maclyn McCarty Colin MacLeod
Avery, McCarty & MacLeod• Conclusion
– first experimental evidence that DNA was the genetic material
1944 | ??!!
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
1952 | 1969Hershey
Why useSulfur
vs.Phosphorus?
Protein coat labeledwith 35S
DNA labeled with 32P
bacteriophages infectbacterial cells
T2 bacteriophagesare labeled with
radioactive isotopesS vs. P
bacterial cells are agitatedto remove viral protein coats
35S radioactivityfound in the medium
32P radioactivity foundin the bacterial cells
Which radioactive marker is found inside the cell?
Which molecule carries viral genetic info?
Hershey &
Chase
Blender experiment• Radioactive phage & bacteria in blender
– 35S phage• radioactive proteins stayed in supernatant• therefore viral protein did NOT enter bacteria
– 32P phage• radioactive DNA stayed in pellet• therefore viral DNA did enter bacteria
– Confirmed DNA is “transforming factor”
Taaa-Daaa!
Hershey & Chase
Alfred HersheyMartha Chase
1952 | 1969Hershey
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%
1947
That’s interesting!What do you notice?
RulesA = TC = G
Structure of DNA
Now scientists agreed that DNA was in fact the genetic material….
But what did it look like?
Let’s take a closer look at DNA.
Structure of DNA• Watson & Crick
– developed double helix model of DNA• other leading scientists working on question:
– Rosalind Franklin– Maurice Wilkins– Linus Pauling
1953 | 1962
Franklin Wilkins Pauling
Watson and Crick1953 article in Nature
CrickWatson
Rosalind Franklin (1920-1958)
DNA
Stands for
Deoxyribonucleic Acid
proteinsproteins
DNADNA
Nucleic Acids• Function:
– genetic material• stores information
–genes–blueprint for building proteins
»DNA RNA proteins• transfers information
– blueprint for new cells– blueprint for next generation
AA
A
A
TC
G
CG
TG
C
T
Nucleic Acids• Examples:
– RNA (ribonucleic acid)• single helix
– DNA (deoxyribonucleic acid)• double helix
• Structure:– monomers = nucleotides
RNADNA
Nucleotides• 3 parts
– nitrogen base (C-N ring)
– pentose sugar (5C)• ribose in RNA• deoxyribose in DNA
– phosphate (PO4) group
Are nucleic acidscharged molecules?
Nitrogen baseI ’m the
A,T,C,G or Upart!
Types of nucleotides
• 2 types of nucleotides– different nitrogen bases – purines
• double ring N base • adenine (A)• guanine (G)
– pyrimidines• single ring N base • cytosine (C)• thymine (T)• uracil (U)
Purine = AGPure silver!
Nucleic polymer• Backbone
– sugar to PO4 bond
– phosphodiester bond• new base added to sugar of
previous base• polymer grows in one direction
– N bases hang off the sugar-phosphate backbone
Dangling bases?Why is this important?
Pairing of nucleotides• Nucleotides bond between
DNA strands– H bonds– purine :: pyrimidine
– A :: T• 2 H bonds
– G ::: C• 3 H bonds
Matching bases?Why is this important?
DNA molecule
Shape = Double helix– H bonds between bases
join the 2 strands• A :: T• C ::: G
H bonds?Why is this important?
Time for Questions!!!Learning Objectives: 1. Summarize the experiments leading to the discovery of DNA
as the genetic material.
Name that Scientist(s)….1. The double helix structure of DNA was first described by
________.2. The first major experiment that led to the discovery of DNA as
the genetic material was conducted by ______. He used heat-killed bacteria in mice.
3. The scientist who identified the transforming agent in Griffith’s famous experiment as DNA was _______.
4. These scientists preformed the famous “blender experiment” to demonstrate that DNA is the genetic material in viruses.
5. This scientist’s X-ray diffraction data helped Watson and Crick solve the structure of DNA.
Answers: 1. Watson and Crick 2. Griffith 3. Avery 4. Hershey and Chase 5. Rosalind Franklin
Learning Objective 2:
Diagram and Label the basic structure of DNA.
1. Use the following words to label this piece of DNA:
– Deoxyribose – Phosphate – Adenine – Thymine – Cytosine – Guanine
2. Circle a nucleotide
3. Put a star by the purines
4. Underline the pyrimidines
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