Download Deoxyribonucleic Acid

DNA
Deoxyribonucleic
Acid
Frederick Griffith
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worked with pneumonia causing bacteria
two strains
Smooth (S) strain
Rough (R) strain
Used four different trials
Group 1
• He injected the first group of mice with
the “S” Strain.
• Those mice died
Figure 16.1 Transformation of bacteria
Group 2
• He injected the second group with the “R”
strain.
• Those mice lived.
Figure 16.1 Transformation of bacteria
Group 3
• He heated up a sample of “S” bacteria to
kill them and injected the mice in Group 3
with this.
• Those mice lived.
Figure 16.1 Transformation of bacteria
Group 4
• He mixed some of the “heat-killed ‘S’”
bacteria with some of the “R” bacteria and
injected the mice with that mixture.
• Those mice died.
Figure 16.1 Transformation of bacteria
Figure 16.1 Transformation of bacteria
Results
• He took a sample of the blood from the
mice in group 4 and found that the
harmless “R” bacteria had been
“transformed” into fatal “S” bacteria.
• What was this genetic material that
transformed the bacteria?
Proteins or Nucleic Acids?
• We knew both existed, but it was unknown
which was responsible for this genetic
transformation
Avery
• 2 samples
• 1. Avery took the “heat-killed ‘S’ and ‘R’”
mixture and injected those bacteria with protein
digesting enzymes.
• 2. He took the same mixture and injected those
bacteria with nucleic acid digesting enzymes.
• He assumed that the one that was digested and
did not infect would be the genetic material.
Avery
• Sample 1: DNA present, proteins digested
• Sample 2: DNA digested, proteins present
Avery continued
• He found that sample 1 infected and killed
the mice, but sample 2 did not.
• He concluded that DNA must be the
genetic material.
Skeptics
• Most people still did not believe DNA could
be responsible for genetics because there
are only 4 different bases (nucleotides)
while proteins are made of 20 different
bases (amino acids).
• How could a 4 letter “alphabet” account for
the differences between living things?
Hershey-Chase Experiment
Bacteriophage-virus that
infects bacteria
Hershey and Chase (1952)
Used viruses to prove that
DNA is the genetic
material.
– Nucleic Acid + Protein
coat
– bacteriophages or
phages.
Figure 18.4 The lytic cycle of phage T4
Figure 16.2a The Hershey-Chase experiment: phages
Figure 16.2ax Phages
Building Blocks Of DNA
Nucleotides
• Deoxyribose sugar
• A phosphate group
• A nitrogenous base
How many different nucleotides are there?
2 Samples
• Sample One:
Radioactively labeled
the protein coat of the
virus.
• Sample Two:
Radioactively labeled
the DNA in the virus.
Hershey and Chase
• Figured that whichever sample made
radioactive bacteria would tell him which
was the genetic material (DNA or
proteins).
Protein Labeled
DNA Labeled
Protein Labeled
DNA Labeled
Structure of DNA
Building Blocks Of DNA
Nucleotides
• Deoxyribose sugar
• A phosphate group
• A nitrogenous base
How many different nucleotides are there?
Figure 16.3 The structure of a DNA stand
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Bases
Franklin
• Rosalind Franklin was able to use x-ray
crystallography to make a diffraction
image of a DNA molecule
Figure 16.4 Rosalind Franklin and her X-ray diffraction photo of DNA
Watson and Crick
• used Chargaff’s base pair rules and
Franklin’s images to make a 3-D model of
the DNA double helix
• never really did any experiments
themselves, just used other peoples’
results
• they received the Nobel Prize and all the
credit… hardly acknowledged Franklin
Figure 16.0 Watson and Crick
Figure 16.5 The double helix
DNA Structure
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Double stranded HELIX
Like a “twisted ladder”
Uprights of ladder are S-P
“Rungs” are nitrogen bases