Download Topic 7 The Discovery of DNA & Its Roles

Topic 7
The Discovery of DNA
& Its Roles
October 7-14, 2005
Biology 1001
Introduction to DNA
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DNA is deoxyribonucleic acid, a polymer of the four
nucleotide monomers adenine, guanine, cytosine, & thymine
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DNA is the molecule of heredity
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Arranged as a double helix
DNA is precisely replicated during cell division
Each cell in the organism has a copy and a copy is transmitted from
parent to offspring
DNA encodes the amino acid sequences of all the proteins in
an organism
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All biochemical, anatomical, physiological, and behavioural
characteristics of organisms are at least partially determined by DNA
Science as a process
 A series of classic experiments elucidated the molecular basis
of inheritance
 Griffith (1928) – The genetic material transforms bacteria
 Avery & MacLeod (1944) – The transforming agent is DNA
 Hershey & Chase (1952) – DNA is the hereditary material of viruses
 Watson & Crick (1953) – Modeled the 3D structure of DNA
 Meselson & Stahl (1958) – DNA replication is semi-conservative
7.1 The Genetic Material Is DNA
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A. Griffith’s (1928) Experiment
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Worked with two strains of Streptococcus pneumoniae, a pathogenic one
that causes pneumonia (S) and a nonpathogenic one (R)
R bacteria mixed with heat-killed S bacteria became pathogenic
The pathogenicity was inherited by the offspring of these R bacteria
Griffith reasoned that the R bacteria were able to take up a chemical from
the environment
Figure 16.2!
7.1 The Genetic Material Is DNA
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Griffith called this process transformation, but he did not know
what the transforming chemical was
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Avery and colleagues spent 14 years testing various chemicals
from the S bacterial remains to see which would transform
nonpathogenic bacteria into pathogenic ones (R into S)
 Only DNA worked
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Their discovery was met with considerable skepticism because
it was widely held that proteins were a better candidate for the
genetic material
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The question than became whether this finding was
generalizable to other organisms, or specific to bacteria
7.1 The Genetic Material Is DNA
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B. The “Blender” Experiment of Hershey and Chase (1952)
Studied the T2 bacteriophage that infects E. coli
 T2 is a virus comprised of DNA & protein
 Infected E. coli produce new viruses; viral DNA OR protein is responsible
32P and protein with 35S
 Separately radiolabelled each component; DNA with
 Allowed infection to proceed, then blended & centrifuged the bacteria
 32P was found in the pellet, 35S in the supernatant
 DNA is the hereditary material in viruses
7.2 The Discovery of the Model of DNA
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What was known to Watson & Crick
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DNA is the hereditary material
DNA is a polymer of nucleotides
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Nucleotides contain a pentose sugar
(deoxyribose), a phosphate group, and a
nitrogenous base (A,C,G, or T)
A & G are purines, C & T are pyrimidines
The sugars and phosphates form the backbone of
the polymer and give it directionality
The proportion of each base varies from species
to species – first evidence of diversity of DNA
But in each species, the proportion of adenine
equals that of thymine, and the proportion of
guanine equals that of cytosine
These are Chargaff’s rules A=T and G=C
7.2 The Discovery of the Model of DNA
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The challenge was to devise a 3D structure that would account
for DNA’s role in inheritance
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Watson had also seen Rosalind Franklin’s X-ray diffraction data
suggesting that the molecule was helical and wide enough to
accommodate two strands of DNA – a double helix
Franklin reasoned that the sugar phosphate backbone faced outward,
allowing the hydrophobic bases to occupy the interior of the molecule
What remained to be determined was the specific pairing of bases
holding the two strands together
7.2 The Discovery of the Model of DNA
• In 1953, Watson and Crick reported their model in a one page
paper to Nature entitled Molecular Structure of Nucleic Acids
– Its key feature was the arrangement of bases between the two strand
strands of the helix
– Watson and Crick proposed that adenine paired with thymine, and
guanine with cytosine, with hydrogen bonds between bases holding
together the two strands of the helix
Figure 16.7 The Structure of DNA
Review - The 3D Structure of DNA
• Features of the model
– Two strands of DNA arranged in a double-helix,
antiparallel with the bases inward and the sugar phosphate
backbone outward
– The two strands are held together by hydrogen bonding
between base pairs, A with T and C with G
– The helix is “right-handed” with a 3.4 nm spacing between
adjacent turns of the helix, and a 0.34 nm spacing between
adjacent base pairs
– The model fits the X-ray diffraction data and Chargaff’s
rules