Transcript DNA

DNA
Discovery and Structure
Timeline
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1866
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1903
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1928
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1952
Gregor Mendel experiments with
pea plants
Sutton develops the chromosome
theory of inheritance
Frederick Griffith explores genetic
material with mouse experiments
Martha Chase and Alfred Hershey
experiment with bacteriophages to
prove that DNA is the genetic
material
Timeline (cont.)
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1952
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1952
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1952
Edwin Chargaff discovered that in
DNA samples the amount of adenine
was always equal to the amount of
thymine and the amount of cytosine
was always equal to the amount of
guanine
Maurice Wilkins and Rosalind
Franklin photographed DNA using
X-ray crystallography
Watson and Crick model DNA
Griffith’s Experiment
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He studied Streptococcus pneumoniae, a
bacteria that causes pneumonia in mice
He found that there were two types: S strain
and R strain
In the first experiment, he injected the S
strain into the mice
The mice developed pneumonia and died
In the second experiment, he injected the R
strain into the mice
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There was no change in the mice
In the next experiment, he took the S strain,
heat-killed it, and injected it into the mice
There was no change in the mice
In the last experiment, he took the heat-killed
S strain, combined it with the R strain, and
injected it into the mice
The mice developed pneumonia and died
Griffith concluded that there was some factor
that was passed from the S strain to the R
strain that enabled the R strain to cause
pneumonia
Chase and Hershey’s
Experiment
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They worked with bacteriophages, viruses
that infect bacteria
Bacteriophages are made up of only two
substances: DNA and protein
When a bacteriophage infects a bacterial cell,
part of the phage enters the cell and part of it
stays outside
The part that enters the cell is what takes
control of the bacteria
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They wanted to find out if the part that
entered the cell (the genetic material) was
DNA or protein
They put radioactive DNA into a
bacteriophage and let it infect a cell
Tests showed that the bacteria became
radioactive
Next, they put radioactive protein into a
bacteriophage and let it infect a bacterial cell
Tests showed that the bacteria did not
become radioactive
These experiments proved that DNA was the
genetic material
DNA Structure
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DNA is made up of nucleotides
Nucleotides are made up of three parts:
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Deoxyribose (a 5-carbon sugar)
Phosphate group
Nitrogenous base
Bases
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Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
Adenine and Guanine are purines
Purines have a two-ring structure
Cytosine and Thymine are pyrimidines
Pyrimidines have a one-ring structure
Double Helix
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The shape of DNA is described
as a double helix
A double helix resembles a
“twisted ladder”
The “backbone” of the ladder is
made up of alternating sugar
and phosphate groups
The “rungs” of the ladder are
made up of the bases joined by
weak chemical bonds
Complementarity
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Bases that always pair with each other are
said to be complementary
The space between the two sides of the DNA
ladder is only large enough to contain 3 rings
This means that A (two rings) cannot pair with
G (two rings)
This also means that C (one ring) cannot pair
with T (one ring) because the rings would be
too far apart
Complementarity (cont.)
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Chemical bonds formed:
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A forms two
G forms three
T forms two
C forms three
So the only combinations possible are:
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A pairs with T
C pairs with G
Complementary Sequence
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Give the complementary sequence for the
following strand of DNA:
ATTCGACTGA
T AAG C T G AC T
Complementary Sequence
(cont.)
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Give the complementary sequence for the
following strand of DNA:
ATCCGGT
TAGGCCA