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DNA
the molecule of life
DNA…the history
• 1865…Gregor Mendel presented his
laws of heredity to the Natural Science
Society in Brunn, Austria. He proposed
that invisible “factors” are passed from
one generation to the next.
DNA…the history
• 1868…Fredrich Miescher, a Swiss
biologist, isolates nuclein, a compound
that includes nucleic acid, from pus cells
obtained from discarded bandages.
Miescher was trying to identify the
chemicals in cells.
DNA…the history
• 1871…DNA isolated from the sperm of
trout found in the Rhine River.
• 1879…Albrecht Kossel began his
studies of nuclein, leading to his
discovery of nucleic acids.
DNA…the history
• 1882…Walther Flemming reported his
discovery of chromosomes and mitosis.
• 1883…August Weismann asserts that the
male & female parent contribute equally to the
heredity of offspring; that sexual reproduction
generates new combinations of hereditary
factors; that chromosomes must be the
bearers of heredity.
DNA…the history
• 1887…Edouard-Joseph-Louis-Marie van
Beneden discovered that each species
has a fixed number of chromosomes.
• 1896…E.B. Wilson elaborated on
August Weismann’s chromosome theory
of heredity.
DNA…the history
• 1900…science of genetics is born when
Mendel’s work was rediscovered by
three scientists—DeVries, Von
Tschermak, and Correns—independently
researching scientific literature for
precedents to their own “original” work.
DNA…the history
• 1902…Walter Stanborough Sutton
stated that chromosomes are paired and
may be the carriers of heredity. He
suggested Mendel’s “factors” are located
on chromosomes. Sutton gave these
“factors” the name we use today: genes.
DNA…the history
• 1907…Thomas Hunt Morgan works with
fruit flies to show that chromosomes
have a definite function in heredity. This
work also establishes mutation theory
and leads to an understanding of how
inheritance works.
DNA…the history
• 1909…Phoebus Levene discovered that the
sugar ribose is found in some nucleic acids—
those we now call RNA.
• 1929…Phoebus Levene discovered a
previously unknown sugar, deoxyribose, in
nucleic acids; those nucleic acids are now
known as deoxyribonucleic acids, or DNA.
DNA…the history
• 1934…Desmond Bernal showed that
giant molecules can be studied using
X-ray crystallography.
• 1935…Andrei Nikolaevitch Belozersky
isolated DNA in the pure state for the
first time.
DNA…the history
• 1938…proteins and DNA studied in
various labs with X-ray crystallography.
DNA…the history
• 1944…Oswald Avery, Colin
MacLeod, & Maclyn McCarty
determined that DNA is the
hereditary material involved in
transformation in bacteria.
DNA…the history
• The prevailing hypothesis of the time—
the tetranucleotide hypothesis—
suggested that DNA was made up of
equal amounts of four bases—adenine,
guanine, cytosine, and thymine—but that
it was organized in a way that was too
simple to enable it to carry genetic
information. (proposed by Phoebus
Levene)
DNA…the history
• 1950…Erwin Chargaff (an American
biochemist) was not satisfied with this
hypothesis because of the minimal data
that supported it. He and his students
collected many samples and used a
relatively new technique called paper
chromatography to analyze the DNA
samples.
DNA…the history
• The analysis showed that there were
NOT an equal number of all the
bases…but, they found that certain
bases were equal to each other.
DNA…the history
• Chargaff found that in DNA the amounts
of adenine and thymine are about the
same, as are the amounts of guanine
and cytosine.
DNA…the history
• Chargaff’s Rule:
– The # of adenine bases is equal to the # of
thymine bases and the # of cytosine bases
is equal to guanine bases.
• Ratio of A = T
• Ratio of C = G
• A + T + C + G = 100%
– The proportion of A:T and C:G holds true for
both strands.
DNA…the history
• In human DNA,
– A = 30.9%
– T = 29.4 %
– C = 19.8 %
– G = 19.9 %
• These equalities remained unexplained
until the discovery of the shape/structure
of the DNA molecule 3 years later.
DNA…the history
• 1952…Alfred Hershey and Martha
Chase showed that only the DNA of a
virus enters a cell in significant amounts.
This result supported a role for DNA as
the genetic material, and refuted a role
for protein.
DNA…the history
• 1952…Rosalind Franklin, at King’s
College in London, produced the clearest
X-ray images of DNA that anyone had
ever obtained. She established that
DNA’s structure depended on an
external backbone with the bases on the
inside.
DNA…the history
• 1953…James Watson (American
biologist) and Francis Crick (British
physicist) proposed the double-stranded,
helical, complementary,
anti-parallel model
for DNA.
DNA…the history
• 1958…(April 16) Rosalind Franklin died
of ovarian cancer. She was 37.
DNA…the history
• 1962…Watson, Crick, and Maurice
Wilkins were awarded the Nobel Prize
for Medicine.
Biological
macromolecules
Polymer
Monomer
example
Carbohydrate
Simple
Sugar
starches
Lipid
Glycerol + Fatty
acids
Fats, oils, waxes
Protein
Amino acids
Hair, muscle
Nucleic acids
Nucleotides
DNA, RNA
DNA…the molecule
Each DNA nucleotide consists of a
phosphate group, the sugar deoxyribose,
and a nitrogen base.
P
D
N
nucleotide
DNA…the molecule
There are four nitrogen bases in DNA…to
make four different nucleotides.
»Adenine
»Cytosine
»Guanine
»Thymine
DNA…the molecule
Scientists knew that
• Adenine = Thymine
• Cytosine = Guanine
within a DNA sample (Chargaff’s Rule)
DNA…the molecule
Adenine bonds only
to Thymine; Cytosine
bonds only to
Guanine.
The bases are
COMPLEMENTARY.
DNA…the molecule
Each pair of bases is held together by a
type of weak chemical bond…
a hydrogen bond!
P
T
P
A
D
D
P
A
T
D
A
T
D
A
T
D
D
P
P
D
P
P
D
P
DNA…the molecule
Two strands in a spiral
shape
=double helix
DNA…the molecule