The DNA Structure
Three Major Approaches
1. Structural (architecture of the molecules)
2. Biochemical (interactions between cell
metabolism and heredity)
3. Informational (transfer of information,
translation of information)
The Discovery of DNA
• Friedrich Miescher (Swiss,
1844-1895)
• Intelligent from early age but
very shy
• His father was a professor of
anatomy
• Studied at the University of
Göttingen under Ernst Felix
Immanuel Hoppe-Seyler,
considered the father of
biochemistry
• He examined used bandages
obtained from a hospital caring
for the wounded of the Crimean
War in hope of finding something
interesting
• He discovered a substance
containing both phosphorus and
nitrogen, made up of molecules
that were apparently very large,
in the nuclei of white blood cells
found in pus
• Named the substance nuclein
because it seemed to come from
cell nuclei. In 1874 when
Miescher separated it into a
protein and an acid molecule. It
is now known as
deoxyribonucleic acid (DNA)
• This substance was so unusual
that Hoppe-Seyler repeated the
work himself before allowing
Miescher to publish the paper on
the discovery
• He found an excellent (and more
pleasant) source of nuclear material
in the sperm of the salmon
• The nuclei are large in any sperm
cells, remarkably so in the salmon’s
• From these he first extracted a pure
DNA
• In 1889 a pupil of his, the
German pathologist
Richard Altmann (18521900) introduced the term
“nucleic acid”
Inborn Errors of Metabolism
• Archibald Garrod (British,
1857-1936)
• His father was a physician
• Received his medical
training at St.
Bartholomew's Hospital,
London
• The relationship between
genes and proteins was
first proposed by Garrod in
1908
• Garrod, a prominent
physician at St.
Bartholomew's Hospital in
London, understood both
the new science of
biochemistry and the
emerging discipline of
genetics
• He studied a harmless but rare disorder in
the general population but frequent in
children of first-cousin marriages:
alkaptonuria
• A patient with this disorder produces urine
that when exposed to air turns distinctively
dark, because these people lack the
enzyme found in normal individuals who
are able to convert the reddening agent,
alkapton, to another substance
• Following
Mendel’s laws,
Garrod
concluded that
alkaptonuria is a
congenital
disorder, not the
result of a
bacterial infection
as was
commonly
thought
• He observed that inherited diseases reflect
a patient's inability to make a particular
enzyme, which he referred to as “inborn
errors of metabolism”
• Garrod's hypothesis was ahead of its time
• In the 1930s George Beadle and Boris
Ephrussi linked the synthesis of pigments
to eye color in fruit flies to specific
mutations
1910-1925: Development of
Cytological Genetics
• Cytogenetics is the study of chromosomes
and chromosome abnormalities
X-ray Crystallography
• X-ray crystallography is a technique that exploits
the fact that X-rays are diffracted by crystals
• It is not an imaging technique: X-rays have the
proper wavelength to be scattered by the
electron cloud of an atom of comparable size
• Based on the diffraction pattern obtained from Xray scattering off the periodic assembly of
molecules or atoms in the crystal, the electron
density can be reconstructed
• In 1912 Max von Laue
reported the diffraction of X
rays by a crystal (for which
he received a Nobel Prize
in physics in 1914)
• For progress in pharmaceutical research
the structure of complex organic
molecules has to be understood
• By studying the chemical reactions that a
compound and its degradation products
could enter into with other compounds of
known structure, chemists were able to
deduce the structures of many complex
organic molecules
• X-ray crystallography allowed to determine
molecular structure from the compound itself. In
this method, structural information is obtained by
mathematical analysis of the intensity of X rays
scattered (or diffracted) from parallel planes in a
crystal, as recorded photographically or by an
electronic detector
• In 1915 a unique father-son team, William Henry
Bragg (1862–1942) and his son, William
Lawrence Bragg (1890–1971), won the Nobel
Prize in physics for their seminal roles in X-ray
crystallography
William Lawrence Bragg (left)
and William Henry Bragg
• William Henry's original interest was in what
diffraction showed about the nature of X-rays,
and he was a skilled experimenter and designer
of instruments
• William Lawrence was more concerned with
what X-rays revealed about the crystalline state,
and he possessed a powerful ability to
conceptualize physical problems and express
them mathematically
• Until electronic computers were developed
during World War II, these calculations were
incredibly laborious
“Transforming Principle”
• In 1928, Frederick Griffith (British,
conducted an experiment that showed the
transformation of living cells by a
“transforming principle”, which was later
discovered to be DNA
Tetranucleotide Hypothesis
• Phoebus Levene
(Russian-American,
1869-1940)
• He worked with
Albrecht Kossel and
Emil Fischer, the
nucleic acid and
protein experts at the
turn of the 20th. century
• He conducted experiments that in 1931
suggested that the four components of
DNA occur in approximately equal ratios
• He suggested the possibly that DNA was
made of a repeating tetramer
• If so, the implication was that the structure
of DNA was too simple and too regular to
contribute to genetic variation: attention
thereafter focused on protein as the
probable hereditary substance
One gene-One enzyme
Hypothesis
• This proposal of the one
gene-one enzyme
hypothesis was developed
by the Americans George
Beadle (1903-1989) and
Edward Wells Tatum (19091975) in 1938
• Beadle and Tatum hypothesized that if there
really was a one-to-one relationship between
genes and specific enzymes, it should be
possible to create genetic mutants that are
unable to carry out specific enzymatic reactions
• They exposed spores of Neurospora crassa (a
bread mold) to X-rays or UV radiation and
studied the resulting mutations
• The mutant molds had a variety of special
nutritional needs. Unlike their normal
counterparts, they could not live without the
addition of particular vitamins or amino acids to
their food
• Normal Neurospora requires only one vitamin
(biotin), but mutants were created that also
required thiamine or choline
• Genetic analysis showed that each mutant
differed from the original, normal type by
only one gene
• Biochemical studies showed that the
mutants seemed to be blocked at certain
steps in the normal metabolic pathways
• Their cells contained large accumulations
of the substance synthesized just prior to
the blockage point, just as Garrod's
patients had accumulated alkapton
• As Beadle and Tatum had predicted, they
created single gene mutations that
incapacitated specific enzymes, so that
the molds with these mutations required
an external supply of the substance that
the enzyme normally produced, and the
substance that the enzyme normally used,
piled up in the cell
• These results confirmed their one geneone enzyme hypothesis
• They received the 1958 Nobel Prize in
Physiology and Medicine
“Transforming Principle”
identified as DNA
• In the early 1940s Oswald T. Avery and
Maclyn McCarty, a colleague at the
Rockefeller Institute Hospital, began
concentrating on the problem of
pneumococcal transformation
• Avery's work focused first on purifying the
transforming substance. Using refined
versions of Colin M. MacLeod's
preparation techniques, Avery and
McCarty isolated biologically active
“transforming principle” from samples of
pneumococci
• Then attention turned to its chemical
analysis. Proteases (enzymes that
deactivate proteins) and lipases (enzymes
which destroy lipids) were found not to
inactivate the transforming principle
• Avery concluded that the substance was
essentially protein and lipid free. He found
that the substance was rich in nucleic
acids, but ribonuclease, an enzyme that
destroys ribonucleic acid (RNA), did not
inactivate the substance either
• It was not a carbohydrate like the polysaccharide
capsular material, as carbohydrates are not
precipitated by alcohol (the “transforming
principle” was)
• Alcohol was, however, a well-known precipitant
for deoxyribonucleic acid (DNA). Further, the
transforming substance had a high molecular
weight, as did DNA, and gave a strong reaction
to the Dische test for DNA. Thus, the
transforming substance, producing permanent,
heritable change in an organism, was DNA
What is Life?
• By the end of WWII,
many physicists,
saddened by the
prospects of atomic
bombs, turned their
attention toward biology
• One of them was Erwin
Schrödinger (Austrian
1887-1961)
• Schrödinger published in 1945 a book
titled What is Life? that planted the idea for
searching “the secret of life”
• This marked the beginning of reductionism
in biology
Base Ratios
• In 1951 Edwin Chagraff (19052002) noted “regularities” in the
base composition of nucleic
acids, which reflected the
existence in all DNA
preparations of certain
structural principles
• In particular, for duplex DNA
he identified the constant
proportion of bases: %A = %T
and %C = %G
X-ray Crystallography Applied
to Nucleic Acids
• Between 1940's and 1950's: Maurice
Wilkins (1916-) and Rosalind Franklin
(1920-1958) worked on X-ray/DNA
The Hershey-Chase Experiment
• The Americans Alfred Hershey (19081997) and Martha Chase (1930-2003)
published in 1952 a now classical paper
• They showed DNA to be the carrier of
genetic information in virus reproduction,
working with T2 phage
• They demonstrated by labeling the capsid
proteins and nucleic acid of bacteriophage
T2 with different radioactive isotopes that
only the DNA of the phage had to enter
the cell for virus replication to occur
Enter Watson and Crick
• James Watson (American, 1928-)
• Francis Crick (British, 1916-)
Principle
Data
Source
X-ray crystallography
Stacked layers of subunits
in spirals; long chain, no
ruling out of two chains,
sugar-phosphate in the
outside
Wilkins and Franklin
(but mostly Franklin)
Organic chemistry
4 nucleotides
Levene
Biochemistry
α-helix, model building
Pauling
Chromatography
Base ratios
Chargaff
Chemical bonding
Right form of the bases
J. Donahue
Mathematics
Attractive forces between
DNA bases
J. Griffith
What did Watson and Crick
Really Contribute?
• Major original breaks by W & C were the
A-T and C-G having the same molecular
shape
• That lead to understanding constancy for
the diameter of the DNA molecule, the use
of Chargaff’s ratios
• The application of model building which
lead to the complementary model