1900 - 1953 - Converging on DNA
While two World Wars killed millions and pushed medicine to new
limits, various fields of science began to converge to explore the
miracle and mechanism of reproduction: the nature and structure of
The science of genetics was finally born when
Mendel's work was
rediscovered by three scientists - Hugo DeVries, Erich Von Tschermak,
and Carl Correns - each one independently researching scientific
literature for precedents to their own "original" work.
William Sutton observed homologous pairs of chromosomes in grasshopper
Major outbreaks of disease in overcrowded industrial cities led to the
introduction of large-scale sewage purification systems based on
It was first shown that key industrial chemicals (glycerol, acetone,
and butanol) could be generated using bacteria.
Beijerinck identified free-living aerobic nitrogen fixers.
E. Wildiers discovered "a new substance indispensable for the
development of yeast." Such growth factors eventually became known as
Walter Stanborough Sutton stated that chromosomes are paired and may
be the carriers of heredity. He suggested that Mendel's "factors"
are located on chromosomes. After observing chromosomal movements
during meiosis, Sutton developed the chromosomal theory of heredity.
Sutton noticed that chromosomes occur as pairs, and that gametes (egg
cells) receive only one chromosome from each pair when they form
during meiosis. This corroborated Mendel's theory that
the genetic "factors" were segregated. Sutton gave
the name we use today: "genes."
Archibald Garrod made the connection between
and the biochemical pathways of reproduction in the individual
organism. He went on to inspire a distinguished line of research
physicians whose work in the ensuing ninety years became indispensable
to the growth of human genetics.
Walter Sutton and Theodor Boveri, working independently, proposed that
each egg or sperm cell contains only one of each chromosome pair. This
connected two phemonema: the patterns by which pairs of Mendel's
factors assort themselves and the precisely similar sorting and
recombination of the chromosomes in the formation of the germ cells
and the fertilization of the egg.
William Bateson demonstrated that some characteristics are not
independently inherited. This introduced the concept now called 'gene
linkage' and led to the need for 'genetic maps' that describe the
order of the linked genes.
Edmund Wilson and Nellie Stevens proposed the idea that separate X and
Y chromosomes determine sex. They showed that a single Y chromosome
determines maleness, and two copies of the X chromosome determine
William Bateson and Reginald Crudell Punnett, along with others,
demonstrated that some genes modify the action of other genes.
Paul Erlich investigated atoxyl compounds and discovered the
beneficial properties of Salvarsan - the first chemotherapeutic agent.
1907 Thomas Hunt Morgan began his work with fruit flies that
will prove that chromosomes have a definite function in heredity,
establish mutation theory, and lead to a fundamental understanding of
the mechanisms of heredity.
Calmette and Guerin developed a vaccine against TB. This vaccine,
called BCG, was not used until 1921.
A.E. Garrod described "inborn errors of metabolism" based on his
analysis of family medical histories. This was a first recognition of
a role for genetics in biochemistry, but the idea remained
unappreciated until the work of Beadle and Tatum in the 1940's.
1909 Wilhelm Johannsen coined the terms 'gene' to describe
the carrier of heredity; 'genotype' to describe the genetic
constitution of an organism; and 'phenotype' to describe the actual
organism, which results from a combination of the genotype and the
various environmental factors.
Phoebus Levene discovered that the sugar ribose is found in some
nucleic acids, those we now call RNA.
William Bateson first applied Mendel's laws to animals.
Thomas Hunt Morgan proved that genes are carried on chromosomes,
establishing the basis of modern genetics. With his co-workers, he
pinpointed the location of various fruit fly genes on chromosomes,
establishing the use of Drosophila fruit flies to study heredity.
Morgan's group also demonstrated the existence of sex-linked genes,
and over the next ten years expanded the idea to other trait linkages,
using "crossing-over" to help determine the location of genes.
1911 Thomas Hunt Morgan explained the separation of certain
inherited characteristics that are usually linked as caused by the
breaking of chromosomes sometimes during the process of cell division.
Morgan began to map the positions of genes on chromosomes of the fruit
Lawrence Bragg discovered that X-rays can be used to study the
molecular structure of simple crystalline substances. This discovery
led to the development of X-ray crystallography, which made it
possible to further explore the three-dimensional structures of
nucleic acids and proteins.
Alfred H. Sturtevant, a student of Morgan's, constructed the first
gene map by analyzing mating results for fruit flies with six
different mutant factors each known to be recessive and X-linked. He
traced each mutation and its normal alternate in relation to each of
the other mutants, and thus calculated the exact percentage of
crossing-over between the genes.
Frederick Twort discovered a "disease" of bacteria called "glassy
transformation" and showed the disease agent is transmissible,
filterable, invisible with a light microscope and does not grow in the
absence of the living host bacterium. Twort proposed three possible
explanations, including a filterable virus and an enzyme.
George Harrison Shull, a pioneering corn breeder and Princeton
genetics professor, published the inaugural issue of the scientific
1917 Plough demonstrated the rearrangement of chromosomes
known as 'crossing over'.
F. D'Herelle described "an invisible microbe" that antagonizes the
bacillus that causes dysentery and coined the term "bacteriophage" for
the antagonist. Phage caused plaques on bacterial lawns, analogous to
colonies on agar plates. Later plaques will prove useful in preparing
pure cultures and characterizing different strains of the
bacteriophages or bacterial viruses.
Sewall Wright analyzed the inheritance of coat colors in guinea pigs,
mice, rats, rabbits, horses, and other mammals. Wright showed that
production of the pigment determining coat color in mammals requires
biochemical steps, taking place in fixed temporal order. He suggested
that each step is mediated by a different, specific enzyme.
Herbert M. Evans found (incorrectly) that human cells contain 48
The German army used acetone produced by plants to make bombs. Yeast
was grown in large quantities to produce glycerol, and activated
sludge was produced in large quantities for sewage treatment
A world-wide epidemic of influenza killed 20 million people, more than
killed in The World War.
Plant hybridization became widespread in the United States, greatly
improving the productivity of agriculture.
Hermann J. Muller, another of Thomas Hunt Morgan's students, wrote a
paper about the nature of the gene that proved to be astonishingly
prescient. He conceived of the gene as a particle that, despite its
ultramicroscopic size, exhibits a complex structure of different
Politicians encouraged by the eugenics movement passed the U.S.
Immigration Act of 1924, limiting the influx of poorly educated
immigrants from Southern and Eastern Europe on the grounds of
suspected genetic inferiority.
Nikolai Vavilov led Russian plant hunters on the first attempt to
"cover the globe" in search of wild plants and primitive cultivators.
For his scientific curiosity, he was later thrown in prison, dying
there of malnutrition in 1943.
Congress voted to cut the Seed Distribution Program, which had
consumed more than 10% of the USDA's total budget in 1921. The
decades-old flood of free seed to the agricultural community suddenly
Thomas Hunt Morgan published 'The theory of the gene', the culmination
of work on the physical basis for Mendelian genetics based on breeding
studies and optical microscopy.
Hermann Muller discovered that X-rays induce genetic mutations in
fruit flies 1,500 times more quickly than under normal circumstances.
This discovery provided researchers with a way to induce mutations, an
important tool for discovering what genes do on their own.
Henry Agard Wallace, US Secretary of Agriculture during the Franklin
Roosevelt's first two terms, and Vice-president during his third,
founded the Hi-Bred Company - a hybrid corn seed producer and
marketer known today as Pioneer Hi-Bred International, Inc.
Fredrick Griffiths noticed that a rough type of bacterium changed to a
smooth type when an unknown "transforming principle" from the smooth
type was present. Sixteen years later, Oswald Avery identified that
"transforming principle" as DNA.
Lewis Stadler showed that ultraviolet radiation can also cause
Alexander Fleming noticed that all the bacteria in a radius
surrounding a bit of mold in a petrie dish had died. The age of
penicillin thus began, although it would be almost 15 years before it
was made available to the community for medicinal use.
elucidated the physical laws governing how atoms are arranged within
molecules. He also described sickle cell anemia, defining it a
1929 Phoebus Levene discovered a previously unknown sugar,
deoxyribose, in nucleic acids that do not contain ribose; those
nucleic acids are now known as deoxyribonucleic acids, or DNA.
Thirty states in the U.S. had adopted compulsory sterilization laws.
Germany established eugenics laws, sterilizing 56,244 individuals as
T.S. Painter announced in a brief article in Science that he had
charted perceptible differences among chromosomes under the microscope
- differences detailed enough to correlate crossing-over of genes as
shown in the statistical tables with physical interchanges in the
material of the chromosomes.
Desmond Bernal showed that giant molecules, such as proteins, can be
studied using X-ray crystallography.
Martin Schlesinger purified bacteriophage and found about equal
amounts of protein and DNA. Which of these was the informational
molecule remained unclear.
Wendell Meredith Stanley crystallized tobacco mosaic virus, the first
such purification of a virus. He believed, incorrectly, that protein
was the active agent of the virus
George Beadle and Boris Ephrussi transplanted tissue between larvae of
fruit flies bred with various mutations of eye color and observed the
mature flies development.
Andrei Nikolaevitch Belozersky isolated DNA in the pure state for the
1936 Wendell M. Stanley isolated nucleic acids from the
tobacco mosaic virus, which later (1955) will be found to cause the
1937 Frederick Charles Bawden discovered that tobacco mosaic
virus contains RNA.
Howard Florey and Ernst Chain of Oxford University in England isolated
the antimicrobial agent penicillin.
Proteins and DNA were studied in various labs with X-ray
The term "molecular biology" was coined.
Gauteret cultivated carrot callus cultures.
Andrei Nikolaevitch Belozersky started his experimental work showing
that both DNA and RNA are always present in bacteria.
Large-scale production of penicillin was achieved.
Countries in the West made the transition from animal power to
mechanical power on farms.
George Beadle and Edward Tatum experimented with Neurospora, a mold
that grows on bread in the tropics, developing the
"one-gene-one-enzyme" hypothesis: each gene is translated into an
enzyme to perform tasks within an organism. They examined
X-ray-damaged mold specimens that would not grow on the sample medium,
but would grow if they added a certain vitamin. They hypothesized
that the X-rays had damaged the genes that synthesized the proteins.
The Rockefeller Foundation, collaborating with the Mexican government,
initiated the Mexican Agricultural Program. This was the first use of
plant breeding as foreign aid.
Cortisone was first manufactured in large amounts.
1943 Salvador Luria and Max Delbruck performed "the
fluctation test," the first quantitative study of mutation in
bacteria. This was the beginning of bacterial genetics as a distinct
Oswald Theodore Avery, Colin MacLeod and Maclyn McCarty determined
that DNA is the hereditary material involved in transformation in
pneumococcus bacteria. At first this theory gained little attention
because scientists believed that DNA was too simple a molecule to
contain all of the genetic information for an organism. Most
scientists believed that only proteins were complex enough to express
all of the genetic
Waksman isolated streptomycin, an effective antibiotic for TB.
Frederick Sanger used a new method called chromatography to determine
the amino acid sequences of the bovine insulin molecule.
Barbara McClintock, working in Cold Spring Harbour, New York, discovers that genes can be transposed from one position to another on a chromosome. McClintock was awarded the Nobel Prize for Physiology or Medicine in 1983 for her discovery.
Max Delbruck and Salvador Luria developed a simple model system using
phage for studying how genetic information is transferred to host
bacterial cells. They organized a course to study a type of bacterial
virus the T phages that consists of a protein coat containing DNA.
Delbruck and Luria's course attracted many scientists to Cold Spring
Harbor, which soon became a center for new ideas about explaining
heredity at the cellular and molecular levels.
The U.N. Food and Agriculture Organization (FAO) was formed in Quebec,
1945 - 1950
Isolated animal cell cultures were grown in laboratories.
D.C. Salmon, a U.S. military adviser stationed in Japan, sent home
Norin 10 - the source of the dwarfing gene that later helped produce
the Green Revolution wheat varieties.
Edward Tatum and Joshua Lederberg showed that bacteria sometimes
exchange genetic material directly, in a process they called
Max Delbruck and Alfred Day Hershey independently discovered that the
genetic material from different viruses can be combined to form a new
type of virus. This process was another example of genetic
McClintock first reported on
"transposable elements" - known today as "jumping genes." The
scientific community failed to appreciate the significance of her
discovery at the time.
Erwin Chargaff found that in DNA the amounts of adenine and thymine
are about the same, as are the amounts of guanine and cytosine. These
relationships are later known as "Chargaff's Rules" and serve as a key
principle for Watson and Crick in assessing various models for the
structure of DNA.
Earle and Enders studied monkey, mouse, and chick cells in cell
Artificial insemination of livestock using frozen semen, a long-time
dream of farmers, was successfully accomplished.
Esther M. Lederberg discovered lambda phage, a virus of E. coli.
Joshua Lederberg and Norton Zinder showed that bacteria sometimes
exchange genes by an indirect method, which they termed
"transduction," in which a virus mediates the exchange by snaring bits
of DNA from one bacterial cell and transporting the bacterial genes
into the next cell it infects.
Alfred Hershey and Martha Chase performed the infamous "blender
experiments" using phages. They postulated that if one could
separately "tag" both the DNA and protein in the phages, then one
could follow the DNA and proteins through the phages' replication
process. Using virus particles with DNA tagged or 'labeled' with 32P
phosphorous and protein labeled with 35S, Hershey and Chase showed
that only the DNA of the virus enters the cell in significant amounts.
This result supported a role for DNA has the genetic material, and
refuted a role for protein.
J. Lederberg introduces the term plasmid to describe the bacterial
structures he has discovered that contain extrachromosomal genetic
William Hayes discovered conjugation, the process whereby one
bacterial cell pipes a copy of some of its genes into a second
Electron microscopy showed the insides of cells to be filled with
minute but well-formed anatomical structures, including vast numbers
of a complex molecular organ now termed the ribosome.
Jean Brachet suggested that RNA, a nucleic acid, plays a part in the
synthesis of proteins.
James Watson and Francis Crick proposed the double-stranded, helical,
complementary, anti-parallel model for DNA.
William Hayes discovered that plasmids can be used to transfer
introduced genetic markers from one bacterium to another.
L. Cavalli, J. Lederberg, and E. Lederberg discovered the F factor in
Compiled from various sources, including:
Brock, Thomas D. 1961. Milestones in Microbiology. Science Tech
Publishers. Madison, Wisconsin. Pp. 273.
Brock, Thomas D. 1990. The Emergence of Bacterial Genetics. Cold
Spring Harbor Laboratory Press. Cold Spring Harbor, New York. Pp.
Bunch, Bryan and Hellemans, Alexander. 1993. The Timetables of
Technology. Simon & Schuster. New York, New York. Pp. 490.
Hellemans, Alexander, and Bunch, Bryan. 1988. The Timetables of
Science. Simon & Schuster. New York, New York. Pp. 660.
Go to next timeline: 1953 - 1976: Expanding the Boundaries of DNA
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