Avery–MacLeod–McCarty experiment was an experimental
demonstration, reported in 1944 by Oswald Avery, Colin MacLeod, and
Maclyn McCarty, that
DNA is the substance that causes bacterial
transformation, in an era when it had been widely believed that it was
proteins that served the function of carrying genetic information
(with the very word protein itself coined to indicate a belief that
its function was primary). It was the culmination of research in the
1930s and early 20th Century at the Rockefeller Institute for Medical
Research to purify and characterize the "transforming principle"
responsible for the transformation phenomenon first described in
Griffith's experiment of 1928: killed
Streptococcus pneumoniae of the
virulent strain type III-S, when injected along with living but
non-virulent type II-R pneumococci, resulted in a deadly infection of
type III-S pneumococci. In their paper "Studies on the Chemical Nature
of the Substance Inducing Transformation of Pneumococcal Types:
Induction of Transformation by a Desoxyribonucleic Acid Fraction
Isolated from Pneumococcus Type III", published in the February 1944
issue of the Journal of Experimental Medicine, Avery and his
colleagues suggest that DNA, rather than protein as widely believed at
the time, may be the hereditary material of bacteria, and could be
analogous to genes and/or viruses in higher organisms.
Avery and his colleagues showed that
DNA was the key component of
Griffith's experiment, in which mice are injected with dead bacteria
of one strain and live bacteria of another, and develop an infection
of the dead strain's type.
With the development of serological typing, medical researchers were
able to sort bacteria into different strains, or types. When a person
or test animal (e.g., a mouse) is inoculated with a particular type,
an immune response ensues, generating antibodies that react
specifically with antigens on the bacteria.
Blood serum containing the
antibodies can then be extracted and applied to cultured bacteria. The
antibodies will react with other bacteria of the same type as the
original inoculation. Fred Neufeld, a German bacteriologist, had
discovered the pneumococcal types and serological typing; until
Frederick Griffith's studies bacteriologists believed that the types
were fixed and unchangeable from one generation to the next.
Griffith's experiment, reported in 1928, identified that some
"transforming principle" in pneumococcal bacteria could transform them
from one type to another. Griffith, a British medical officer, had
spent years applying serological typing to cases of pneumonia, a
frequently fatal disease in the early 20th century. He found that
multiple types—some virulent and some non-virulent—were often
present over the course of a clinical case of pneumonia, and thought
that one type might change into another (rather than simply multiple
types being present all along). In testing that possibility, he found
that transformation could occur when dead bacteria of a virulent type
and live bacteria of a non-virulent type were both injected in mice:
the mice would develop a fatal infection (normally only caused by live
bacteria of the virulent type) and die, and virulent bacteria could be
isolated from such infected mice.
The findings of
Griffith's experiment were soon confirmed, first by
Fred Neufeld at the Koch Institute and by
Martin Henry Dawson at
the Rockefeller Institute. A series of Rockefeller Institute
researchers continued to study transformation in the years that
followed. With Richard H.P. Sia, Dawson developed a method of
transforming bacteria in vitro (rather than in vivo as Griffith had
done). After Dawson's departure in 1930, James Alloway took up the
attempt to extend Griffith's findings, resulting in the extraction of
aqueous solutions of the transforming principle by 1933. Colin MacLeod
worked to purify such solutions from 1934 to 1937, and the work was
continued in 1940 and completed by Maclyn McCarty.
1 Experimental work
4 Further reading
5 External links
Pneumococcus is characterized by smooth colonies and has a
polysaccharide capsule that induces antibody formation; the different
types are classified according to their immunological specificity.
The purification procedure Avery undertook consisted of first killing
the bacteria with heat and extracting the saline-soluble components.
Next, the protein was precipitated out using chloroform and the
polysaccharide capsules were hydrolyzed with an enzyme. An
immunological precipitation caused by type-specific antibodies was
used to verify the complete destruction of the capsules. Then, the
active portion was precipitated out by alcohol fractionation,
resulting in fibrous strands that could be removed with a stirring
Chemical analysis showed that the proportions of carbon, hydrogen,
nitrogen, and phosphorus in this active portion were consistent with
the chemical composition of DNA. To show that it was
DNA rather than
some small amount of RNA, protein, or some other cell component that
was responsible for transformation, Avery and his colleagues used a
number of biochemical tests. They found that trypsin, chymotrypsin and
ribonuclease (enzymes that break apart proteins or RNA) did not affect
it, but an enzyme preparation of "deoxyribonucleodepolymerase" (a
crude preparation, obtainable from a number of animal sources, that
could break down DNA) destroyed the extract's transforming power.
Follow-up work in response to criticism and challenges included the
purification and crystallization, by
Moses Kunitz in 1948, of a DNA
depolymerase (deoxyribonuclease I), and precise work by Rollin
Hotchkiss showing that virtually all the detected nitrogen in the
DNA came from glycine, a breakdown product of the nucleotide
base adenine, and that undetected protein contamination was at most
0.02% by Hotchkiss's estimation.
Maclyn McCarty (with Watson and Crick)
The experimental findings of the Avery–MacLeod–McCarty experiment
were quickly confirmed, and extended to other hereditary
characteristics besides polysaccharide capsules. However, there was
considerable reluctance to accept the conclusion that
DNA was the
genetic material. According to Phoebus Levene's influential
DNA consisted of repeating units of the
four nucleotide bases and had little biological specificity.
therefore thought to be the structural component of chromosomes,
whereas the genes were thought likely to be made of the protein
component of chromosomes. This line of thinking was reinforced
by the 1935 crystallization of tobacco mosaic virus by Wendell
Stanley, and the parallels among viruses, genes, and enzymes; many
biologists thought genes might be a sort of "super-enzyme", and
viruses were shown according to Stanley to be proteins and to share
the property of autocatalysis with many enzymes. Furthermore, few
biologists thought that genetics could be applied to bacteria, since
they lacked chromosomes and sexual reproduction. In particular, many
of the geneticists known informally as the phage group, which would
become influential in the new discipline of molecular biology in the
1950s, were dismissive of
DNA as the genetic material (and were
inclined to avoid the "messy" biochemical approaches of Avery and his
colleagues). Some biologists, including fellow Rockefeller Institute
Fellow Alfred Mirsky, challenged Avery's finding that the transforming
principle was pure DNA, suggesting that protein contaminants were
instead responsible. Although transformation occurred in some
kinds of bacteria, it could not be replicated in other bacteria (nor
in any higher organisms), and its significance seemed limited
primarily to medicine.
Scientists looking back on the Avery–MacLeod–McCarty experiment
have disagreed about just how influential it was in the 1940s and
Gunther Stent suggested that it was largely ignored, and
only celebrated afterwards—similarly to Gregor Mendel's work decades
before the rise of genetics. Others, such as
Joshua Lederberg and
Leslie C. Dunn, attest to its early significance and cite the
experiment as the beginning of molecular genetics.
A few microbiologists and geneticists had taken an interest in the
physical and chemical nature of genes before 1944, but the
Avery–MacLeod–McCarty experiment brought renewed and wider
interest in the subject. While the original publication did not
mention genetics specifically, Avery as well as many of the
geneticists who read the paper were aware of the genetic
implications—that Avery may have isolated the gene itself as pure
DNA. Biochemist Erwin Chargaff, geneticist
H. J. Muller
H. J. Muller and others
praised the result as establishing the biological specificity of DNA
and as having important implications for genetics if
DNA played a
similar role in higher organisms. In 1945, the
Royal Society awarded
Avery the Copley Medal, in part for his work on bacterial
Between 1944 and 1954, the paper was cited at least 239 times (with
citations spread evenly through those years), mostly in papers on
microbiology, immunochemistry, and biochemistry. In addition to the
follow-up work by McCarty and others at the Rockefeller Institute in
response to Mirsky's criticisms, the experiment spurred considerable
work in microbiology, where it shed new light on the analogies between
bacterial heredity and the genetics of sexually-reproducing
organisms. French microbiologist André Boivin claimed to extend
Avery's bacterial transformation findings to Escherichia coli,
although this could not be confirmed by other researchers. In
Joshua Lederberg and
Edward Tatum demonstrated
bacterial conjugation in E. coli and showed that genetics could apply
to bacteria, even if Avery's specific method of transformation was not
general. Avery's work also motivated
Maurice Wilkins to continue
X-ray crystallographic studies of DNA, even as he faced pressure from
funders to focus his research on whole cells, rather than
Despite the significant number of citations to the paper and positive
responses it received in the years following publication, Avery's work
was largely neglected by much of the scientific community. Although
received positively by many scientists, the experiment did not
seriously affect mainstream genetics research, in part because it made
little difference for classical genetics experiments in which genes
were defined by their behavior in breeding experiments rather than
their chemical makeup. H. J. Muller, while interested, was focused
more on physical rather than chemical studies of the gene, as were
most of the members of the phage group. Avery's work was also
neglected by the Nobel Foundation, which later expressed public regret
for failing to award Avery a Nobel Prize.
By the time of the 1952 Hershey–Chase experiment, geneticists were
more inclined to consider
DNA as the genetic material, and Alfred
Hershey was an influential member of the phage group. Erwin
Chargaff had shown that the base composition of
DNA varies by species
(contrary to the tetranucleotide hypothesis), and in 1952 Rollin
Hotchkiss published his experimental evidence both confirming
Chargaff's work and demonstrating the absence of protein in Avery's
transforming principle. Furthermore, the field of bacterial
genetics was quickly becoming established, and biologists were more
inclined to think of heredity in the same terms for bacteria and
higher organisms. After Hershey and Chase used radioactive
isotopes to show that it was primarily DNA, rather than protein, that
entered bacteria upon infection with bacteriophage, it was soon
widely accepted that
DNA was the material. Despite the much less
precise experimental results (they found a not-insignificant amount of
protein entering the cells as well as DNA), the Hershey–Chase
experiment was not subject to the same degree of challenge. Its
influence was boosted by the growing network of the phage group and,
the following year, by the publicity surrounding the
proposed by Watson and Crick (Watson was also a member of the phage
group). Only in retrospect, however, did either experiment
definitively prove that
DNA is the genetic material.
^ a b c d Avery, Oswald T.; Colin M. MacLeod; Maclyn McCarty
(1944-02-01). "Studies on the Chemical Nature of the Substance
Inducing Transformation of Pneumococcal Types: Induction of
Transformation by a Deoxyribonucleic Acid Fraction Isolated from
Pneumococcus Type III". Journal of Experimental Medicine. 79 (2):
137–158. doi:10.1084/jem.79.2.137. PMC 2135445 .
PMID 19871359. Archived from the original on 7 October 2008.
^ Fruton (1999), pp. 438–440
^ Lehrer, Steven. Explorers of the Body. 2nd edition. iuniverse 2006 p
^ Griffith, Frederick (January 1928). "The Significance of
Pneumococcal Types". The Journal of Hygiene. 27 (2): 113–159.
doi:10.1017/S0022172400031879. JSTOR 4626734.
PMC 2167760 . PMID 20474956.
^ Dawes, Heather (August 2004). "The quiet revolution". Current
Biology. 14 (15): R605–R607. doi:10.1016/j.cub.2004.07.038.
PMID 15296771. Retrieved 2009-02-25.
^ Neufeld, Fred; Levinthal, Walter (1928). "Beitrage zur Variabilitat
der Pneumokokken". Zeitschrift für Immunitatsforschung. 55:
^ Dawson, Martin H. "The Interconvertibility of 'R' and 'S' Forms of
Pneumococcus", Journal of Experimental Medicine, volume 47, no. 4 (1
April 1928): 577–591.
^ Dawson, Martin H.; Sia, Richard H. P. (1930). "The Transformation of
Pneumococcal Types In Vitro". Proceedings of the Society for
Experimental Biology and Medicine. 27: 989–990.
^ Fruton (1999), p. 438
^ The Oswald T. Avery Collection: "Shifting Focus: Early Work on
Bacterial Transformation, 1928–1940." Profiles in Science. U.S.
National Library of Medicine. Accessed February 25, 2009.
^ Fruton (1999), p. 439
^ Witkin EM (August 2005). "Remembering Rollin Hotchkiss
(1911–2004)". Genetics. 170 (4): 1443–7. PMC 1449782 .
^ a b c Morange (1998), pp. 30–39
^ a b Fruton (1999), pp. 440–441
^ Stanley, Wendell M. (1935-06-28). "Isolation of a Crystalline
Protein Possessing the Properties of Tobacco-Mosaic Virus" (PDF).
Science. New Series. 81 (2113): 644–645.
JSTOR 1658941. Archived from the original (PDF) on September 27,
2006. Retrieved 2009-02-26.
^ On the intersecting theories of viruses, genes and enzymes in this
period, see: Creager, Angela N. H. The Life of a Virus: Tobacco Mosaic
Virus as an Experimental Model, 1930–1965. University of Chicago
Press: Chicago, 2002. ISBN 0-226-12025-2
^ a b c d Deichmann, pp. 220–222
^ Deichmann, pp. 207–209
^ Deichmann, pp. 215–220
^ Boivin; Boivin, André; Vendrely, Roger; Lehoult, Yvonne (1945).
"L'acide thymonucléique hautement polymerise, principe capable de
conditioner la spécificité sériologique et l'équipement
enzymatique des Bactéries. Conséquences pour la biochemie de
l'hérédité". Comptes rendus. 221: 646–648.
^ Lederberg, Joshua; Edward L. Tatum (1946-10-19). "
in Escherichia Coli". Retrieved 2009-02-26.
^ Deichmann, pp. 227–231
^ a b c Morange (1998), pp. 44–50
^ a b c Fruton (1999), pp. 440–442
^ Chargaff E (June 1950). "Chemical specificity of nucleic acids and
mechanism of their enzymatic degradation". Experientia. 6 (6):
201–9. doi:10.1007/BF02173653. PMID 15421335.
^ Hotchkiss, Roland D. "The role of deoxyribonucleotides in bacterial
transformations". In W. D. McElroy; B. Glass. Phosphorus Metabolism.
Baltimore: Johns Hopkins University Press. pp. 426–36.
^ Hershey AD, Chase M (May 1952). "Independent functions of viral
protein and nucleic acid in growth of bacteriophage". The Journal of
General Physiology. 36 (1): 39–56. doi:10.1085/jgp.36.1.39.
PMC 2147348 . PMID 12981234.
Deichmann, UTE (2004). "Early responses to Avery et al.'s paper on DNA
as hereditary material". Historical Studies in the Physical and
Biological Sciences. 34: 207–32.
Fruton, Joseph S. (1999). Proteins, enzymes, genes: the interplay of
chemistry and biology. New Haven, Conn: Yale University Press.
Matthew Cobb; Morange, Michel (1998). A history of molecular biology.
Cambridge: Harvard University Press. ISBN 0-674-00169-9.
Lehrer, Steven (2006). Explorers of the Body: Dramatic Breakthroughs
in Medicine from Ancient Times to Modern Science. United States:
iUniverse. ISBN 0-595-40731-5.
Lederberg J (February 1994). "The transformation of genetics by DNA:
an anniversary celebration of Avery, MacLeod and McCarty (1944)".
Genetics. 136 (2): 423–6. PMC 1205797 .
McCarty, Maclyn (1986). The transforming principle: discovering that
genes are made of DNA. New York: Norton.
Stegenga, Jacob (2011). "The chemical characterization of the gene:
vicissitudes of evidential assessment". History and Philosophy of the
Life Sciences. 33 (1): 105–127. PMID 21789957.
DNA: The Search for the Genetic Material Avery, MacLeod and McCarty's
Experiment for the Advanced Science Hobbyist.
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