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Brachet, Jean Louis

BRACHET, JEAN LOUIS

(b. Etterbeek, Belgium, 19 March 1909; d. Braine-l’Alleud, Belgium, 8 February 1988),

embryology, cytology, molecular biology.

In the course of the 1930s, Brachet established that RNA and DNA are both universal constituents of both animal and plant cells. During the two following decades, Brachet, Raymond Jeener, Hubert Chantrenne and colleagues decisively demonstrated that a specific RNA fraction, the messenger RNA(mRNA), enables the transfer of genetic information from the nucleus to cytoplasmic microsomes, where the synthesis of protein occurs.

Biographical Sketch. The youngest of the two sons of the Belgian embryologist Albert Brachet and Marguerite Guchez, one of the first Belgian women to have undertaken medical studies (though interrupted after the fifth year, as she fell in love with her teacher), Jean Brachet grew up and started his career during a period darkened by the two world wars. In August 1914, as Albert Brachet was working at the marine station of Roscoff (France), the Brachet family was surprised by the German invasion and stayed in France until the end of World War I. As his father was teaching at the Collège de France and at the School of Medicine, the five-year-old Jean Brachet learned to read and write in Paris.

Back in Belgium after the war, Brachet pursued his studies in Brussels, graduating in medicine in 1934 at the University of Brussels. Fascinated by the histology course of Pol Gerard and surprised by the fact that the role of the nucleus in the cell was so poorly understood, Brachet decided to study this problem. He approached Albert Dalcq about working in his laboratory; Dalcq proposed several areas of research, and Brachet selected the problem of the localization of thymonucleic acid (now called DNA) at different stages of egg formation. Performed in tandem with medical studies, his researches led to the publication of his first paper in 1929. This early start prefigured a long series of more than 400 articles dealing with various molecular aspects of embryonic development during sixty years of intensive scientific activity.

After his graduation, Brachet became an assistant professor at the Faculty of Medicine and pursued his scientific work in parallel with teaching duties. In 1934 Brachet married Françoise de Barcy, an excellent musician, with whom he had three children: Etienne, who became a physician and also taught at the University of Brussels; Lise, a dentist and talented painter; and Philippe, who was a successful researcher in molecular biology at the Centre national de la recherche scientifique (CNRS) in France. Brachet’s wife took care of most daily domestic aspects, enabling him to fully devote himself to scientific research.

A member of the international brigades, Brachet’s older brother Pierre was killed in 1936, a loss that deeply affected his parents, who were barely recovering from the loss of Albert Brachet in 1930.

In 1937 Brachet obtained a grant from the Rockefeller Foundation, which enabled him to work at Princeton University as well as at the Marine Biological Laboratory of Woods Hole, where he met the best American embryologists and cell biologists of the time. Brachet was deeply influenced by the pervasive pragmatism and open attitude of American researchers, which contrasted with continental European traditions.

Back in Brussels, Brachet joined the Faculty of Science to teach animal morphology in the context of the undergraduate program of zoology, and soon also taught general biology to chemistry students. At this point, Brachet joined efforts with Raymond Jeener, who was teaching physiology and who was also attracted by biochemical approaches. This partnership was the seed of a quickly growing interdisciplinary research group, ultimately leading to the founding of the first department of molecular biology in Belgium.

However, in July 1942, the promising researches of Brachet and Jeener were brutally interrupted, as the University of Brussels closed down in protest against the Germans’ expulsion of all Jewish professors. Jeener and Brachet then pursued their research in the laboratories of colleagues from other Belgian institutions. Brachet was arrested in December 1942 and detained as a hostage together with other intellectuals. Liberated in 1943 but prevented from pursuing his experimental researches until the end of the war, Brachet focused on writing a book. Titled Embryologie Chimique and published in 1944, this book is devoted to a critical synthesis of what was known about the biochemistry of embryonic development, pointing to unsolved issues and delineating specific strategies to address them. This book stimulated the interest of the young French-speaking scientists who regularly visited

Brachet and Jeener’s laboratory settlement at the outskirts of Brussels from the end of World War II.

At that time, most opponents to the German invasion were affiliated with the Communist Party, which was the most efficient resistance organization. Brachet remained a member of the Communist Party until 1949, when he was asked to support the accomplishments of Trophim Lyssenko in the Soviet Union. After visiting Moscow and talking with several Russian scientists, including Lyssenko himself, Brachet realized that their claims relied much more on ideology than on scientific analysis. His public critical account angered Belgian communist leaders. Threatened with expulsion, Brachet offered and was finally authorized to resign from his party membership to avoid open conflict.

This relatively brief episode as a member of the Belgian Communist Party had long-term detrimental effects on Brachet’s scientific carrier, in particular by complicating access to a U.S. visa until his retirement. However, this did not preclude Brachet and his colleagues from

obtaining several grants from the Rockefeller Foundation, which enabled them to equip their laboratory with the analytical instruments (centrifuge, spectrophotometer, and the like) needed to remain at the forefront of molecular biology research.

From the 1960s on, Brachet spent several months each year in Italy, first in the International Laboratory of Genetics and Biophysics headed by Adriano Buzzati Traverso in Naples, and later in the laboratory of Molecular Embryology directed by Alberto Monroy, located nearby in Arco Felice. Together with his Italian colleagues Brachet also regularly worked at the Anton Dorhn Zoological Station, renowned among embryologists for the availability of fresh marine material, as well as for its friendly atmosphere. After his retirement in 1977, Brachet split his time between Brussels and the Naples area. During this period he remained extremely active, pursuing experimental embryological research and giving regular scientific presentations.

From Experimental to Chemical and Molecular Embryology. At the beginning of the twentieth century, experimental embryology was well established in most academic centers of western Europe. At the Free University of Brussels, Albert Brachet pioneered experimental studies of vertebrate development. He called his approach Causal Embryology, also the title of one of his major books. Jean Brachet learned embryology with his father’s academic heir, Dalcq, whose studies initially focused on the cytological analysis of spermatogenesis, parthenogenesis, and mitosis. During the 1930s Dalcq turned his attention to the first steps of embryonic development, from fecundation to gastrulation. His experimental studies were synthesized and contextualized in a book, Form and Causality in Early Development (1938).

In parallel, Joseph Needham at Cambridge was developing extensive chemical analyses of the developing eggs, focusing in particular on respiratory metabolism. Published in 1931, Needham’s monumental Chemical Embryology compiled most contemporary data on the chemical constitution of eggs and embryos, and discussed the main directions of a research program aiming at providing chemical bases to major concepts of experimental embryology such as gradient, morphogenetic field, induction, and organizer.

Deeply influenced by Needham, Brachet embraced his general attempt to explain embryonic development in terms of chemical processes, and titled his first book Embryologie Chimique (1944, translated into English in 1950). In 1934–1935 Brachet spent a year in Needham’s laboratory and participated in the hunt for the illusive organizer molecule, which would explain the phenomenon of neural induction uncovered during the early 1920s by Hilde Mangold and Hans Spemann. Conrad H. Waddington, Brachet, and collaborators observed that even artificial substances such as methylene blue could provoke neural induction, leading them to postulate an inherent neural potency, which could be stimulated by various means (Waddington referred to these stimulants as “evocators”).

It is in this context that Brachet initiated his studies on the characterization of nucleic acids in eggs and developing embryos. At that time, two classes of nucleic acids where distinguished. On the one hand, thymonucleic acids (later called DNA) were found in large quantities in thymus of animals. The use of a specific staining technique developed by Robert Feulgen revealed a spatial and quantitative correlation of DNA with chromosomes across cell divisions. On the other hand, zymonucleic acids (later called RNA) were initially isolated from yeast cultures and wrongly considered to be plant specific.

Two main theories were then advanced. According to Jacques Loeb, nucleic acids would be synthesized de novo from elementary constituents (phosphoric acid, sugar, purine and pyrimidine bases). In contrast, the embryolo-gist Emile Godlewski thought that the cytoplasm of the egg would contain a reserve of nucleic acids, which would progressively migrate into nuclei, where chromosomes were known to be located. Experimental results at the time were confusing. On the one hand, purine or phosphoric acid dosages suggested a global conservation of nucleic acids. On the other hand, the quantification of deoxyribose suggested a net synthesis of deoxyribonucleic acids.

To solve this contradiction, Brachet hypothesized the presence of ribonucleic acids in animal eggs. To test this hypothesis, he developed a novel cytochemical technique to estimate cellular contents in DNA and RNA. This technique, known as the Unna-Brachet method combines two stains (pyronine and methyl green) revealing RNA in pink, DNA in green. The use of ribonuclease permitted double-checking whether stained material was RNA or not, as this enzyme removed the color where RNA was present. Comparison with parallel Feulgen staining provided further cross-checking of the proportion of staining really due to DNA.

This sophisticated protocol resulted in a considerable increase in spatial and temporal sensitivity and enabled Brachet to demonstrate the presence of RNA in all organisms tested, first in sea urchin eggs and soon in many different animal and plant cell types. Brachet further established that DNA remained present at all stages of egg and embryo formation, with notable DNA synthesis during cell duplications.

The comparative analysis of RNA and DNA contents of various animal and plant cell types soon led Brachet to emphasize another correlation, which later proved to be of primary importance. Indeed, in parallel with the group of Torbjörn Oskar Caspersson in Sweden, Brachet realized that the DNA content per cell is roughly conserved in the diverse cell types of a given organism, whereas RNA content varies much more widely and is particularly high in cells strongly involved in protein synthesis.

Molecular Explorations at the Rouge-Cloître. During the 1940s the blending of various biological, chemical, and physical approaches, accompanied by the development of novel instruments such as the ultracentrifuge and the electron microscope, as well as by novel experimental protocols, led to a profound transformation of biology. In this respect, several Belgian scientists notably contributed to the progressive delineation of cellular ultra-structures. At the Free University of Brussels, the physicist Émile Henriot invented an ultracentrifuge prototype propelled and supported by compressed air. Beckman later commercialized a modified version of this centrifuge. Henriot also contributed to the development of electron microscopy and to its adaptation to biological purposes. Independently from Ernst Ruska, Henriot’s assistant Lucien Marton built a transmission electronic microscope in Brussels, which was later developed for the RCA Company. However, the proper biological exploitation of this transmission electronic microscope would have to wait until the development of novel techniques to prepare very thin slices of biological tissues, notably by Albert Claude and Keith Robert Porter at the Rockefeller Institute for Medical Research in New York.

These two novel analytic instruments enabled Claude, George Emil Palade, and Christian De Duve to progressively uncover the structural and functional organization of cells, progressively separating mitochondria, lysosomes, ribosomes, and other constituents from the cytoplasm. These achievements earned them the 1974 Nobel Prize in Physiology and Medicine.

Inspired by Claude’s results, and despite German occupation and the consequent practical difficulties, Jeener, Brachet, and a young doctoral student in biochemistry, Hubert Chantrenne, took advantage of Henriot’s centrifuge to isolate small cytoplasmic particles, and demonstrated that almost the totality of RNA in adult cells was located in macromolecular granules (soon called microsomes), in association with hydrolytic or respiratory enzymes. Together with the established correlation between RNA and protein synthesis, this observation led Brachet, Jeener, and Chantrenne to propose that the microsomes constituted the site of protein synthesis.

After the end of the war, the trio, soon joined by Maurice Errera and René Thomas, would pursue this lead, refining centrifuge-based partitions and their chemical characterization, further consolidating the connection between microsome, RNA, and protein synthesis.

At this point, radioactive isotopes started to be used by biochemists to track molecular processes in living cells. The use of radioactive labeling was at the basis of the famous blender experiment by Alfred Hershey (1908–1997) and Martha Chase (1927–2003), which established that DNA (and not proteins) constituted the hereditary material in bacteriophages in 1952.

As radioactive compounds were barely available in post-war Belgium, Jeener produced radioactive phosphorus (32 P) in a lead-protected container located in the laboratory garden, using radium and beryllium salts obtained from the Union Minière company as a neutron source. Using self-prepared32 P as tracer, Jeener, Brachet, and collaborators engaged in an analytical study of the metabolism of ribonucleic acid and soon showed that32 P was incorporated at very different rates into the various ribonucleic fractions of the cell. Jeener also noticed that a depletion of Uracil, a nucleic acid base that is specifically needed for RNA synthesis, caused a concomitant arrest in cell growth.

Independently from Heinz Frankel-Conrad, Gerhard Schramm, and Alfred Gierer, Jeener further demonstrated that the genetic material of tobacco mosaic virus is constituted of RNA. He was then one of the very few researchers to use analogs of nucleic acid bases to analyze their necessity for viral growth. Another collaborator of Brachet and Jeener, Adrienne Ficq, developed a novel auto-radiographic technique to localize tritium (3 H) and radioactive Carbon (14 C) in histological preparations, thereby completing the current array of nucleic acid and protein in situ visualization techniques.

Focusing on DNA, Thomas, a doctoral student of Brachet, measured and compared the ultraviolet absorption spectrum of native DNA to a theoretical spectrum computed on the basis of the spectra of single nucleotides. Thomas observed a discrepancy that vanished following mild DNA treatments, without effects on covalent bounds. This strongly suggested the establishment of weak bonds between DNA molecules and the formation of a secondary structure, an hypothesis which would find independent confirmation in the DNA double helix model proposed by James Watson and Francis Crick in 1953.

The publication of the double-helix model in Nature in 1953 had a high impact on the scientific community but gave little clue about the detailed mechanisms of DNA replication and protein synthesis. The following decade witnessed an intensive hunt for the mechanisms of protein synthesis. Guidelines were provided by Crick in his landmark paper “On Protein Synthesis,” presented at the 1957 annual meeting of the Society of Experimental Biology (published in 1958). Indeed, in this paper, Crick crisply stated the problem of protein synthesis in terms of the specification of the linear amino-acid sequence of proteins by the linear sequence of nucleotides in DNA, stressing the need for (yet to identified) adapter molecules. The solution of the genetic code from studies using in vitro

protein synthesis systems, with notable contributions from Har Gobind Khorana, Marshall W. Nirenberg, and Robert W. Holley, who shared the 1968 Nobel Prize of Physiology and Medicine.

Although Brachet and collaborators did not directly contribute to deciphering the genetic code, their work certainly played a role in the pervasive implication of RNA in the synthesis of proteins. For example, working with large unicellular organisms (in particular the giant Acetabularia alga), Brachet and Chantrenne demonstrated that protein synthesis can persist for days in enucleated cells, and even undergo substantial morphogenetic events (for example, the generation of a cap). They further established that this protein synthesis remains dependant on the presence of RNA in the cytoplasm. The role of RNA in protein synthesis found a spectacular confirmation in the work of François Jacob, Jacques Monod and collaborators, who delineated the main properties of the bacterial messenger RNA around 1960.

The Golden Age of Molecular Biology at the University of Brussels. From the 1950s, Brachet’s small unit could secure equipment with the support of the Rockefeller Foundation and was frequently visited by researchers from United States, England, Italy, and France. This boosted the scientific research activities of Brachet and his colleagues, which can only be partially mentioned here.

Pursuing the mRNA thread, Chantrenne set out to isolate genuine eucaryote mRNA. Under Chantrenne’s direction, Arsène Burny, Gérard Marbaix, and Georges Huez succeeded in extracting, purifying, and characterizing the mRNA coding for hemoglobin from rabbit reticulocytes (red blood cell precursors, devoid of nucleus but nevertheless synthesizing large amounts of hemoglobin). This series of delicate biochemical and biophysical experiments culminated in 1971 with the demonstration by John Gurdon and Marbaix that the injection of rabbit hemoglobin mRNA in frog enucleated oocytes indeed results in the production of rabbit hemoglobin protein.

Taking advantage of two postdoctoral training stays in genetics with Harriet Ephrussi-Taylor in 1953–1954 and with Alfred Hershey in 1957–1958, René Thomas launched a novel research program in bacterial genetics, which led him to uncover one of first cases of positive genetic regulation (trans-activation of most prophage genes by hetero-immune superinfection), and later to develop an original method to model and analyze the behavior of complex regulatory networks, using a logical algebra.

Around the mid-1960s, the multidisciplinary group was so renowned that it was selected by the European Atomic Energy Community (Euratom) as one of the four European centers for the training of biologists, in the context of a European project to study the effects of radiations on living organisms. This led the Free University of Brussels to build new research and teaching facilities on the periphery of Brussels, at Rhode-Saint-Genèse. Inaugurated in 1965, this institute was large enough to host the different units burgeoning from the original little group, spanning diverse biological fields such as microbiology, biochemistry, genetics, embryology, electronic microscopy, virology, parasitology, and immunology.

The mid-1960s were also the time of harsh linguistic conflicts in Belgium, leading to the split of several Belgian universities into autonomous French- and Dutch-speaking entities. In the case of the Free University of Brussels, both resulting entities collaborated in the development of the new molecular biology campus in Rhode-Saint-Genèse and shared several facilities, including a library and a restaurant.

By then Brachet’s scientific accomplishment and international recognition earned him a continuous flow of academic honors, including many affiliations with foreign academies (Denmark, Boston, Edinburgh, Washington, Milan, London, Bologna, and Rome), a wealth of scientific prizes (including the Belgian Franqui Prize, the Schleiden Medal, and the Heineken Prize), nine Honoris Causa doctorates, as well as several prestigious Belgian decorations.

Under the leadership of Brachet and his colleagues, the new Department of Molecular Biology (DBM) quickly grew in size and reputation during the 1970s and 1980s, hosting more than three hundred researchers, technicians, and students. This department has played a crucial role in the development of molecular biology in Belgium, contributing to the formation of many outstanding scientists such as Jeff Schell and Marc Van Montagu, who promoted the development of a large and successful department of plant molecular biology in Ghent.

Brachet and his colleagues were actively involved in the discussions about the building of international bodies and laboratories to foster the development of molecular biology in Europe, ultimately resulting in the foundation of the European Molecular Biology Laboratory (EMBL) and that of the European Molecular Biology Organisation (EMBO).

BIBLIOGRAPHY

The Archives of the Université Libre de Bruxelles house the papers of Brachet, including notes from his Chemical Embryology, a collection of reprints, and some of his correspondence.

WORKS BY BRACHET

“Recherche sur le comportement de l’acide thymonucléique au cours de l’oogénèse chez les diverses espèces animales.” Archives de Biologie39 (1929): 677–697.

With Conrad H. Waddington and Joseph Needham. “Studies on the Nature of the Amphibian Organization Center: III. The Activation of the Evocator.” Proceedings of the Royal Society B120 (1936): 173–198.

“Étude histochimique des protéines au cours du développement embryonnaire des Poissons, de Amphibiens, et des Oiseaux.” Archives de Biologie 51 (1940): 167–202.

“La détection histochimique et le microdosage des acides pentosenucléiques.”Enzymologia 10 (1941): 87–96.

“La localisation des acides pentosenucléiques dans les tissus animaux et les oeufs d’Amphibiens en voie de développement.” Archives de Biologie 53 (1942): 207–257.

With Raymond Jeener. “Recherches sur des particules cytoplasmiques de dimensions macromoléculaires riches en acides pentosenucléique.” Enzymologia 11 (1943): 196.

Embryologie chimique. Paris: Masson & Liège, 1944. 2nd ed., 1945. Published as Chemical Embryology. Translated by Lester G. Barth. New York: Interscience, 1950a.

“The Metabolism of Nucleic Acids during Embryonic Development.” Cold Spring Harbor Symposia on Quantitative Biology 12 (1947a): 18–27.

“Nucleic Acids in the Cell and the Embryo.” Symposia of theSociety for Experimental Biology 1 (1947b): 207–224. “The Localization and the Role of Ribonucleic Acid in the Cell.” Annals of the New York Academy of Sciences 50 (1950b): 861–869.

With Hubert Chantrenne. “Protein Dynthesis in Nucleated and Non-Nucleated Halves of Acetabularia mediterranea Studied with Carbon-14 Dioxide.” Nature 168 (1951): 950.

Le rôle des acides nucléiques dans la vie de la cellule et de l’embryon. Paris: Masson, 1952.

Biochemical Cytology. New York: Academic Press, 1957.

The Biochemistry of Development. London: Pergamon Press, 1960a.

The Biological Role of Ribonucleic Acids. Amsterdam: Elsevier, 1960b.

“Ribonucleic Acids and the Synthesis of Cellular Proteins.” Nature 186 (1960c): 198.

Introduction to Molecular Embryology. New York: Springer Verlag, 1974.

“From Chemical to Molecular Sea Urchin Embryology.” American Zoologist 15 (1975): 485–491.

With Henri Alexandre. Introduction to Molecular Embryology, revised and enlarged ed. Berlin: Springer Verlag, 1985a.

Molecular Cytology, 2 vols. New York: Academic Press, 1985b. “Reminiscences about Nucleic Acid Cytochemistry and Biochemistry.” Trends in the Biochemical Sciences 12 (1987): 244–246.

“Recollections on the Origins of Molecular Biology.” Biochimica et Biophysica Acta 1000 (1989): 1–5.

OTHER SOURCES

Académie Royale de Belgique. Florilège des Sciences en Belgique. Brussels: Author, 1968–1980.

Alexandre, Henri. “Jean Brachet and His School.” InternationalJournal of Developmental Biology 36 (1992): 29–41. Bechtel, William. Discovering Cell Mechanisms.The Creation ofModern Cell Biology. Cambridge, U.K.: Cambridge University Press, 2005.

Brachet, Albert. Traité d’Embryologie des Vertèbrés. Paris: Masson, 1921.

Brachet, Lise. Le Professeur Jean Brachet, Mon Père. Paris: L’Harmattan, 2004. A short biography of Jean Brachet by his daughter, with a collection of testimonies from colleagues, in French.

Burian, Richard M. “Underappreciated Pathways toward Molecular Genetics as Illustrated by Jean Brachet’s Chemical Embryology.” In The Philosophy and History of Molecular Biology: New Perspectives, edited by Sahotra Sarkar. Dordrecht: Kluwer, 1996.

———, and Denis Thieffry, eds. History and Philosophy of theLife Science, Vol. 19: Research Programs of the Rouge-Cloître Group. Brussels: University of Brussels Press, 1997. A collection of essays on the work of Jean Brachet and colleagues at the University of Brussels.

Chantrenne, Hubert. The Biosynthesis of Proteins. New York: Pergamon Press, 1961.

———, Arsène Burny, and Gérard Marbaix. “The Search for the Messenger RNA of Hemoglobin.” In Progress in Nucleic Acid Research and Molecular Biology, Vol. 7. New York: Academic Press, 1967.

———. “J. Brachet (1909–1988).” In Selected Topics in theHistory of Biochemistry: Personal Recollections, III, ed. G. Sameness and R. Jaenicke. Amsterdam: Elsevier, 1990a.

———. “Notice sur J. Brachet.” Annuaire 1990 de l’AcadémieRoyale de Belgique(1990b): 3–87. The most complete biographical notice on Brachet, including a comprehensive list of Brachet’s writings, in French.

Dalcq, Albert. Form and Causality in Early Development. Cambridge, U.K.: Cambridge University Press, 1938.

Gurdon, John B., Charles D. Lane, Hugh R. Woodland, et al. “Use of Frog Eggs and Oocytes for the Study of Messenger RNA and its Translation in Living Cells.” Nature 233 (1971): 177–182.

Halleux, Robert; Jan Vandersmissen; Andrée Despy-Meyer; and Geert Vanpaemel, eds. Histoire des sciences en Belgique, 2 vol. Brussels: La renaissance du Livre (Dexia), 2001.

Jeener, Raymond, and D. Szafartz. “Relations between the Rate of Renewal and the Intracellular Localization of Ribonucleic Acid.” Archives of Biochemistry 26 (1950): 54–67.

———, and J. Rossels. “Incorporation of 2-Thiouracil-35S in the ribose nucleic acid of tobacco mosaic virus.” Biochimica et Biophysica Acta 11 (1953): 438.

———, and Paul Lemoine. “Occurrence in Plant Infected with Tobacco Mosaic Virus of a Crystallizable Antigen Devoid of Ribonucleic Acid.” Nature 171 (1953): 935–937.

Judson, Horace Freeland. The Eighth Day of Creation: Makers of the Revolution in Biology, 2nd ed. Plainview, NY: Cold Spring Harbor Laboratory Press, 1996.

Morange, Michel. The History of Molecular Biology. Translated by Matthew Cobb. Cambridge, MA: Harvard University Press, 1998.

Mulnard, Jacques G. “The Brussels School of Embryology.” International Journal of Developmental Biology 36 (1992): 17–24.

Rheinberger, Hans-Jörg. Toward a History of Epistemic Things:Synthesizing Proteins in the Test Tube. Stanford, CA: Stanford University Press, 1997.

Thieffry, Denis, and Richard Burian. “Jean Brachet’s Alternative Scheme for Protein Synthesis.” Trends in Biochemical Sciences26, no. 3 (1996): 114–117.

Thomas, René. “Bacteriophage λ: transactivation, positive control and other odd findings.” BioEssays 15 (1973): 285–289.

———. “Molecular Genetics under an Embryologist’s Microscope: Jean Brachet, 1909–1988.” Genetics 131 (1992): 515–518.

Denis Thieffry

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