WHAT IS A GENE? M. DEMEREC DEPARTMENT OF GENETICS, CARNEGIE INSTITUTION OF WASHINGTON, COLD SPRING HARBOR, N. Y. Demerec, M. 1933. What is a gene? Journal of Heredity. 24:368-378. ESP Electronic Scholarly Publishing http://www.esp.orgNew material in this electronic edition is © 2002, Electronic Scholarly Publishing Project http://www.esp.org The original work, upon which this electronic edition is based, is © 1933, American Genetic Association and is reprinted here with permission. This electronic edition is made freely available for educational or scholarly purposes, provided that these copyright notices are included. The manuscript may not be reprinted or redistributed, in any form (printed or electronic), for commercial purposes without written permission from the copyright holders. Electronic Scholarly Publishing Project Foundations Series: Classical Genetics Series Editor: Robert J. Robbins The ESP Foundations of Classical Genetics project has received support from the ELSI component of the United States Department of Energy Human Genome Project. ESP also welcomes help from volunteers and collaborators, who recommend works for publication, provide access to original materials, and assist with technical and production work. If you are interested in volunteering, or are otherwise interested in the project, contact the series editor: [email protected]. Bibliographical Note This ESP edition, first electronically published in 2000, is a newly typeset, unabridged version, based on the original 1933 edition published in the Journal of Heredity. The numbering of the references in the Literature Cited section has been changed from those of the original publication to correct for a systematic error throughout. Reference 3 has been renumbered as 3a and references 4 through 22 have been renumbered to 3 through 21 (leaving no reference 22). These changes were indicated as hand-written corrections (by Demerec?) on the original reprint of the paper in our possession. Production Credits Scanning of originals: ESP staff OCRing of originals: ESP staff Typesetting: ESP staff Proofreading/Copyediting: ESP staff Graphics work: ESP staff Copyfitting/Final production: ESP staffiii INTRODUCTION In this paper, Demerec sums up what is known about the gene in the early 1930s. He begins by noting: Our present information about genes is largely obtained by indirect, genetic methods. By studying the transmission of hereditary characteristics from one generation to another, genes were established as the mechanism instrumental in effecting this transmission; by following the inheritance of groups of characters, genes were placed in chromosomes and their location determined in respect to one another; by investigating different combinations of characters, evidence was obtained on the interaction of genes. The results of these indirect studies form the basis of our present gene concept. Studies of the effect of x-ray radiation and of unstable genes are the best means now available for broadening this concept. Were we to attempt to define the term “gene,” we could state that it is a minute organic particle, capable of reproduction, located in a chromosome and responsible for the transmission of a hereditary characteristic. This definition includes only statements of fact for which there is sufficient experimental evidence to prove them beyond reasonable doubt. In this presentation we shall endeavor to analyze the above definition, to discuss several directions in which this definition could be extended, and to present the concept of the gene which is used as our working hypothesis. That genes control phenotypes and are carried on chromosomes are taken as given. To Demerec, the real question is, “what is the physical nature of the gene?” How big is it, what is it made of, is it a single molecule or a multi-molecular structure? Demerec believes that the available evidence suggests that genes are uni-molecular, and he notes: If a gene is a complex organic molecule it would be expected to be similar in composition to other complex molecules, viz. molecular groups constituting this molecule (whatever these groups may be) would he arranged in chains and side chains. Figure 4 may serve as a crude diagrammatic example of what the structure of the gene molecule may look like. Amazingly, Figure 4 offers the “tentative structure of thymus nucleic acid” – DNA! But Demerec is using this only as an example of a complex molecular structure, not as the actual chemical structure of a gene: The diagram is not intended to give any implication as to the number, the type, or the arrangement of the molecules in a gene group. Itsiv purpose is to illustrate the molecular structure of a complex organic molecule. This drawing was produced twenty years before the correct Watson-Crick model of DNA structure had been developed and therefore Demerec’s DNA is in a “tetranucleotide” form, which would not allow the encoding of genetic information. Still, in this early paper Demerec is tantalizingly close to the actual chemical nature of the gene. He summarizes the present conception of the gene as: We visualize genes as single organic molecules. The gene string, then, gives us an interesting picture of a group of molecules held together by some unknown force in a string, each molecule possessing a power of self propagation, and each one individually and all of them together having an almost magic power of governing life processes of cells in which they are located, and therefore of governing life processes of the organism of which these cells are an integral part. With the publication of Thomas Morgan’s book, The Theory of the Gene, the fundamental mechanisms for the transmission of hereditary information was well established. Attention then began to shift to the chemical nature of the gene itself – an investigation that could not really get underway until Watson and Crick worked out the structure of DNA. Robert J. Robbins Seattle, Washington 2002v Frontispiece A Chimeral Delphinium Due to Gene Instability The gene which controls color-development in the lavender delphinium is unstable and occasionally mutates to purple. This is the explanation of the variegated flowers produced by certain strains of the delphinium. In the plant shown above a change in the lavender-gene at a very early stage in development has resulted in a flower-spike that is half purple
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