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will later form sepals and petals); the B function genes are active in an overlap ping region coincident with the cells that will form petals and stamens; and the C function genes are active in the center of the developing flower, where the stamens and ovarywill later form. Cells are instructed of their positions by their ABC genes-for example, if a cell has both A and Bgenes active,it knows its relative radial position in the flower and knows to participate in the formation of a petal, while another cell, more central in the developing flower, may have only C genes active and thus be instructed to form part of the floral ovary. We have tested this theory by combining different mutations in single plants and by using molecularly cloned copies of the genes to alter positional information in transgenic plants. An example of this is shown in Fig. 3, where function B has been activated throughout the entire flower, with the result that peripheral cells have both A and B genes active, rather than only Agenes, and thus have made petals where sepalswould normally be found. Central cells in this plant have B and C active,rather than only C, and thus have formed stamens. The theory explains in detail how floral cells know their positions and, thus, to some degree how it is that our ancestors created the floral and crop varieties that we prize and depend upon today. It also shows how we and our descendants may carry on the work of the past, breeding new flowers for aesthetic appreciation and new crops to serve fundamental human needs. Indeed, because so much of human evolution has depended on interactions with flowering plants (which are the source of most of our food), an understanding of the development of floral patterns may be a beginning to an understanding of the evolution of our own visual abilities and thus of the origin and nature of the aesthetic appreciation of flowers and floral patterns in the visual arts. DNA DENDRIMERS Thor W. Nilsen, Poly Probe, Inc., 15 Bala Avenue, P.O. Box 2675, Bala Cynwyd, PA 19004, U.S.A. E-mail: .WWw site: . The work reported in this Abstract was awarded the 1996LVMH Vinci of Excellence Prize. Dendritic molecules are highly branched arborescent structuresthat have found applications in such products as chemical reagents, lubricants and contrast media for magnetic resonance . A new class of these molecules is comprised of dendrimers constructed entirely from unique nucleic acidmonomers designed such that sequential hybridization adds successive layers of monomer, resulting in a geometric expansion of both the molecule’s mass and free single-stranded ends (Fig. 4 ) . My contribution to DNA dendrimers began in Spring 1986.The publication of the Polymerase Chain Reaction (PCR) in December 1985 [l]had dramatically changed my thoughts regarding the detection of nucleic acids. Until that time, nucleic acid blot assay was the standard method for detecting nucleic acids. PCR assay opened my mind to the possibilities of alternative detection methods. I immediately recognized the power of the PCR lay in the geometric expansion of target molecules -however, this is also the source of one of its greatest drawbacks: the difficulty of quantifymg the initial number of target molecules in a sample. PCR. A key component of any alternative I began working on alternatives to Fig. 4. ThorW. Nilsen. Dendritic molecules: (topleft) Initiator. (top,right)One-layer dendrimer. (bottom)Two-layerdendrimer).A new classof dendriticmoleculeshas been created from uniquenucleicacidmonomers.Each monomeris a heterodmer of two single-stranded nucleic acidoligomerspossessinga central double-strandedwaist and four single-stranded arms. Several differentmonomershave been constructedsuch that, when assembledin the appropriate order, theywill form mostlyhollowsphereshavingmultiple single-stranded DNA arms available for bindingat the molecularsurface.The molecular structure grows exponentiallyas each sequentiallayer is added.Moleculeswith two layers have 36 free ends, moleculeswith three layers have 108 free ends, moleculeswith fourlayers have 324 free ends, etc. 4 e /N/T/ATOR) 44 LVMH Abstrac6 would have to be superior quantitation ability-the new method should be capable of single-targetmolecule detection . I had known for some time of signal amplificationmethods for blot assay based on buildingan aggregate of signal molecules.The spark for the...


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