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Because of their ablity to produce large amounts of offspring in a short amount of time, fruit flies were ideal specimens for early genetic experiments.

metaphase stage in mitosis at which chromosomes are aligned along the cell equator several hundred times without separating from each other by mitosis and cell division. Instead, the newly replicated DNA strands line up parallel to one another to form a tight bundle, called polytene chromosomes. Polytene chromosomes are less condensed than normal metaphase chromosomes, and can be around 2 millimeters long, large enough to easily be examined in detail under a low-power microscope. When stained with certain dyes, polytene chromosomes display characteristic banding patterns along their length. Drosophila geneticists have made maps of the banding patterns and have learned to use them as landmarks to help them locate genes of interest. Polytene chromosomes make Drosophila an excellent organism for the sub-branch of genetics known as cytogenetics, which is genetic analysis through directly visualizing the chromosomes themselves.

One of the most important areas of research to come from studies on Drosophila is that of embryonic development. By analysis of mutants, developmental biologists have elucidated a complex and precise picture of how genes orchestrate the development of a fertilized egg into an adult fly. Many of the genes exercising the master control over these processes encode transcription factors, which are proteins that regulate when and where particular genes are transcribed to produce messenger RNA. The initial set of genes in the hierarchy acts in the oocyte, even before it is fertilized by a sperm. Their main function is to define the spatial polarity of the oocyte, determining which is the front and rear (posterior and anterior polarity), and which is the belly and back (dorsal and ventral polarity). These genes, in turn, activate genes that divide the embryo into segments and subsegments, which will eventually become the body segments of the adult animal. Later-acting genes, termed homeotic genes, act within each segment to define its identity, for example a wing or a leg. Remarkably, the genes and regulatory pathways involved in Drosophila development are highly conserved (that is, very similar genes and pathways are present), not only in other invertebrates, but also in mammals, including humans. see also Chromosome, Eukary-otic; Development, Genetic Control of; In Situ Hybridization; Model Organisms; Morgan, Thomas Hunt; Muller, Hermann; Mutagenesis; Mutation; Transcription Factors; Transposable Genetic Elements.

Paul J. Muhlrad

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