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Concept
Gap gene
Résumé
A gap gene is a type of gene involved in the development of the segmented embryos of some arthropods. Gap genes are defined by the effect of a mutation in that gene, which causes the loss of contiguous body segments, resembling a gap in the normal body plan. Each gap gene, therefore, is necessary for the development of a section of the organism.
Gap genes were first described by Christiane Nüsslein-Volhard and Eric Wieschaus in 1980. They used a genetic screen to identify genes required for embryonic development in the fruit fly Drosophila melanogaster. They found three genes – knirps, Krüppel and hunchback – where mutations caused deletion of particular stretches of segments. Later work identified more gap genes in the Drosophila early embryo – giant, huckebein and tailless. Further gap genes including orthodenticle and buttonhead are required for the development of the Drosophila head.
Once the gap genes had been identified at the molecular level it was found that each gap gene is expressed in a band in the early embryo generally correlated with the region that is absent in the mutant. In Drosophila the gap genes encode transcription factors, and they directly control the expression of another set of genes involved in segmentation, the pair-rule genes. The gap genes themselves are expressed under the control of maternal effect genes such as bicoid and nanos, and regulate each other to achieve their precise expression patterns.
Expression of tailless is activated by torso protein in the poles of the embryo. Tailless is also regulated in a complex manner by the maternal-effect gene bicoid.
Both embryonically transcribed hunchback and maternally transcribed hunchback are activated by bicoid protein in the anterior and is inhibited in the posterior by nanos protein. Embryonically transcribed hunchback protein is able to exhibit the same effects on Krüppel and knirps as maternally transcribed hunchback.
The Krüppel gene is activated when the bicoid protein gradient declines steeply, at the central part of the embryo.
Source officielle
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