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.
25-513: Krüppel is a gap gene in Drosophila melanogaster , located on the 2R chromosome, which encodes a zinc finger C2H2 transcription factor . Gap genes work together to establish the anterior-posterior segment patterning of the insect through regulation of the transcription factor encoding pair rule genes . These genes in turn regulate segment polarity genes . Krüppel means "cripple" in German, named for
50-488: 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
75-443: A consistent phenotype despite variations in genotype or environment. It has been proposed that canalization is a manifestation of cross regulation of gap genes expression and can be understood as arising from the actions of attractors in the gap gene dynamical system. Hunchback (gene) Hunchback is a maternal effect and zygotic gene expressed in the embryos of the fruit fly Drosophila melanogaster . In maternal effect genes,
100-481: A low enough level so that it can act as an activator, but Knirps is not yet present to inhibit. In this way the initial gradients of morphogens can lead to the establishment of a specific region within the blastoderm. It can be compared to a narrow bandwidth filter in engineering. The Krüppel protein is a transcription factor , and has been shown to act as a repressor . It functions in collaboration with other gap genes and their localized protein products to regulate
125-503: Is close to Hunchback , and a recently discovered enhancer is farther away. When Bicoid binds to these enhancers, the expression of Hunchback increases proportionally to the Bicoid concentration in the anterior pole. A separate regulatory region downstream of the Hunchback enhancers governs the posterior expression of zygotic Hunchback . Here, Hunchback expression is proportional to
150-499: Is expressed in a stripe in the posterior region of the embryo. Hunchback and Knirps are both transcription factors that regulate Krüppel expression. High levels of Hunchback inhibit expression, whereas low levels of Hunchback activate expression. Knirps acts as a repressor to inhibit expression. This results in Krüppel being expressed in a stripe in the center of the embryo's A-P axis, where Hunchback concentration has dropped to
175-428: Is inhibited by high levels of hunchback, high levels of giant, and tailless, which establishes the anterior boundary of Krüppel expression. Krüppel is also inhibited by knirps and activated by low levels of bicoid and low levels of hunchback, which establishes the posterior boundary of Krüppel expression. The knirps gene appears to be spontaneously activated. It is repressed by hunchback. Hunchback repression thus defines
200-422: Is restricted to this domain largely through interactions with the maternal effect genes Bicoid and Nanos , and fellow gap gene Hunchback and Knirps. Bicoid maternal transcripts are deposited at the anterior end of the embryo, while Nanos maternal transcripts are located at the posterior. Hunchback mRNA transcripts are present throughout the embryo. Bicoid and Nanos both encode morphogens that have
225-527: The Pegasus gene ( Ikzf5 ) of the Ikaros family zinc finger group. Ikaros family genes encode transcription factors that have implications in thrombocytopenia , a blood clotting deficiency, acute myeloid leukemia , a blood and bone marrow cancer, and are involved in mammalian retinal and immune system development. Ikaros family genes have also been implicated as an indicator for chronic graft-versus-host disease ,
250-493: 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
275-512: The RNA or protein from the mother’s gene is deposited into the oocyte or embryo before the embryo can express its own zygotic genes. Hunchback is a morphogen, meaning the concentration gradient of Hunchback at a specific region determines the segment or body part it develops into. This is possible because Hunchback is a transcription factor protein that binds to genes’ regulatory regions, changing RNA expression levels. Maternal Hunchback RNA enters
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#1733093150450300-423: The ability of both hunchback and tailless to bind to the enhancer regions of knirps. The gap genes code for transcription factors that regulate the expression of pair-rule genes and homeotic genes by competing for binding to their enhancer regions. It has been demonstrated that gap gene expression in the Drosophila blastoderm exhibit a property called canalization, a property of developing organisms to produce
325-428: 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. Krüppel is regulated by five regulatory proteins: bicoid, hunchback, tailless, knirps and giant. Krüppel
350-408: The anterior and posterior regions, which later give rise to the respective head and abdomen. In the syncytial blastoderm, Bicoid and Nanos RNA bind to protein ropes involved in cellular locomotion and intracellular transport called microtubules that ferry the RNA to the anterior and posterior regions, respectively. Hunchback does not bind to microtubules and therefore diffuses uniformly throughout
375-405: The anterior boundary of the knirps gene. Due to more efficient inhibition of the knirps gene by hunchback, knirps is expressed more posteriorly in the embryo compared to Krüppel. Tailless protein inhibits knirps gene expression in the posterior part of the embryo, allowing the knirps protein to be expressed only in the central part of the embryo (but more posterior compared to Krüppel). This is due to
400-560: The anterior posterior axis and segment identity. Krüppel has shown homology to the mammalian Krüppel -like factors , which play key biological roles in the pathogenesis of many human diseases: cancer, obesity, inflammatory disorders and cardiovascular complications. Moreover, KLFs are known to be involved in inducible pluripotent stem cells generation, and preservation of the pluripotent state of embryonic stem cells . Gap gene Gap genes were first described by Christiane Nüsslein-Volhard and Eric Wieschaus in 1980. They used
425-426: The concentration of Tailless and Huckebein proteins available to bind to the regulatory region. As a bifunctional transcription factor , Hunchback both activates and represses its target segmentation genes, and in doing so, regulates the anterior and posterior embryonic segmentation in the Drosophila embryo. For example, anterior Hunchback expression is known to establish the region that later develops into
450-463: The crippled appearance of mutant larvae, who have failed to develop proper thoracic and anterior segments in the abdominal region. Mutants can also have abdominal mirror duplications. Human homologs of Krüppel are collectively named Krüppel -like factors , a set of proteins well characterized for their role in carcinogenesis. Krüppel is expressed in the center of the embryo during the cellular blastoderm stage of development. Its expression pattern
475-404: 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,
500-408: The embryo at the syncytial blastoderm stage, where the entire embryo has undergone many nuclear divisions but has one communal cytoplasm, allowing for RNA to disperse freely throughout the embryo. This allows the maternal effect genes Hunchback , Bicoid , Nanos , and Caudal to regulate zygotic genes to create different identities for different regions of the body. The first step is establishing
525-492: The embryo. However, Nanos represses the translation of the Hunchback protein. Since Nanos is ferried to the posterior pole, maternal Hunchback is expressed predominantly in the anterior pole. Hunchback is also expressed zygotically in the farmost anterior and posterior poles of the syncytial blastoderm. Anterior zygotic Hunchback expression is controlled by enhancers, regions of DNA that increase gene expression when transcription factors are bound. One enhancer
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#1733093150450550-477: The expression of the primary pair rule genes – even skipped ( eve ), hairy ( h ), and runt. It has been postulated that Krüppel inhibits eve expression to create the posterior boundary of eve stripe two, and evidence has also been found for Krüppel being a player specifically in the formation of hairy stripe 7. The expression patterns of pair rule gene will in turn regulate the segment polarity genes, making Krüppel essential for proper development along
575-417: The expression pattern of pair-rule genes , such as even-skipped , expressed later in development to define distinct segments along the anterior-posterior axis. Pair-rule genes then encode transcription factors that regulate segment polarity genes: the final, most specified group of proteins that coordinate segmentation. The Hunchback gene has a known human ortholog that evolved from a common ancestor,
600-430: The opposite effect on Hunchback mRNA translation – Bicoid activates translation, whereas Nanos represses it. As such, Hunchback mRNA is translated so that Hunchback protein is present in the concentration gradient which decreases along the anterior – posterior axis. This Hunchback gradient indirectly results in an anterior boundary for Knirps expression. Other factors induce a posterior boundary, so that Knirps
625-453: The thoracic and jaw- and mouth-related segments, and posterior Hunchback expression for the development of abdominal segments. Hunchback’s morphogenetic gradient regulates the expression of other gap genes, Krüppel and Knirps , wherein maternal Hunchback expression defines the anterior Knirps and posterior Krüppel borders, while zygotic Hunchback expression establishes the anterior Knirps border. Hunchback also establishes
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