A gap gene is a type of

gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...

involved in the

development Development or developing may refer to: Arts *Development hell, when a project is stuck in development *Filmmaking, development phase, including finance and budgeting *Development (music), the process thematic material is reshaped * Photograph ...

of the segmented

embryos An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sper ...

of some

arthropods Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

. Gap genes are defined by the effect of a

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...

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. Drosophila gap gene in situ

Gap genes were first described by

Christiane Nüsslein-Volhard Christiane (Janni) Nüsslein-Volhard (; born 20 October 1942) is a German developmental biologist and a 1995 Nobel Prize in Physiology or Medicine laureate. She is the only woman from Germany to have received a Nobel Prize in the sciences. Nü ...

and

Eric Wieschaus Eric Francis Wieschaus (born June 8, 1947 in South Bend, Indiana) is an American evolutionary developmental biologist and 1995 Nobel Prize-winner. Early life Born in South Bend, Indiana, he attended John Carroll Catholic High School in Birming ...

in 1980. They used a

genetic screen A genetic screen or mutagenesis screen is an experimental technique used to identify and select individuals who possess a phenotype of interest in a mutagenized population. Hence a genetic screen is a type of phenotypic screen. Genetic screens c ...

to identify genes required for embryonic development in the fruit fly ''

Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly" or "pomace fly". Starting with Ch ...

''. They found three genes – ''knirps,

Krüppel 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 r ...

and

hunchback Kyphosis is an abnormally excessive convex curvature of the spine as it occurs in the thoracic and sacral regions. Abnormal inward concave ''lordotic'' curving of the cervical and lumbar regions of the spine is called lordosis. It can result fr ...

'' – 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 In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The func ...

, and they directly control the expression of another set of genes involved in segmentation, the

pair-rule gene A pair-rule gene is a type of gene involved in the development of the segmented embryos of insects. Pair-rule genes are expressed as a result of differing concentrations of gap gene proteins, which encode transcription factors controlling pair- ...

s. The gap genes themselves are expressed under the control of

maternal effect A maternal effect is a situation where the phenotype of an organism is determined not only by the environment it experiences and its genotype, but also by the environment and genotype of its mother. In genetics, maternal effects occur when an org ...

genes such as ''

bicoid Homeotic protein bicoid is encoded by the ''bcd'' maternal effect gene in ''Drosophilia''. Homeotic protein bicoid concentration gradient patterns the anterior-posterior (A-P) axis during ''Drosophila'' embryogenesis. Bicoid was the first pro ...

'' and '' nanos'', and regulate each other to achieve their precise expression patterns.

Gene activation

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. Krüppel is regulated by five regulatory proteins: bicoid, hunchback, tailless, knirps and giant. Krüppel 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 the anterior boundary of the knirps gene. Due to more efficient inhibition of the knirps gene by hunchback, knirps is expressed more posterially 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 the ability of both hunchback and tailless to bind to the enhancer regions of knirps.

Mechanism of action

The gap genes code for transcription factors that regulate the expression of

s 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 as canalization, a property of developing organisms to produce 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.

References

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External links

*The Interactive Fly: http://www.sdbonline.org/fly/aignfam/gapnprl.htm Developmental genes and proteins

Gene activation

Mechanism of action

See also

References

External links