A gap gene is a type of gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

 involved in the development of the segmented

Segmentation (biology)

Segmentation in biology refers to either a type of gastrointestinal motility or the division of some animal and plant body plans into a series of repetitive segments. This article will focus on the segmentation of animal body plans, specifically using the examples of the phyla Arthropoda,...

 embryos of some arthropods. Gap genes are defined by the effect of a mutation

Mutation

In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...

 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

Christiane Nüsslein-Volhard

Christiane Nüsslein-Volhard is a German biologist who won the Albert Lasker Award for Basic Medical Research in 1991 and the Nobel Prize in Physiology or Medicine in 1995, together with Eric Wieschaus and Edward B...

 and Eric Wieschaus in 1980. They used a genetic screen

Genetic screen

A genetic screen is a procedure or test to identify and select individuals who possess a phenotype of interest. A genetic screen for new genes is often referred to as forward genetics as opposed to reverse genetics, the term for identifying mutant alleles in genes that are already known...

 to identify genes required for embryonic development in the fruit fly Drosophila melanogaster

Drosophila melanogaster

Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...

. They found three genes – knirps, Krüppel

Krüppel

Krüppel is a gap gene originally described in Drosophila melanogaster, which encodes a zinc finger transcription factor with four tandemly repeated zinc finger domains....

 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

Gene expression

Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...

 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

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 phenotype of its mother...

 genes such as bicoid and nanos

Nanos

Nanos can refer to:* George Peter Nanos, the former director of the Los Alamos National Laboratory* Apostolos Nanos, an Olympic archer* Nanos Research, a Canadian polling firm* Nikita Nanos, the founder of Nanos Research...

, 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 expresed 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 gene proteins code for transcription factors and they compete for binding in the enhancer regions of the next set of genes, the pair-rule genes.
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 recently 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