A pair-rule 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 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 insects. Pair-rule genes are expressed as a result of differing concentrations of

gap gene 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 ...

proteins, which encode transcription factors controlling pair-rule gene expression. Pair-rule 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 the normal developmental pattern in alternating segments. Pair-rule 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 ''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 ...

''. In normal unmutated ''Drosophila,'' each segment produces bristles called denticles in a band arranged on the side of the segment closer to the head (the

anterior Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

). They found five genes – ''even-skipped, hairy, odd-skipped, paired'' and ''runt'' – where mutations caused the deletion of a particular region of every alternate segment. For example, in ''even-skipped,'' the denticle bands of alternate segments are missing, which results in an embryo having half the number of denticle bands. Later work identified more pair-rule genes in the ''Drosophila'' early embryo – ''fushi tarazu'', ''odd-paired'' and ''sloppy paired''. Once the pair-rule genes had been identified at the molecular level it was found that each gene is expressed in alternate ''parasegments'' – regions in the embryo that are closely related to segments, but are slightly out of register. Each parasegment includes the posterior part of one (future) segment, and an anterior part of the next (more posterior) segment. The bands of expression of the pair-rule genes correspond to the regions missing in the mutant. The expression of the pair-rule genes in bands is dependent both upon direct regulation by the

s and on regulatory interactions between the pair-rule genes themselves.

References

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External links

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

See also

References

External links