Homeobox

A homeobox is a DNA sequence, around 180 base pairs long, that regulates large-scale anatomical features in the early stages of embryonic development. Mutations in a homeobox may change large-scale anatomical features of the full-grown organism. Homeoboxes are found within genes that are involved in the regulation of patterns of anatomical development (morphogenesis) in animals, fungi, plants, and numerous single cell eukaryotes. Homeobox genes encode homeodomain protein products that are transcription factors sharing a characteristic protein fold structure that binds DNA to regulate expression of target genes. Homeodomain proteins regulate gene expression and cell differentiation during early embryonic development, thus mutations in homeobox genes can cause developmental disorders. Homeosis is a term coined by William Bateson to describe the outright replacement of a discrete body part with another body part, e.g. antennapedia—replacement of the antenna on the head of a fruit fly with legs. The "homeo-" prefix in the words "homeobox" and "homeodomain" stems from this mutational phenotype, which is observed when some of these genes are mutated in animals. The homeobox domain was first identified in a number of Drosophila homeotic and segmentation proteins, but is now known to be well-conserved in many other animals, including vertebrates. The existence of homeobox genes was first discovered in Drosophila by isolating the gene responsible for a homeotic transformation where legs grow from the head instead of the expected antennae. Walter Gehring identified a gene called antennapedia that caused this homeotic phenotype. Analysis of antennapedia revealed that this gene contained a 180 base pair sequence that encoded a DNA binding domain, which William McGinnis termed the "homeobox". The existence of additional Drosophila genes containing the antennapedia homeobox sequence was independently reported by Ernst Hafen, Michael Levine, William McGinnis, and Walter Jakob Gehring of the University of Basel in Switzerland and Matthew P.