Developmental
regulation proceeds through the sequential activation of a series of regulatory
switches that in turn activate networks of other genes. Many different
classes of regulatory genes share a common DNA sequence known as the homeobox,
which predates the origin of animals. The best-studied class of homeobox-containing
genes are the Hox genes, usually found clustered next to each other along
animal chromosomes. A homeobox is a DNA sequence found within genes that
are involved in the regulation of development (morphogenesis) of animals,
fungi and plants. Genes that have a homeobox are called homeobox genes
and form the homeobox gene family. In eukaryotes, a homeobox encodes a
protein domain (the homeodomain) which can bind DNA that act as part of
transcription factors to switch on cascades of other genes that induce
cellular differentiation by initiating the cascades of coregulated genes
required to make individual tissues, organs , or body parts. The image
above shows a digital rendering of the Antennapedia homeodomain protein
from the fruit fly (Drosophila melanogaster) bound to a DNA fragment.
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