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Body plans Homeobox Body Plans and Homeobox Sequences In any multicellular organism, development is controlled and coordinated: cells end up where they are meant to be development follows a body plan and is under genetic control The genes which control the body plan are called homeobox genes. Homeobox (Hox) genes are 180 base pairs long. They code for small protein transcription factors (60 amino acids long). Transcription factors bind to DNA allowing it to be transcribed and ‘switch on’ many genes in a cascade effect. Homeobox (or Hox) genes have been discovered in: fungi plants animals from fruit fly to humans The following outline shows how they are important in controlling the body plan of fruit fly. 3 Thorax Abdomen Head Pair of wings on T2 Compound eye Pair of balance organs on T3 Antennae Paired legs on T1, T2 and T3 Hox genes of the fruit fly, Drosophila melanogaster 4 The fruit fly has the body plan of a typical insect, i.e. the adult has a segmented body. It has: head section thorax (three segments T1, T2 and T3) abdomen Each of the thorax sections has a pair of legs, there is a pair of wings on T2 and a pair of balance organs on T3. Hox genes control: polarity of the body polarity of the segments development of individual segments The Antp (antennapedia) homeobox gene codes for a transcription factor that switches on all of the genes that allow leg development. It is switched on in thorax sections 1, 2 and 3 and usually switched off in the head segment. If switched on in the head segment, a pair of legs develop instead of antennae. It produces the antennapedia condition. Wild type and mutant Drosophila with antennapedia mutation. 6 Similarly, the Ubx (ultrabithorax) homeobox gene usually stops development of wing production on thorax segment 3. If mutated, an extra pair of wings is grown instead of the usual balance organs. Animations showing the role of homeobox genes in Drosophila melanogaster (the fruit fly) : http://www.dnalc.org/view/16760-Animation-37Master-genes-control-basic-body-plans.html The situation is similar in humans where Hox A11 and Hox D11 switch on the genes for development of the forelimb. It has been suggested that thalidomide may have switched off the homeobox genes for limb development. This caused the birth defects typical of thalidomide use. Thalidomide can certainly insert itself into DNA and inhibits production of new blood vessels in limb buds. Baby born to a mother who had taken thalidomide during pregnancy. Note the malformation of the foot and the right arm.