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Body Plans of Eukaryotes In any multicellular organism, development is controlled and coordinated and, more often than not, cells end up where they are meant to be. The development follows a body plan and is under genetic control. The genes which control the body plan are called homeobox genes. Homeobox genes are 180 base pairs long and code for small protein transcription factors (60 amino acids long – remember the triplet coding?). These 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 and animals from fruit fly to humans. The following outline shows how they are important in controlling the body plan of fruit fly. The fruit fly has the body plan of a typical insect, i.e. the adult has a segmented body with a head section, a thorax (three segments T1, T2 and T3) and an 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 the polarity of the body, polarity of the segments and the development of individual segments. Thorax Abdomen Head Pair of wings on T2 Compound eye Antennae Pair of balance organs on T3 Paired legs on T1, T2 and T3 Hox genes of the fruit fly, Drosophila melanogaster. Public domain image (PhiLiP). 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 (producing the antennapedia condition). 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. You can view a series of animations showing the role of homeobox genes in Drosophila melanogaster (the fruit fly) by connecting to: http://www.dnalc.org/view/16760-Animation-37-Master-genes-control-basic-bodyplans.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 and so 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. SJWMS Biology Apoptosis or Programmed Cell Death Cells are not immortal. They can only undergo a limited number of cell division (usually about 50) and then die. It is not the type of death that is caused by damage or trauma, but an orderly series of events leading to cell death. This ordered or programmed cell death is called apoptosis. During apoptosis the cytoskeleton is broken down by enzymes, the nuclear envelope and DNA fragments and the cell membrane begins to bleb – instead of invaginating as it does during phagocytosis, it forms little balloons that are pinched off from the cell surface membrane. The blebs (vesicles) are then phagocytosed. The key events are summarised in the diagram. 1. Normal cell, capable of dividing for a number of cycles (the Hayflick constant, about 50 divisions). After this, a series of ordered steps results in the death of the cell – not the same as necrosis! 2. Cell begins to break down. Enzymes digest the cytoskeleton and the cytoplasm becomes dense. The chromatin condenses and DNA fragments. Blebs (extrusions of the cytoplasm) appear containing cytoplasmic material. The process is controlled by a range of signalling molecules (cytokines, hormones and even chemicals like nitric oxide) 3. Cell shrinks as it disintegrates. The and is part of normal development. Consider blebs containing cytoplasm and the changes that take organelles are phagocytosed place in the (engulfed) by phagocytes and metamorphosis of digested. Process is quick and prevents amphibians. The larvae damage to surrounding tissues (if (tadpoles) are aquatic, necrosis, enzymes and toxic products have external gills and a can cause extensive damage). tail. The adults have lungs, legs and do not have a tail. The tissues of The events of apoptosis or programmed cell death the gills and tail are reabsorbed due to apoptosis. Similarly, during limb development in humans, the tissues have to be rearranged. Apoptosis allows for the separation of the digits (fingers and toes). Failure of this leads to syndactyly, i.e. fused digits (toes in the case of the illustration). The balance between too much cell death and too little is important. Too little and tissue accumulates (tumour); too much and there is cell loss and tissue degeneration. Failure of apoptosis leads to syndactyly or fused digits, as seen here on the foot. Image licensed under the Creative Commons Attribution ShareAlike 3 Licence SJWMS Biology