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Homeotic Genes & Gene Cascades •Ever wondered why so many organisms have two eyes, two ears or a set of fore and rear limbs in approximately the same place? •Ever wondered how the various parts of a fetus manage to grow at just the right time in relation to other parts? •Imagine a human fetus that grew arms where its legs should be, or developed fully formed and functioning newborn ears on a 10 week old fetus. •There are a series of genes, called homeotic genes, and their role it is to control the genes that play a function in early development. Homeotic Genes •Homeotic genes are a kind of master gene that controls the early development of organisms. •These genes play an important role in the time and place when certain genes are switched on or off during embryonic development. •Due to their ability to control other genes, a mutation in a homeotic gene can have devastating effects. •Studying mutations in homeotic genes assisted scientists in understanding their role. Homeotic Genes Normal fly with antennae in the correct position. Homeotic genes functioning normally. Normal fly with legs in the place of antennae. Homeotic genes have been mutated. http://www.esb.utexas.edu/palmer/bio303/group25/DROSOPHILA/homeotic_genes.htm •Both the fruit fly embryo and the mouse embryo show similar genes controlling the development of various body parts. •In the fruit fly the homeotic (HOM) genes are on chromosome 3. •In mammals, the homeotic genes (HOX1, HOX2, HOX3, HOX4) are found on four different chromosomes. http://www.people.virginia.edu/~rjh9u/homeo.html Homeotic Genes •There is little doubt that this well preserved section of DNA appeared very early on in the evolution of living things. •It was an important tool because it let the cells know where they were during the early stage of embryonic development so that they could then go on and differentiate in order to develop a certain part of the organism. •Once an early cell at the anterior end of the early embryo could identify that it was actually at the anterior end, then it could go on with safely and accurately growing the head of a normal fetus. •Homeotic genes work in modules or levels. Once one module or level has developed, further modules/levels are formed within it to take on more and more specialised jobs. •For example limb buds develop at one level before fingers or toes which develop at a later level. Gene Cascades •The protein made by each gene triggers one or more genes into either turning on or off. •Genes that perform similar functions are grouped together •If the genes do not become active at a particular time there will be a major disruption of the developmental process. http://www.biology.pomona.edu/martinez/zoo-Hom.htm •Disruption of many of these genes results in the death of the embryo. Homeobox Genes •A common exon sequence, discovered in 1984 and found in all homeotic genes studied so far. •Scientists believe that all organisms have identical – or at least very similar homeobox sequences. They identify the sequence using a laboratory prepared DNA probe that binds to similar sequences. •Homeobox genes are 180 base pairs in length and code for a series of amino acids that are positively charged. •As DNA is negatively charged the positively charged section of the protein is attracted to the DNA. This is how the homeotic gene binds to DNA. http://www.mun.ca/biology/desmid/brian/BIOL2060_W2003/CellBiol21/CB21_30.html The End… the rest we will leave for the experts…