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The timing of gene expression Summarise the role of gene cascades determining limb formation in birds and mammals Gene cascade: a sequence in which genes are turned on and expressed. Genes are turned on and off in a particular order and only in the correct cells Protein produced by one gene acts as a transcription factor to turn on the next gene, and so on As a result genes are switched on in the right sequence and in the right places to form working parts Limb Formation: During embryonic development, tiny bulges bud out – the beginning of limbs. Position of limb buds is determined by HOX genes that start the gene cascade. Genes at the start of the cascade are switched on and as each gene is expressed, the next is turned on. In humans, limbs develop in weeks 5 and 6 and in a precise pattern: shoulders to fingers, thumb to little finger, back of the hand to the palm. In chick embryos, buds become recognizable wings and legs by the 10th day. Homeotic genes: Any of a group of genes that control the pattern of body formation during early embryonic development of organisms Hox genes: Homeobox genes (Hox) are master genes that produce proteins that activate or express a large number of genes for the formation of body structures (e.g. begin the gene cascade for limb formation) Hox 9-13 control limb development in birds and mammals Homeotic genes were first discovered through mutations which caused limbs or organs to develop in the wrong parts of the body Drosophila with one pair of wings (a) and a mutant fly with two pairs of wings (b) Describe the evidence which indicates the presence of ancestral vertebrate gene homologues in lower animal classes Gene homologue: Similar DNA sequences in different organisms. Homeotic genes often are homologous, coding for the same function in many different organisms similar homologue genes have been found in every eukaryote studied including: invertebrates (e.g. fruit flies and worms), vertebrates (e.g. frogs, chickens and humans), yeasts and plants they control the development of similar body parts in the developing embryos of many different species presence of these similar DNA sequences in organisms suggests a common ancestry of all eukaryotic organisms Orgnization and expression of Hox genes in Drosophilla and in the mouse Identify data sources gather process and analyse information from secondary sources and use available evidence to assess the evidence that analysis of genes provides for evolutionary relationships Globin protein Globin proteins carry oxygen, e.g. haemoglobin Studies of DNA sequences of globin proteins across species such as insects and primates reveal similar amino acid sequences that suggest an evolutionary relationship Differences can help map the evolutionary relationship, e.g. myoglobin in insects has alpha chains whereas haemoglobin in mammals has alpha and beta chains. Identify data sources gather process and analyse information from secondary sources and use available evidence to assess the evidence that analysis of genes provides for evolutionary relationships DNA repair DNA repair genes are very similar in all organisms from yeasts to humans. Shows the repair process evolved early and has remained unchanged since then