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BIOL 205
Ian D Chin- Sang

BIOL205:week 12, lecture 33 Development : Chapter 12 What are the number, identity and function of genes in development? 1. What genes are important? 2. Where in the developing animal and at what times are they active? 3. How is the expression of developmental genes regulated? 4. Through what molecular mechanisms do gene products affect development? Pattern formation a key to developmental positioning and identity • You distinguish an arm from a leg by shape: • An abstract quality as both are made of same components: muscle, nerves, circulatory system • But shape is consistent in normal individuals More complex patterns → a cell has to define what is up, down, left, right (anterior, posterior, ventral, dorsal) ex: moths: each cell on wing is programmed to be different colour - have to be a system in place that tells cell where it needs to go and once it gets there it needs to be programmed to generate unique transcription to produce a particular colour Third thoracic segment:“halteres” Pattern Formation or Homeotic Mutants identified in Drosophila → little stubs that • Homeotic (greek homeos- the same) were once wings • Wildtype –one pair of wings on second thoracic segment (lost through evolution) now used • Bithorax mutant –wings develop on third thoracic segment for balancing • Antennapedia mutant – legs develop where antenna normally develop Transplantation experiments confirmed that “organizing activities play a role in long term pattern formation in higher organisms Limb bud development Zone of Polarizing activity (ZPA) → creates a drosophila embryo that has little hairs onit:looks like a hedgehog Polydactyly Homeotic mutants of Drosophila melanogaster Dominant gain of function –are inherited! • Bithorax mutant • Ultrabithorax (Ubx) normally acts in developing hindwing to promote hindwing development and repress forewing development • Ubx loss of function in third thoracic segment: hindwing transformed into forewing • Ubx gain of function in second thoracic segment converts forewing into hindwing. • Antennapedia mutants (Antp) dominant gain of function ofAntp in antenna Hox genes regulate the identity of body parts • Two gene complexes control segment identity • Order on chromosome corresponds to order of body regions from head to tail Hox genes are expressed in spatially restricted domains Final expression of Hox genes in adult depends on controlled of their expression early in embryogenesis Expressionof8 Hoxgenes inembryo detected by in situ hybridization(posterior end folded over) Hox proteins have a sequence in common Likely arose by tandem gene duplication of an ancestral gene Homeodomain:proteindomainof60 amino acids found inallhomeotic genes – a helix-turn- helixmotif→ motifcommoninDNAbindingproteins Homeodomain Protein - helix-turn-helixmotif - 3 α –helices -helices 2 & 3 formH-T-H - mayformheterodimers:severaldifferent types of homeodomainproteins cancome together and activate numerous domains The order of Hox genes parallels the order of body parts in which they are expressed Mouse : 4 large hox gene complexes on 4 chromosomes - order of genes on chromosomes again matches body position anterior-posterior or head to tail Hox genes regulate the identity of serially repeated structures in vertebrates Negative mutation Dominnant Hoxmutation 1995 –Nobel prize in Physiology or Medicine → Large scale mutagenesis screen to determine all the genes controlling the final expression of Hox genes Drosophila Development -motherdeposits mRNA and proteins into unfertilized egg to give it info right fromthe start
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