Lec42n - Lecture 42 Development, Part II 1. Pattern...

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Lecture 42 Development, Part II 1. Pattern formation (Fly) 2. Differentiation Campbell; 6 th Ed.; 406-418. 7 th Ed.; 412-425; 431-432. 8 th Ed.: 368-372; 412-416; 445 Introduction . Pattern formation is the process of setting up the basic body plan, so tissues and organs can develop in the right place. This plan is established before the actual structures develop. An example of an early event in pattern formation in animals is gastrulation. The basic body plan of the organism, with the archenteron that will form the digestive tract and the 3 primary germ layers that will develop into all the tissues, is already in place, even though all cells look pretty much the same. Unfortunately, our knowledge of how development proceeds from this stage in animals is pretty rudimentary. However, we know much more about pattern formation in Drosophila (fruit fly). We know at least the general outline of all the steps in development, from the fertilized egg to the adult. Maybe surprisingly, some (but not all) of what we’ve learned from Drosophila is turning out to apply to animal development as well. PART I: Pattern formation in Drosophila (fruit fly) (Fig. 21.10/21.12/18.17) . After fertilization, the fly zygote develops into an embryo. The embryo hatches into a larva. This crawls around as a maggot in fruit. Eventually, the larva metamorphoses (or turns into) an adult. An early event in pattern formation is setting up the anterior-posterior axis. (Remember that the anterior end of an organism is the head). Last time I said that bicoid mRNA is a cytoplasmic determinant; a molecule important in development that’s stored in the egg (where in this case it’s concentrated at the anterior end) before fertilization. Scientists learned that the function of bicoid is to set up the anterior- posterior axis. The following experiment gave the first hint that this was true (Fig. 21.12/21.14/18.19). This experiment is an example of a very powerful approach in genetics; learning the normal function of a gene by seeing what goes wrong when that gene is inactivated (or made defective) by mutation. Scientists found that when the bicoid gene was mutated, an embryo with two back ends joined together (and no front end) formed. This suggested that Bicoid protein might control development of anterior structures. Later experiments proved this, and showed the key role of Bicoid in establishing the anterior-posterior axis. Of course, these mutant embryos don’t survive long, and die soon after scientists are able to see that they have 2 back ends. This is an example of an embryonic lethal mutation; a case where defects in the dead embryo give information on the normal function of the affected gene in early development. The
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Lec42n - Lecture 42 Development, Part II 1. Pattern...

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