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Fiona Rawle

Genetic Regulation of the development of the Drosophila body plan Chapter 21: iGenetics (Russel) Embryonic development in Drosophila: Development commences with a single fertilized egg, giving rise to cells that have different developmental fates. Development of body structures relies upon three processes: (i)Establishment of egg polarity by molecular gradients (prefertilization) (ii)Compartmentalization of the embryo into distinct regions or segments. (iii)Establishment of segment identity (head vs. thorax vs. abdomen) Development of body structures relies upon three processes: (i) Polarity, (ii) Compartmentalization, (iii) Identity Sequential Process – occur in a highly structured way....stepwise progression. Genetic Programs – each process dependent upon the expression of particular sets of genes. Gene Regulatory Hierarchy – those genes that are expressed early regulate the expression of genes that follow. Developmental genes involved in generating the Drosophila body plan • Three major classes of developmental genes: (i) Maternal effect genes • Materially provided mRNA (produced by maternal cells) transferred to and stored in the egg prior to fertilization. • Produce molecular gradients and establish egg polarity. (ii) Segmentation genes • Gap Genes, PairRule Genes, and Segment Polarity Genes • Determine distinct body region or segments in the embryo and adult fly (iii) Homeotic genes • Specify the identity of each segment with respect to the adult body part to which it gives rise during metamorphosis. I. Maternal Effect Genes • Required early in embryonic development (syncytial blastoderm). • Establish embryo polarity on which subsequent developmental (segmentation) events depend. • Maternal effect gene mutants result from mutant mothers unable to produce functional mRNA molecules. • Focus on two key material genes (AP axis) (i) bicoid (bcd) is responsible for patterning the anterior of the embryo. Encodes a transcription factor – a protein that interacts with RNA polymerase to either activate or repress transcription. (ii) nanos (nos) is responsible for patterning the posterior of the embryo. Encodes a protein that acts as a translational repressor – prevents mRNA from being translated into protein. Gradients of bicoid mRNA and BICOID protein in the egg - bicoid gene encodes a protein referred to as a morphogen, it participates in patterning development - Maternal bicoid mRNA is localized to the anterior region of the embryo. BICOID protein spreads from this source and from a concentration gradiant BICOID protein gradient specifies the head and thorax. - Acts as transcription factor – activating and repressing the expression of other genes along the AP axis. - Represses caudal, a gene required later in development for the formation of posterior structures. Gradient of nanos RNA and NANOS protein in the egg nanos is the key maternal effect gene involved in the formation of the posterior structures of the embryo. NANOS protein forms a posterior to anterior concentration gradient. nanos encodes also encodes a transcription factor. NANOS represses the translation of hunchback mRNA. HUNCHBACK protein is required in an anterior to posterior gradient for the abdomen to form. II. Segmentation Genes • Function immediately after the maternal effect genes during embryonic
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