BIO 349, Drosophila D/V Patterning and Gene Regulatory Networks
BIO 349, Drosophila D/V Patterning and Gene Regulatory Networks BIO 349
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This 3 page Class Notes was uploaded by Clayton Payne on Thursday February 11, 2016. The Class Notes belongs to BIO 349 at University of Texas at Austin taught by Dr. Steven Vokes in Spring 2016. Since its upload, it has received 36 views. For similar materials see Developmental Biology in Biology at University of Texas at Austin.
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Date Created: 02/11/16
BIO 349 CLASS NOTES 28 212 Drosophila DorsalVentral Patterning ● Germ cells vs. Somatic cells ○ Pole cells develop at the posterior end of a Drosophila embryo. These pole cells eventually become germ cells. ○ A female’s germ cell (oogonium) divides to form 1 oocyte (egg) and several nurse cells during oogenesis. Nurse cells provide maternal mRNAs to the oocyte prior to fertilization. ○ The nurse cell/oocyte complex is surrounded by a layer of follice cells, which are somatic. The oocyte and follicle cells are separated by a thin layer of extracellular space called the perivitelline space. Germ cells vs. somatic cells are an important distinction when considering genetic crosses and tissue transplant experiments ● After fertilization, the embryo’s nucleus goes through 10 cycles of replication and division, and localize to the outer layer of the embryonic cell. In the 11th cycle, zygotic transcription begins, as well as cellularization of the syncitial nuclei. ● Prefertilization events 1. Nurse cells secrete Gurken mRNAs into the oocyte, where they are localized between the nucleus and posterior side of the cell. Nurse cells also secrete Dorsal mRNAs, which remain translationally repressed throughout the oocyte until cellularization. Note that Dorsal RNA is evenly distributed throughout the oocyte. 2. Gurken mRNAs are translated and secreted into the posterior perivitelline space, where they bind Torpedo, a membrane receptor protein expressed in follicle cells. Posterior follicle cells then send a signal back to the oocyte, which recruits PAR1 proteins to the posterior end. PAR1 organizes and polarizes the microtubules, with the capped () ends being anterior and the dynamic (+) ends being posterior. 3. Motor proteins transport the nucleus along microtubules to a more dorsal position. Gurken mRNAs are translated again and secreted into the dorsal perivitelline space, where the again bind and activate Torpedo. 4. Torpedo activates a cascade which represses Pipe expression in dorsal follicle cells. ● Postfertilization events 1. In ventral follicle cells, where Pipe is synthesized, Pipe adds sulfate groups to membrane proteins facing the perivitelline space. These sulfated proteins recruit a GD complex, which recruits Snake protein. The GD/snake complex cleaves and activates Easter, which in turn cleaves and activates Spatzle, generating a Spatzle gradient along the ventral axis. Active Spatzle binds Toll, a receptor in the membrane of embryo cells facing the perivitelline space. Note that Toll is evenly distributed throughout the egg but only activated ventrally. 2. Dorsal is synthesized from mRNA about 90 minutes after fertilization, just before cellularization begins, and forms an inactive complex with Cactus protein. 3. The activated Toll receptor activates Pelle kinase, which phosphorylates Cactus. Phosphorylated Cactus is degraded. 4. Unbound Dorsal is able to be translocated to the nucleus, and does so in the ventral embryonic cells where Cactus has been degraded. Once translocated, Dorsal acts as a transcription factor for genes which drive a ventral cell fate. Gene Regulatory Networks ● Temporally and spatially precise transcription of genes is essential for proper development and cellular function. Organisms have evolved mechanisms to ensure genes are kept under tight transcriptional control. ● Doublenegative linear pathways: let’s say you want to Gene C to be transcribed after Gene A is transcribed, and ONLY after Gene A is transcribed. The easiest way for this to happen would be fore gene product A to act as a transcription factor for gene C. However this poses a problem: in the absence of gene product A, Gene C may undergo “leaky” expression. Thus, a more secure way of regulating is to have Gene C be repressed by gene product B, and Gene B repressed by gene product A. Thus, Gene C is actively repressed until Gene A is transcribed, at which point the repression can be lifted. ● Positive feedback loops: L et’s say you want Gene B transcribed after Gene A, but you want lots and lots of product B. Have product B be a trxn factor for Gene A, and B be a trxn factor for Gene B. This is good for amplifying signal but the loop poses the problem of becoming unstable. ● Negative feedback loops: L et’s say you want to limit the signal to a very brief period of development. Have product B repress Gene A, which can no longer drive transcription of B.
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