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Drosophila Development

Drosophila Development. Each egg is surrounded by a chorion . The anterior end has two filaments to allow oxygen to enter the cell. Sperm enter through the micropyle at the anterior end. Early Drosophila Development. It takes 1 day for the embryo to develop into a larva .

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Drosophila Development

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  1. Drosophila Development • Each egg is surrounded by a chorion. • The anterior end has two filaments to allow oxygen to enter the cell. • Sperm enter through the micropyle at the anterior end.

  2. Early Drosophila Development • It takes 1 day for the embryo to develop into a larva. • The larva hatches, feeds, and sheds its skin twice. • After 5 days, the larva becomes immobile and forms a pupa. • During the pupal stage, cells in the imaginal discs differentiate into adult structures.

  3. Maternal Gene Activity in Development Materials transported into the egg during oogenesis play a major role in embryonic development.

  4. Maternal-Effect Genes • Maternal-effect genes contribute to the formation of healthy eggs; effects of mutations in these genes may not affect the phenotype of the female making the eggs but may be seen in the next generation. • A maternal-effect mutation causes a mutant phenotype in the offspring of a female with a mutant genotype.

  5. The dorsal Gene:Offspring of dl/dl Females are Dorsalized and Inviable

  6. Excision of Intron Sequences

  7. Splicing • Removal of introns must be very precise. • Conserved sequences for removal of the introns of nuclear mRNA genes are minimal. • Dinucleotide sequences at the 5’ and 3’ ends of introns. • An A residue about 30 nucleotides upstream from the 3’ splice site is needed for lariat formation.

  8. Types of Intron Excision • The introns of tRNA precursors are excised by precise endonucleolytic cleavage and ligation reactions catalyzed by special splicing endonuclease and ligase activities. • The introns of nuclear pre-mRNA (hnRNA) transcripts are spliced out in two-step reactions carried out by spliceosomes.

  9. The Spliceosome • Five snRNAs: U1, U2, U4, U5, and U6 • Some snRNAs associate with proteins to form snRNAs (small nuclear ribonucleoproteins)

  10. What are Logo plots?

  11. Logo for a) Splice acceptor b) Splice Donor c) Initiator Met

  12. AG/GT CAG/NT exon 1 intron 1 exon 2

  13. KEY SLIDE…Know it.

  14. Chapter 21The Genetic Control of Animal Development

  15. Sex Determination in Drosophila and C. elegans • The sex determination signal in both animals is the ratio of X chromosomes to autosomes. If the ratio is 1.0 or greater, the animal is a female; if the ratio is 0.5 or less, the animal is a male.CLASSIC Definition • But wrong • In Drosophila, the key genes in sex determination encode proteins that regulate RNA processing.

  16. Sex Determination in Drosophila • Components of the sex-determination pathway include • A system to ascertain the X:A ratio , • A system to covert this ratio into a developmental signal, and • A system to respond to this signal by producing either male or female structures.

  17. NOT CORRECT..from book Ascertaining the X:A Ratio • The system that ascertains the X:A ratio involves interactions between maternally synthesized proteins in the egg cytoplasm and embryonically synthesized proteins encoded by several X-linked genes. • The X-linked gene products are called numerator elements and are twice as abundant in XX embryos as in XY embryos. • The autosomal gene products are called denominator elements and antagonize the products of the numerator elements.

  18. NOT CORRECT..from book

  19. The Sex-lethal (Sxl) Gene • Sxl is the mater regular of the sex determination pathway in Drosophila. • The X:A ratio is converted into a molecular signal that controls the expression of the X-linked Sxl gene.

  20. Function of SXL • SXL regulates splicing of its own transcript to maintain SXL protein expression in XX embryos. • SXL also regulates splicing of the transformer (tra) gene.

  21. Differentiating in Response to the Signal • TRA, along with TRA2, regulate splicing of doublesex (dsx) and fruitless (fru). • In XX embryos, where TRA is present, dsx transcripts are processed to encode a DSX protein that represses the genes for male development. • In XY embryos, where TRA is absent, dsx transcripts are processed to encode a DSX protein that represses the genes for female development.

  22. Loss-of-Function Mutations in Sex-Determination Genes in Drosophila • Mutations in Sxl prevent SXL protein from being made in males; homozygous mutants would develop into males but die as embryos. • Mutations in transformer and transformer2 cause both XX and XY animals to develop into males. • Mutations in dsx cause both XX and XY embryos to develop into intersexes.

  23. Key Points • In Drosophila the pathway that controls sexual differentiation involves some genes that ascertain the X:A ratio, some that convert this ratio into a developmental signal, and others that respond to the signal by producing either male or female structures. • The Sex-lethal (Sxl) gene plays a key role in Drosophila sexual development by regulating the splicing of its own transcript and that of another gene (tra).

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