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Excision of Intron Sequences. Splicing  Removal of introns must be very precise.  Conserved sequences for removal of the introns of nuclear mRNA genes.

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Presentation on theme: "Excision of Intron Sequences. Splicing  Removal of introns must be very precise.  Conserved sequences for removal of the introns of nuclear mRNA genes."— Presentation transcript:

1 Excision of Intron Sequences

2 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.

3 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.

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

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6 What are Logo plots?

7 Logo for a) Splice acceptor b) Splice Donor c) Initiator Met

8 AG/GTCAG/NT exon 1 intron 1 exon

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13 SXL makes a functional protein in XX zygotes SXL makes a non-functional protein in XY zygotes

14 Where is RNA binding domain of SXL protein? E is Establisment form Made cycle 10 L is Late form made During cycle 14

15 SXL makes a functional protein in XX zygotes SXL makes a non-functional protein in XYzygotes Event is initiated by Exons E1+ 4 No early form of protein, thus no SXL 1/2/4 form SXL 1/2/4 form

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17 Cycle10-14 Cycle14+ Zygotic transcription starts

18 ~Cycle 9 Cycle14-and up Cycle10-14 Zygotic transcription starts

19 KEY SLIDE…Know it. Gro-maternal Dpn-Zygotic Development Zygotic-X chromosome factors:XSEs 2x1x Maternal gro Down regulated by Zygotic XSE proteins Zygotic deadpan Represses SxlPe with Maternal gro 2x difference

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21 Chapter 19 The Genetic Control of Animal Development

22 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.

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

24 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- NONE FOUND. NOT CORRECT..from book

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26 The Sex-lethal (Sxl) Gene  Sxl is the master 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.

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28 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.

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30 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.

31 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.

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33 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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