2Splicing 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.
3Types 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.
4The Spliceosome Five snRNAs: U1, U2, U4, U5, and U6 Some snRNAs associate with proteins to form snRNAs (small nuclear ribonucleoproteins)
21Chapter 19 The Genetic Control of Animal Development
22Sex 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 DefinitionBut wrongIn Drosophila, the key genes in sex determination encode proteins that regulate RNA processing.
23Sex Determination in Drosophila Components of the sex-determination pathway includeA system to ascertain the X:A ratio ,A system to covert this CONCEPTUAL ratio into a developmental signal, andA system to respond to this signal by producing either male or female structures.
24Ascertaining the X:A Ratio NOT CORRECT..from bookAscertaining the X:A RatioThe 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.
26The 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.
30Differentiating 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.
31Loss-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.
33Key PointsIn 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).