 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.

 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.

Materials transported into the egg during oogenesis play a major role in embryonic development.

 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.

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

 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.

 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.

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

What are Logo plots?

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

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

KEY SLIDE…Know it.

 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.

 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.

NOT CORRECT..from book
 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.

NOT CORRECT..from book

 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.

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

 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.

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.

 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 ).