Excision of Intron Sequences Splicing 4 Removal of

Excision of Intron Sequences

Splicing 4 Removal of introns must be very precise. 4 Conserved sequences for removal of the introns of nuclear m. RNA 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.

Types of Intron Excision 4 The introns of t. RNA precursors are excised by precise endonucleolytic cleavage and ligation reactions catalyzed by special splicing endonuclease and ligase activities. 4 The introns of nuclear pre-m. RNA (hn. RNA) transcripts are spliced out in two-step reactions carried out by spliceosomes.

The Spliceosome 4 Five sn. RNAs: U 1, U 2, U 4, U 5, and U 6 4 Some sn. RNAs associate with proteins to form sn. RNAs (small nuclear ribonucleoproteins)

What are Logo plots?

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

CAG/NT AG/GT exon 1 4321123456 intron 1 exon 2 98765432112

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

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

The Sxl protein in different Diptera species. Penalva L O F , and Sánchez L Microbiol. Mol. Biol. Rev. 2003; 67: 343 -359

The Sxl protein in different Diptera species. Penalva L O F , and Sánchez L Microbiol. Mol. Biol. Rev. 2003; 67: 343 -359

The Sxl protein in different Diptera species. Penalva L O F , and Sánchez L Microbiol. Mol. Biol. Rev. 2003; 67: 343 -359

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

Zygotic transcription starts Cycle 10 -14 Cycle 14+

~Cycle 9 Cycle 10 -14 Zygotic transcription starts Cycle 14 -and up

KEY SLIDE…Know it. Zygotic-X chromosome factors: XSEs 2 x 1 x Gro-maternal Dpn-Zygotic 2 x difference Maternal gro Down regulated by Zygotic XSE proteins Zygotic deadpan Represses Sxl. Pe with Maternal gro Development

Chapter 19 The Genetic Control of Animal Development

Sex Determination in Drosophila and C. elegans 4 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 4 But wrong 4 In Drosophila, the key genes in sex determination encode proteins that regulate RNA processing.

Sex Determination in Drosophila 4 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.

NOT CORRECT. . from book Ascertaining the X: A Ratio 4 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. 4 The X-linked gene products are called numerator elements and are twice as abundant in XX embryos as in XY embryos. 4 The autosomal gene products are called denominator elements and antagonize the products of the numerator elements- NONE FOUND.

NOT CORRECT. . from book

The Sex-lethal (Sxl) Gene 4 Sxl is the master regular of the sex determination pathway in Drosophila. 4 The X: A ratio is converted into a molecular signal that controls the expression of the X-linked Sxl gene.

Function of SXL 4 SXL regulates splicing of its own transcript to maintain SXL protein expression in XX embryos. 4 SXL also regulates splicing of the transformer (tra) gene.

Differentiating in Response to the Signal 4 TRA, along with TRA 2, regulate splicing of doublesex (dsx) and fruitless (fru). 4 In XX embryos, where TRA is present, dsx transcripts are processed to encode a DSX protein that represses the genes for male development. 4 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 4 Mutations in Sxl prevent SXL protein from being made in males; homozygous mutants would develop into males but die as embryos. 4 Mutations in transformer and transformer 2 cause both XX and XY animals to develop into males. 4 Mutations in dsx cause both XX and XY embryos to develop into intersexes.

Key Points 4 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. 4 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).