Excision of Intron Sequences

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.

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.

The Spliceosome

Five snRNAs: U1, U2, U4, U5, and U6

Some snRNAs associate with proteins to form snRNAs (small nuclear ribonucleoproteins)

What are Logo plots?

Logo fora) Splice acceptor

b) Splice Donor

c) Initiator Met

AG/GTAG/GT CAG/NTCAG/NT

exon 1 exon 1 intron 1 intron 1 exon 2exon 2

9 8 7 6 5 4 3 2 1 1 2 4 3 2 1 1 2 3 4 5 6

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

Where is RNA binding domain of SXL protein?

E is Establisment formMade cycle 10

L is Late form madeDuring cycle 14

SXL makes a functional protein in XX zygotesSXL makes a non-functional protein in XYzygotes

Event is initiated by Exons E1+ 4No early form of protein, thus no SXL 1/2/4 form

SXL 1/2/4 form

Cycle10-14

Cycle14+

Zygotic transcription starts

~Cycle 9

Cycle14-and up

Cycle10-14Zygotic transcription starts

KEY SLIDE…Know it.

Gro-maternalDpn-Zygotic

Development Development

Zygotic-X chromosome factors:XSEs2x 1x

Maternal groDown regulated byZygotic XSE proteins

Zygotic deadpan Represses SxlPe withMaternal gro

2x difference

Chapter 19The Genetic Control of Animal

Development

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.

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.

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

NOT CORRECT..from book

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.

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.

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.

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.

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).