Transcript Chapter 21 The Genetic Control of Animal Development

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 for
a) Splice acceptor
b) Splice Donor
c) Initiator Met
AG/GT
exon 1
4321123456
CAG/NT
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?
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
Zygotic transcription starts
Cycle10-14
Cycle14+
~Cycle 9
Cycle10-14
Zygotic transcription starts
Cycle14-and up
KEY SLIDE…Know it.
Zygotic-X chromosome factors:XSEs
2x
1x
Gro-maternal
Dpn-Zygotic
2x difference
Maternal gro
Down regulated by
Zygotic XSE proteins
Zygotic deadpan
Represses SxlPe with
Maternal gro
Development
Development
Chapter 19
The 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.
NOT CORRECT..from book
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
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 SexDetermination 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).