Drosophila- patterning the embryo Flashcards
what’s D/V polarity
-A gradient of nuclear localisation of the Dorsal protein:
=High on Ventral side
=Low on Dorsal side.
-The promoters of important Dorsoventral genes require different levels of Dorsal protein to be activated
-pipe activates spatzle
-dorsal side is cytoplasmic
what does different levels of nuclear dorsal lead to
- different genes being expressed.
-The promoters of the snail gene has LOW AFFINITY Dorsal binding sites :
=Only expressed when a high level of nuclear Dorsal is present
=Make MESODERM
-rhomboid has HIGH AFFINITY Dorsal binding sites in its promoter, in addition it is repressed by Snail:
=Expressed laterally on both sides of mesoderm
=Make NEUROECTODERM
-hgih level of dorsal needed for snail activation
-dpp expressed on dorsal side
what do activation thresholds help with
-The nuclear gradient of dorsal lead to expression of different genes along the D/V axis
-This works because the different genes have different “activation thresholds”
whats a promoter
-a control region in the DNA that controls expression of a gene…it “promotes” transcription
what happens with a Low level of nuclear Dorsal
-rhomboid expressed
what happens with a High level of nuclear Dorsal
-Snail expressed, interferes with Rhomboid expression
what affinity does the promoter of snail have for dorsal binding sites
-low
=Only expressed when a high level of nuclear Dorsal is present
=Make MESODERM
-rhomboid has HIGH AFFINITY Dorsal binding sites in its promoter, in addition it is repressed by Snail
=Expressed laterally on both sides of mesoderm
=Make NEUROECTODERM
what does app set up in the dorsal ectoderm
- Decapentaplegic (Dpp), sets up a second signaling center on the dorsal side of the embryo.
-This signal is conserved in VERTEBRATES, where it is known as the Bone Morphogenetic Protein (BMP) signal.
whats the A/P axis
-The Antero/Posterior gradients of Bicoid and Nanos lead to activation of a number of genes encoding transcription factors which are known as GAP GENES.
-mutants of genes have a gap where the protein has just gone
-A primary target is the Hunchback gene, which together with Bicoid, sets up the expression of other GAP genes
-The GAP genes create a ”Combinatory Code” that defines different regions in the embryo.
what are gap genes responsible for
-striped pair-rule expression
-even skipped (eve)
-fushi tarazu (ftz)
what are pair-rule gene expression
-slightly shifted relative to the visible segments
- the anterior part of the even skipped expression stripe forms the posterior of the a larval segment.
-The Pair Rule gene expression expression stripes are known as PARA SEGMENTS
what do the SEGMENTATION genes elaborate
-the pair rule pattern into a segmental pattern
-high level of Eve and Ftz drive expression of a transcription factor ..this leads to formation of 14 stripes, one in each segment.
-The “14-striped” expression of Engrailed and some other genes we will come back to, is directly driving the pattern of the cuticle, for instance the denticles that form the stripy pattern on the larvae.
how is the posterior boundary of the segment formed
-The future visible segments are formed with the Engrailed positive cells forming the POSTERIOR boundary
what are the 2 signalling proteins as well as the transcriptional regulators expressed in adjacent cells
-Wingless (Wg), and Hedgehog (Hh)
-first under the influence of the pair rule genes
-Then as a result of a feedback loop expression of Wg is maintained by Hh, and En+Hh by Wg
what state is the embryo in when intra segment patterning is happening
-the embryo is cellularised.
-So in order to coordinate patterning, these coordinating signals need to act across CELL MEMBRANES.
what are wingless and hedgehog
-Are protein signals that act across cell boundaries
-After their discovery, and the precise pathways they are using to influence transcription in Drosophila, it was found that they are also very important in other multicellular organisms including human.
-Reduced or aberrant activity leads to a variety of congenital defects
-Inappropriate activation of these is at the basis of many types of cancer
whats Bithorax
-flies with 2 sets of wings
-Homeotic mutations; one structure is replaced with another
whats Antennapedia
-2 sets of legs
-Homeotic mutations; one structure is replaced with another
what are hox genes
-In most organisms the Hox genes are a single complex: The Hox Complex (in Drosophila it is broken into 2 pieces)
-Their order on the genome reflects their SPATIAL and TIMING of expression
-3’ first and most anterior; 5’ last and most posterior…(NB DNA is usually shown 5’-3’!
whats the bithorax complex responsible for
-for diversification of the posterior segments
