Lecture 13 - patterning the embryo Flashcards
What creates dorsal/ventral polarity?
a gradient of nuclear localisation of the Dorsal protein:
- High on Ventral side
- Low on Dorsal side
What do the promoters of important dorsoventral genes require?
different level of Dorsal protein need to be activated
Describe the affinity for Dorsal binding sites in the promoters of the snail gene
- Low affinity Dorsal binding sites
- only expressed when a high level of nuclear Dorsal is present
- make MESODERM
What has a high affinity Dorsal binding sites in its promoter?
Rhomboid - in addition it is repressed by Snail
- expressed laterally on both sites of mesoderm
- make NERUOECTODERM
What is a promoter?
a control region in the DNA that controls expression of a gene - it “promotes” transcription
What gene is expressed when there is low levels of nuclear Dorsal?
Rhomboid
What gene is expressed when there is highlevels of nuclear Dorsal?
Snail expressed, interferes with with Rhomboid expression
In the dorsal ectoderm, what does Decapentapleguc (dpp) do?
sets up a second signalling centre on the dorsal side of the embryo
What is the name of the second signal?
Bone morphogenetic protein (BMP)
In what is BMP (bone morphogenetic protein) conserved?
vertebrates
What genes does the antero/posterior gradients of Bicoid and Nanos lead to activation of?
a number of gene encoding transcription factors which are known as GAP genes
What is the primary target of Bicoid & Nanos?
Hunchback gene, which together with Bicoid, sets up the expression of other GAP genes
What do GAP genes create?
a ‘combinatory code’, that defines different regions in the embryo
What GAP genes are responsible for striped pair-rule expression?
Even skipped (eve)
Fishing tarazu (ftz)
What drives the striped expression of the prefigured segments of the embryo?
each stripe is driven by a specific gap gene combination
What are the Pair Rule gene expression stripped known as?
Para segments
What is the effect of the segmentation genes?
go from 7 stripes in a pair-rule gene to 14, as high levels of Ftz or Eve switch on Engrailed
What does high levels of Ftz & Eve lead to?
switches on Engrailed
What directly drives the pattern of the cuticle, e.g. the denticles that form the stripy pattern on the larvae?
The “14-striped” expression of Engrailed and some other genes
What forms the POSTERIOR boundary of the segment?
the future visible segments are formed with the Engrailed positive cells
What has occurred by the time the Posterior boundary of the segment has formed?
the embryo has cellularised - meaning diffusion will not work as no longer a Syncytium.
What has to occur due to transcription factors not being to diffuse anymore, as they are individual cells?
cell-to-cell signalling is required, in order to coordinate the pattern formation within one segment
In addition to the transcriptional regulator (En), what 2 highly important signalling proteins are expressed in adjacent cells?
- Wingless (Wg)
- Hedgehog (Hh)
What are Wingless (Wg) & Hedgehog (Hh) under the influence of?
at first pair-rule genes
- then as a result of a feedback loop, expression of Wg is maintained by Hh, and En+Hh by Wg
What can Wingless & Hedgehog do?
Protein signals that act across cell boundaries - they change expression
- they influence transcription in Drosophila
- also important in other multicellular organisms (including human)
What can inappropriate activation of Wnt & Hedgehog (Hh)?
basis of many types of cancer:
Wnt - colon cancer
Hh - basal cell carcinoma
What specifies the initial 14 segments’ identity?
The homeotic selector genes (HOX genes)
- discovered through mutations is Drosophila
- homeotic mutations, one structure is replaced with another
What is the single complex that HOX genes are found in most organisms?
The HOX complex (in Drosophila broken down into 2 pieces
What does the HOX genes order in the genome reflect?
spatial and timing of expression
What is first 3’ or 5’?
3’ first and most anterior, 5’ last and most posterior (DNA usually shown 5’ on the left)
What do HOX genes do?
these genes that give different segments, different fates and modify along the A/P axis, in most organisms.
Why is the Drosophila an exception to the HOX complex?
they have 2 HOX complexes
How many HOX complexes do most mammals have?
4 HOX complexes
What is the bithorax complex responsible for?
diversification of the posterior segments
What is the bithorax complex made up of?
Ubx, abd-A & Abd-B
What cab incorrect expression of genes lead to?
incorrect development of vertebrate
