Body plan Flashcards
what happens to the spätzle ligand after fertilisation
the toll receptor is activated to produce a gradient of the dorsal transcription factor protein along the dorsal/ventral axis
where is the spätzle ligand localised and what protein is expressed in the nuclei of this area
in the ventral region, expresses the dorsal protein in these cells
describe how the toll signalling pathway means that a spätzle signal can result in the transcription of dorsal proteins
spätzle signal binds to the toll receptor, causes a transduction mechanism that results in dorsal protein being translocated to the nucleus where gene expression can be altered.
how is the dorsal protein kept in the cytoplasm
through an interaction with the cactus protein
what are the two key features of the egg when it is laid
bicoid RNA is localised at the anterior + machinery to produce the spätzle ligand is localised ventrally
explain how bicoid mRNA becomes localised to its final anterior site in the drosophila egg
oocyte becomes localised at posterior end and the posterior end of the oocyte sticks to the posterior follicle cells. EGF-like signal from oocyte induces posterior follicle cells with then induce polarisation of oocyte cytoskeleton. polarised microtubules localised bicoid mRNA to its anterior site
how is the D/V axis formed in drosophila oogenesis
oocyte nucleus moves along polarised microtubules to a dorso-anterior position, signals to follicle cells which mean they become different, the ventral follicle cells can not produce spätzle and the dorsal cannot
what are ‘gap’ genes responsible for
coding for transcription factors, result in broad regional differences. regulate ‘pair-rule’ genes to define 14 parasegments
why is it useful that the drosophila embryo is a syncytium
there are a lot of nuclei in a common cytoplasm, means that transcription factors and other proteins can diffuse and influence gene expression in neighbouring nuclei directly
list 5 gap genes in drosophila embryo
hunchback, giant, kruppel, knirps, tailless
describe what the bicoid protein is responsible for
in high concentration at the anterior end, is a direct regulator of hunchback
explain how the kruppel gene is regulated
there is a threshold level of hunchback protein which when reached will result in kruppel expression. expression is stopped when the repression level of hunchback is reached. results in a central line of kruppel. with lower hunchback levels, threshold for kruppel is lower so will be produced more and have no repression stage -> therefore is expressed more anteriorly like hunchback is
what is the purpose of ‘segment polarity’ genes
stabilises boundaries between parasegments so that they can go down different developmental pathways
give an example of two ‘pair-rule’ genes
eve and ftz proteins, result in 7 clear stripes along the drosophila
how does the eve gene code for 7 unique stripes
the DNA contains 7 regulatory sequences, each transcribed to form a separate line and each regulated by their own activators and repressor
what can pair-rule expression genes be impacted by
the concentration of gap gene transcription factors
What establishes the body plan along the anterior/ posterior axis?
Sequential expression- makes differences in embryos- Starts broad and progressively subdivides
Does maternal or paternal genes set up A/P and D/V axes?
Maternal
Do the A/P and D/V axes develop at different times or simultaneously?
Simultaneously
What are the steps if drosophila Oogenesis?
- Germ line stem cell in germanium, divides asymmetrically to produce a cyst oblast and another stem cell
- Cystoblast divides 4X to give 16 cells
- One becomes oocyte, other 15 are nurse cells
- The nurse cells make RNA and protein that is exported into developing oocyte
- All surrounded by somatic follicle cells- produce materials from the protective membrane ( the viteline membrane) that surrounds the egg
What are the three maternal gene mutations in drosophila? What regions do these mutations affect?
Biccoid mutation - affects anterior
Nanos mutant- affects posterior
Torso mutant- affects terminal regions
What is the biocoid gene needed for?
The development of anterior structures
What happens if bicoid gene from the wild type egg is taken and inserted into the bicoid mutant egg?
Some anterior structures develop - restores some normal development
What happens if you move the bicoid gene from the wild type egg into the centre of a bicoid mutant egg?
Head structures developed at the site of injection and the adjacent segments became thoracic segments, setting up a mirror image body pattern at h the site injection.
Where is bicoid mRNA localised to in a newly laid egg? What happens to this mRNA once it has been translated?
mRNA is localised at anterior of newly laid egg.
The bicoid mRNA is translated after fertilisation into bicoid protein and is present in a gradient.
How is the gradient of bicoid protein produced?
Bicoid protein movement and distribution of bicoid mRNA
What is the french flag model for how cell can develop differently in an order formation? (Pattern formation)?
Each cell in a line of cells has the potential to develop as bile, white or red. The line of cells is exposed to a concentration gradient of some substance and each acquires a positional valve it has acquired and differentiates into blue, white or red.
What is a morphogen?
Substances that can direct the development of cells according to concentration gradients within a group of cells, influencing their positional value and subsequent developmental fates.
What is the posterior pattern controlled by?
The gradients of Nanos and Caudal proteins
What happens when there is a mutation in nanos?
The larvae is shorter than normal because there is no abdomen.
What happens if there is mutation in caudal genes?
Abnormal development of abdominal segments
What causes the gradient of caudal proteins?
Is established by specific inhibition of caudal protein synthesis by bicoid protein.
Bicoid protein inhibits what other protein?
Caudal protein- so where bicoid is high at the anterior end, caudal is low etc.
Where is nanos mRNA localised to?
The posterior pole, once translated after fertilisation protein forms gradient
What signalling pathway is torso involved in?
RTK signalling
What are the two termini of the drosophila embryo defined by?
Cell surface receptor torso activation
How is the eve stripe 2 gene regulated?
by the concentration of gap gene transcription factors:
- activated by bicoid and hunchback expression
- repressed either side of the stripe by giant and kruppel
What are segment polarity genes
stabilise the parasegments boundaries and establish cell fate for the segments. respond to pair rule genes and form 14 transverse stripes - one per parasegments
What are segment polarity genes
stabilise the parasegments boundaries and establish cell fate for the segments
What are the two key segment polarity genes and how are they expressed
Hedgehog and wingless. both act on eachother in neighbouring cells to maintain expression. work together to establish boundaries between segments
what region has the highest concentration of dorsal transcription factor in the nuclei
ventral region
what transcription factor is responsible for the dorsal ectoderm development
Dpp
what transcription factors are expressed for neuroectoderm development
rhomboid, sog
what transcription factors are expressed in the nuclei in the mesoderm
twist, snail
What type of genes are twist and dpp
zygotic
Explain how twist and Dpp interpret the gradient of intra-nuclear dorsal protein
through thresholds:
- there is a threshold where once surpassed, dpp is repressed (threshold occurs as you go more ventrally)
- there is another threshold much further ventrally where twist is activated
Explain the role of dorsal
TF that subdivides the D/V axis:
- exists as a gradient
- high dorsal ventrally activates twist expression
- low dorsal (dorsally) activates rhomboid expression in neuroectoderm cells
- dorsal binds to DNA at certain affinity’s to regulate gene expression
low dorsal conc = binds to higher affinity sites, such as rhomboid
high dorsal conc = binds to lower affinity sites, such as twist
explain the function of Dpp and how it works
patterns the dorsal region. acts as signalling molecule
- member of TGFB family
- when intra-nuclear dorsal is established, starts being expressed
- initially, expressed uniformly in the dorsal region, but then evolves into a gradient due to sog protein
- sog protein restricts Dpp to the dorsal side
at what point is Dpp needed to be expressed and why
after introducing-nuclear dorsal is established, teh embryo becomes cellular, signalling is therefore needed
How can D/V patterning in Xenopus laevis be similar to mammals
BMP4 is a TGFB family protein and is the equivalent of Dpp
chordin has a similar role to sog by inhibiting spread of BMP4 to ventral region
similar to mammals but ventral and dorsal have been inverted due to an evolutionary event
what are some similarities between bicoid and dorsal
- both TF’s
- present at a gradient and are concentration dependent regulators
- bind to promoters with on/off switches
- other activators work with them to define broad regions where target genes are initiated
- boarders of target gene expression are established by repressor
What is the bithorax complex responsible for?
The diversification of posterior segments.
What does the removal of all three genes in the bithorax complex do?
Converts parasegments 5-13 into non parasegments 4
What causes the Antennapedia mutant? What kind of mutant is this?
There is expression of the Antp gene in an anterior segment where it is not normally expressed. This transforms and anterior structure (antenna) into a more posterior one (leg) - Is a homeotic mutations
How do hox genes give each cell a permanent record of its A/P position?
- Hox genes auto activate
- The trithorax group of protein stamp the hox gene chromatin with a heritable record of gene activation or repression.
What is the result of a polycomb mutant?
All segments look like the most posterior abdominal segment.- Maintain repressed hox gene expression .
What does the polycomb group of proteins do?
Maintains repressed hox gene expression.
What do the trithorax group do?
Maintain expression of hox genes
What is co-linearity?
The order of genes on the chromosome corresponds to the order of where the genes are expressed along A/P axis.
What are the hox gene targets?
Micromanager and master control gene
What is a micromanager?
Is a hox gene target. It interacts with the transcriptional network at multiple levels and over many developmental stages directly regulates the expression of numerous target genes.
What is the master control gene?
Directly activates a set of primary regulators at a specific time to initiate a novel organ. These few initial target genes control terminal differentiation genes secondarily to orchestrate organogenesis.
How do somites form particular vertebrae? Give examples
Deletion or ectopic-expression of hox genes changes axial patterning.
Example
- Knock out of hox c8
-knock out of all hox 10 paralogues (similar with all hox 11)
- Ectopic expression of hox a7
What happens when Hoxc8 deletion in the mouse?
Additional rib on the 1st lumbar vertebra. ( A posterior to anterior
What does the knock out of Hox10 paralogues?
More ribs attached to sacral and lumbar region.
What does the knock out of all Hox11 paralogues cause?
Little ribs on sacral and lumbar
What happens when Hox a7 gene is expressed throughout the A/P axis?
Anterior structures are transformed into more posterior ones.
Where do cranial neural crests migrate from and to?
Migrate from the rhombomere into brachial arches
When neural crest cells of rhombomere 4 are replaced with rhombomere 2 what happens?
They enter brachial arch 2, but develop like cells in brachial arch 1.
Genes expressed first and most proximally affect what structures?
Proximal structures.
