Lecture 7 Flashcards
What is the purpose of segmentation and how is the drosophila segmented?
Segmentation is an ancient and conserved way of building bodies. Drosophila are segmented via denticle belts and thorax vs abdomen which each have a certain number of legs for example.
Briefly describe the first manifestation of segmentation in drosophila
Between the 24 hours it takes to form a segmented embryo from a fertilised egg, the Embryo begins to display many folds that can be seen from the outside
What did Volhard and Wischaus do?
In 1979-80, they carried out a saturation mutagenesis on 27,000 drosophila. 98% of the mutations were recessive and 18,000 were embryonic lethal. 139 complement groups were found which meant that they probably saturated the system
How are genes named?
After the mutated phenotype
Describe the gene hierarchy and their phenotypes when mutated
1) Maternal genes e.g. bicoid= los of anterior structures e.g. head and thorax
2) Gap genes e.g. knirps- head, thorax and tail but no middle
3) pair rule genes e.g. paired- loss of pairs= embryo is missing half
4) segment polarity genes e.g. gooseberry= smaller embryo due to loss of naked cuticle= covered in hairs
How was bicoid tested as a morphogen?
Cytoplasm containing bicoid was sucked out using a needle and was inserted into a bicoid mutant
1) anterior= some head regions were restored
2) middle= mirror image with head structures forming in the centre and thoracic segments on either side
What was dine once bicoid was cloned and identified?
Pair rule genes were looked at in bicoid overexpression experiments
1) WT= 7 segmented striped with gaps anteriorly and posteriorly
2) overexpression of bicoid= 7 bands but pushed more posteriorly
How does bicoid induce patterning?
Is a TF which binds to high affinity binding sites in low concentrations and low affinity binding sites in high concs= works like the french flag model
How are gap genes activated and give an example
They are activated by maternal genes e.g. stripe of krupple expression in the middle of the embryo due to being in the goldilocks zone of the bicoid gradient
How are pair rule genes expressed?
expression is controlled stripe by stripe- krupple and giant are transcriptional repressors that turn off even skipped transcription which is activated by bicoid. Many transcriptional activators and repressor work stripe by stripe and are controlled by gap genes
How are segment polarity genes expressed?
Further refinement of segmentation- made of repeating units of segments and parasegments= defined before the segment to act as the boarder to prevent SPG expression overlap. A segment is a denticle belt and naked cuticle.
Describe the relationship between hh and wg in segmentation.
One cell expresses wnt and the other expressed hh- they feedback on each other to maintain one another’s expression. Wg cell produces an asymmetric conc gradient with more wg anteriorly= suppresses denticle formation
What are HOX genes are how are they expressed?
Are homeotic TFs that are expressed along the A/P axis in the same order as the genes in the genome. This occurs by epigenetic changes under the control of gap and pair rule genes
What is antennapedia and what did it lead to the realisation of?
When antenna become legs- the segment is there but are told to become legs. Looked at DNA binding domain for the genes- are the same but produce completely different biological outcomes
Give an example of a long germ band insect
Drosophila- all 14 segments are defined at once within 24hrs- very complex use of gene hierarchy
