Axes of Development 2 Flashcards
What drives early developmental changes?
2 types of genes:
Maternal effect genes
Zygotically acting genes expressed in embryo (not to be confused with maternal inheritance)
At this stage genotype of the mother determines the phenotype of progeny.
What effect do maternal effect genes have on embryo?
They function in early development and directly influence phenotype.
How do maternal effect genes carry out their function?
Pattern formation takes place in oocyte while developing in ovary of mother.
Maternal expression of genes is responsible for creating asymmetries and chemical gradients that the zygote and the embryo uses.
Where are gene products of maternal effect genes located?
They are produced by mother and the gene products are in the ova.
The proteins are already present at fertilisation.
Phenotype of offspring depends on genotype of the mother because source is mother’s genes.
When does the body plan start to take action?
At the stage of the egg. The egg with maternally deposited mRNA has 2 poles. An anterior pole and a posterior pole. The anterior pole has the maternal proteins and this diffuses to the posterior end of the cell creating a gradient of concentration of maternal effect proteins.
This is influenced by maternal effect genes.
What is bicoid gene?
Maternal effect gene that is concentrated in anterior end. Knocking this gene out results in lack of polarity and death.
What is the role of the cytoskeleton in development?
Microfilaments, intermediate filaments, and microtubules have distinct ‘+’and ‘-‘ends which serve as highway systems for intracellular transport.
What do dynein and kinesin do?
Dynein pulls to anterior -ve pole and kinesin to posterior +ve pole.
How does mRNA localisation assist development?
Organisation of cytoskeleton permits localisation of mRNA encoding TFs that will provide positional information. This is by creating high concentration of TFs on the mRNA pole and the diffusion creates a gradient that decreases in concentration in the direction of the posterior pole.
What happens to germline cells during development?
In animals, germline cells are set aside from soma in early development.
What is a morphogen?
A diffusable molecules that determines cell fate in a concentration-dependent manner.
What model explains how cell fate depends on morphogen concentration?
The French Flag Model. Each cell has potential to develop in to blue white or red.
Position of each cell is defined by the concentration of morphogen.
Positional value is interpreted by the cells which differentiate to form a pattern.
Do morphogens get expressed on all poles of the cell?
Yes, some start anteriorly, others posteriorly, others dorsally, and others ventrally.
This sets up gradients that broadly defines areas creating basic body plan map.
*You can imagine the complexity this creates when multiple morphogens can act on exactly the same cells in different ways.
Logic of pattern formation:
Cytoskeleton imposes asymmetry on egg
Germline cells develop
Gradients of morphogens are laid down on both A-P axis and D-V axis.
Broad domains created
Within broad bands more complex gene interactions subdivide further and these identities are remembered.
How is body plan created along A-P axis?
Maternal effect genes then zygotic genes initiate the polarity.
Gap genes divide embryo into broad regions.
Pair-rule genes divide embryo into stripes, defining segment border (on-off, on-off, etc)
Segmentation polarity genes divide segments into anterior and posterior halves.
Homeotic selector genes specify the identity of each segment.
