Hard Terms Flashcards
Mid-Blastula Transition
After the nuclei have reached the periphery, characterised by slowing down of nuclear division, cellularisation and increase in RNA transcription… ■ Zygotic gene transcription is enhanced… Maternal-to Zygotic transition.
■ Phase controlled by
■ Ratio of chromatin to cytoplasm
Drosophila gastrulation
- A furrow of presumptive mesoderm folds inwards to form the ventral furrow which then pinches off to become a ventral tube (no dorsal tube)
- the endoderm at each end folds inwards to form the cephalic furrow and the posterior transverse folds respectively
- the ectoderm along the ventral midline forms the germ band, which extends posteriorly, so that it wraps around the dorsal side of the embryo towards the head (confined by the egg case which doesn’t grow)-this retracts later
- segmentation lines begin to appear on the surface of the embryo–can now distinguish the head, thorax, and abdominal segmental regions
What triggers the invagination of the mesodermal cells
- The mesodermal cells at the ventral side invaginate and buckel inwards to form the ventral tube.
- Staining of nuclei with a protein that is marker for mesoderm.
- Fgf8 receptors present in the mesoderm cells, fgf8 ligands expressed by the cells above mesoderm (mesectoerm).
- Fgf8 binds to fgf8 receptors and triggers the invagination of the mesodermal cells within the embryo.
- fgf= fibreglass growth factor
Pre-Gradient: Pattern formation in Drosophila in the syncytial cytoplasm-by a hierarchy of gene action that sets up gradients of morphogenetic proteins
- Maternal mRNA molecules, from maternal genes are deposited (even tethered) at the anterior and posterior ends of the egg and are translated into proteins, which diffuse throughout the syncytial blastoderm, setting up a gradients
- These proteins enter the cleavage stage nuclei and activate or repress other maternal or zygotic genes, establishing another round of patterning
- Broad regions are established first, then these are refined to to produce segmental identities
- There is a strict temporal sequence of gene action-a real hierarchy
Formation of the terminal regions of a Drosophila embryo by torso signalling
- Huckbein (hkb) and tailless (tll) along with bicoid protein form the acron.
- Huckbein (hkb) and tailless (tll) are activated by torso signal at area of low Capicua expression form telson
- Hkb+Tll act of own = forms telson
- Hkb+Tll+bicoid = forms acorn
Hedgehog signaling: Drosophila early development
in absence of Hh binding to patched, Ci is weathered to mtubules by Cos2 and fused proteins.
•allows PKA and Slimb portions cleave Ci into a transcriptional repressor that blocks transcription of particular genes
•when Hh binds to patched, conformational changes releasing inhibition of smoothened protein.
•Smoothened then releases Ci from m.tubules (P Cos2 and Fused)
•Inactivates cleavage proteins PKA and Slimb
•Ci origin enters nucleus binds CBP proteins and acts as a transcriptional actuator of a particular gene.
Paracrine regulation of wingless and hedgehog transcription
- Reciprocal interactions between neighboring cells:
- Cells secreting hedgehog activate the transcription of wg.
- wg binds to frizzled, activates Dsh inhibts GSK which activates B-cat which activated Hh
- Cells secreting wg activate the transcription of en and hedgehog.
Homeotic Selector Genes
- Homeotic gene contains homeobox sequence that codes for a 60 amino acid homeodomain protein.
- Homeodomain is a helix-turn-helix motif.
- Gives segment it’s identity
- They activate segment-specific traits like wings, legs and balancers on the thorax; antenna and eyes on the head.
- Drosophila has 8 homeotic genes clustered in 2 complexes.
Mutations of homeotic genes
- Mutations in cis-regulators of ultrabithorax gene transforms 3rd thoracic segment to another 2nd thoracic segment
- Misexpression of Antennapedia gene in the head convert antennae into legs in the head.
Mechanism of homeotic gene transctiption
- Homeotic genes are regulated by gap and pair-rule genes which are transient!!!
- The functions of gap and pair-rule genes are taken over by two groups of proteins (Chromatin modifiers):
- Polycomb group of proteins (-ve regulators; repressor)
- Trithorax proteins (+ regulators; activators)
Formation of the primitive Streak
- thickening of epiblast (and the Kohler’s sickle) [3-4hrs]
- cell converge form depression called primitive groove [7-8hrs streak, 15-16hrs groove]
- cells move through groove to the deeper layers cells in anterior layer thicken and form hensons’ node (primitive knot) [19-22hrs] The streak starts at the posterior margin of the epiblast, and elongates anteriorly
Neural plate: formation and future
broad ectodermal thickening on top of the Henson’s node. Neural plate eventually grows and folds over to form a tube called the neural tube
Induction of Organizer & -what Inhibits the Primitive Streak
- Posterior Marginal Zone contains cells which are equivalent to Nieukoop Centre. •Koller’s sickle is equivalent to Nieukoop Center.
- Nodal activity is high in the region where primitive streak is formed.
- In the rest of PMZ cerberus inhibits the formation of primitive streak
Specification of chick anterior-posterior axis by gravity
Rotations of the ovum in the oviduct at 15rph. Centerfugal forces applied to egg. Separate molecules based i density. Yolk rotes and lighter part of yolk in on top and denser darker is on bottom. Therefore blastoderm laughter sure is on the posterior side. Leads to the movement of light yolky region towards the posterior side.
Experiment: Formation of Henson’s node from Koller’s sickle
Just before gastrulation, cells in the anterior end of Koller’s Sicle (epiblast and middle layer) were labelled with green dye. Cells in the posterior portion of Koller’s Sickle were labelled with red dye. As the cells migrated, the anterior cells formed the Henson’s Node and its notochord derivates. The posterior cells formed the primitive streak.
