Ch9 Flashcards
Superficial cleavage
cleavage confined to the cytoplasmic rim
Syncytial cytoplasm
all cleavage nuclei in a common cytoplasm
Energids
nuclei and their associated cytoplasmic islands
Nuclei move to the periphery in cycle
10
Membrane develops around nuclei in cycle
13
Cellular blastoderm
cells (nuclei + membrane) in a single layer around yolk core (around perimeter)
Blastoderm cellularization involves
furrow canals
Gastrulation
- segregation of the mesoderm, ectoderm, and endoderm
- mesoderm (at ventral midline) makes a ventral furrow - layer of mesoderm tissue beneath ventral ectoderm
- endoderm invaginates - 2 pockets and anterior and posterior of the ends of the ventral furrow (pole cells to posterior pocket)
- embryo bends –> cephalic furrow
- ectoderm on surface and and mesoderm converge and extend, migrate to ventral midline to make the germ band
- germ band extends posterioroly to the top (dorsal)
- the nervous system forms from 2 regions of ventral ectoderm
- specification of cell types along anteriorposterior and dorsal-ventral by interaction of cytoplasmic materials
Distinguish the thoracic and abdominal regions by
differences in the cuticle
Anterior and posterior form from
the position of the egg in the ovary
Maternal effect genes
encode translational/transcriptional proteins that activate/repress the expression of zygotic genes
Bicoid and Hunchback
maternal effect genes that regulate the production of anterior structures
Nanos and Caudal
proteins that regulate the formation of posterior parts
Zygotic genes include
gap, pair-rule, polarity, and homeotic sector genes
Gap genes
make broad, overlapping segments
• divide the embryo into broad regions with several parasegment primordia
Differing concentrations of gap genes cause trasncription of
pair-rule genes
Pair-rule genes
- transcribed due to concentrations of gap genes
- divide the embryo into periodic units (7 vertical bands)
- subdivide the broad gap gene regions into parasegments
Pair-rule genes activate the transcription of
segment polarity genes
Segment polarity genes
mRNA and protein divide the embryo into 14 units, establishing periodicity
• responsible for maintaining certain repeated structures within each segment
Homeotic selector genes are regulated by
the products of
• gap genes
• pair-rule genes
• segment polarity genes
Homeotic selector genes
transcription determines the developmental fate of each segment
Axes patterned before
nuclei begin to function
mRNA deposited
Bicoid and hunchback are responsible for
head and thorax formation
Nanos and caudal are responsible for
abdomen formation
Tethered morphogens
bicoid and nanos
Hunchback and caudal are found
throughout the embryo
Fertilization leads to
translation of mRNAs to proteins
Bicoid is high in
the anterior region
• inhibits translation of caudal (in the anterior)
Nanos and pumilio bind
hunchback RNA
• no hunchback translation (in posterior)
Bicoid binds
hunchback enhancer
and stimulates transcription
The Bicoid, Hunchback, and Caudal proteins are
transcription factors whose concentration activates/represses zygotic genes (transcription)
Anterior organizing center
Bicoid
2 genes keep Bicoid
at anterior
• exuperantia and swallow
• absence = bicoid diffuses, less steep gradient, poorly formed head with extended mouth region
Bicoid represses
translation of caudal RNA
Bicoid is a
transcription factor that activates hunchback
Genes for head formation need
- Bicoid for activation
* Hunchback for transcription
Nanos inhibits
hunchback transcription in the posterior
• hunchback bound by Pimilio
• Pumilion joined by Nanos
–> no hunchback translation in posterior
Anterior and posterior termini
anterior = acron posterior = telson
Torso gene
terminal gene
• activated in the ends
• found throughout
Mutation in the torso gene leads to
no acron or telson
The torso gene is activated by the
Torso-like protein
• activates production of kinases that inactivate the transcriptional inhibitor of tailless and huckebin gap genes that specify the termini
Terminal genes alone
both terminal regions telsons
• bicoid also present = 1 acron
The anterior-posterior axis involves 3 sets of genes
- the anterior organizing center
- the posterior organizing center
- the terminal boundary region
Cell fate commitment has 2 steps
- specification
2. determination
Specification
a loose commitment
• flexible (can receive signals from environment eg morphogens)
Determination
irreversible
The transition from specification to determination is mediated by
segmentation genes
Segmentation genes
genes that divide the early embryo into a repeating series of segmental primordia
Mutations of segmentation genes often affect
parasegments
Parasegments
regions of the embryo separated by mesodermal thickenings and ectodermal grooves
• 14 in total
A parasegment consists of
- the posterior of the anterior segment
* the anterior of the segment behind
The transition from an embryo with gradients of morphogens into an embryo with distinct units is mediated by
gap genes
Pair-rule genes are activated by
the products of gap genes interacting with products of neighboring gap genes
Mutations of segment polarity genes leads to
a portion of each segment is deleted and replaced with a mirror image of another portion
Segmentation genes are
transcription factors that use the gradients of the early cleavage embryo to transform the embryo into a periodic, parasegmental structure
Homeotic selector genes are regulated by
gap and pair-rule genes
Homeotic selector genes
determine the identity of each segment
