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29 Cards in this Set
- Front
- Back
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C. elegans life cycle
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small soil dwelling nematode
short life cycle under 3 days exist mainly as hermaphrodites (reproduce by self fertilization) |
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C. elegans characteristics
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transparent
complete cell lineage map; invariant simple anatomy early larval stages can be frozen and revived later First multicellular animal to have its genome completely sequenced |
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C. elegans Anatomy
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Simple
Four molting stages before adult Post-embryonic development essentially additions to the larval body plan |
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C. elegans cell lineage
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complete cell lineage including all 959 somatic cells was determined by John Sulston
organs not usually derived from one sub-lineage |
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C. elegans Embyogenesis
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14 hours of development compressed into about a minute
Early asymmetic cell division; gastrulation;elongation; muscle movements at 2-fold stage |
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C. elegans cleavage
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Embyo is asymmetric and the A/P axis is visible early after fertilization
Asymmetric cell divisions are visible Gastrulation begins at 28 cell stage: gut cells from E move inside Zygotic gene expression starts at 4 cell stage maternal components control development up to the 28-cell stage |
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early cell divisions
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E is the only major sub-lineage that gives rise to a complete organ
P1, P2, P3 like stem cells eventually just give rise to germ cells Hypodermis, neurons and muscles come from multiple sub-lineages |
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P-granules reflect Cell Polarization
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P-granules are NOT developmental determinants
They are required for germ cell development They are assymmetrically localized in the posterior of cells BEFORE division at 28 cell stage they are only in P4 which gives rise to germ cells |
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A/P axis formation
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A/P axis determined by the site of sperm entry
centrosome brought in by sperm, interacts w/the actomyosin cortex of egg results in cortical flow towards the anterior and cytoplasmic flow towards the posterior PAR proteins become unevenly distributed P1 cell is always in the posterior |
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PAR proteins
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crucial for the first assymetric cell division
important in establishing cell polarity in many cell types in many organisms |
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D/V Axis can be Reversed by Manipulation
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The AB cell can be mechanically rotated while it is dividing
ABa and ABp switch positions and P1 displaced Animals develop normally except D/V and L/R axes are reversed |
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Conclusions from D/V axis Manipulation Experiment
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D/V pattern is not fixed at this stage (same with L/R axis)
D/V patterning is determined by cell-cell interactions since the future development of the cells can be changed ABa develops as ABp and vice versa D/V reversal can be reversed as the 2 cell to 4 cell stage therefore L and R also still not specified |
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L/R axis can also be reversed
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specification of the L/R axis occurs at the six-cell stage
embryo manipulation at this stage can reverse the L/R axis Mechanisms determining handedness unknown although spindle orientations and centrosomes play a role |
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Important Cell- cell interactions in early cell fate determination (ABp)
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observation: removal of P1 results in no pharynx
P1 plays a role in determining ABp fate since pharynx is derived from ABp If ABp doesn't contact P2 it develops as ABa P2 specifies ABp via the Notch-Delta pathway: GLP-1 APX-1 |
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Important Cell-Cell Interactions In Early cell Fate Determination (EMS)
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Observation: removal of P2 results in no gut
P2 takes part in the fate of EMS since gut is derived from EMS If EMS removed from a late 4 cell embryo it can form gut cells, but not if it is removed from an EARLY 4 cell embryo Adding P2 to an Early EMS isolate restores gut development Involves the Wnt Signaling pathway: MOM-2 = Wnt; MOM-5= Frizzled |
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Wnt Signal from P2 Results in Redistribution of POP-1
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POP-1 is downregulated in the posterior of the cell receiving the signal (EMS)
results in the formation of two different daughter cells: MS and E If P2 signal is missing both are MS If pop-1 is mutated, both are E |
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POP-1
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POP: Posterior Pharynx defective
POP-1 is the TCF/Pangolin (vertebrate/drosophilia) transcription factor homolog high level of POP-1 in general may specify anterior cell fate |
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Cells that Form the Same Organ Eventually Cluster Together
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Occurs at about the 80 cell stage
cells in a particular cluster may express an organ identity gene Ex: pha-4 (expressed in cells that give rise to pharynx) PHA-4 is a transcription factor |
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C. elegans Hox Genes
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C. elegans has many homeobox genes: onloy four similar to the Hox genes
also in similar order on the genome A/P order of expression for 3 of the genes correlates w/their order on the chromosome no segmentation in the A/P axis in C. elegans These genes are important for regional specification in post-embryonic development |
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RNA interference (RNAi)
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method of silencing gene fxn at the RNA level
Phenomena first discovered in plants Originally used antisense RNA to block translation but this wasn't very effective Discovered that dsRNA was most effective at silencing genes Process thought to exist to help defend cell against viruses |
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Timing of Development is Under Genetic Control
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Invariant lineage makes it easy to study the timing of development
heterochronic genes affect the timing of development ex: lin-4 and lin-14 |
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LIN-14 and LIN-4 Control the timing of development
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levels of LIN-14 protein determine the stage of development (temporal gradient)
LIN-14 at different concentrations may specify different fates for cells LIN-14 could supply the basis of a precisely ordered temporal sequence of cell activities LIN-4 represses LIN-14 translation |
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lin-4 Encodes a microRNA (miRNA)
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miRNAs are short specialized RNAs
They prevent genes from being translated Some of the same machinery used in RNAi is used by miRNAs Primary miRNA transcript is 100s of nts long and ocntains an inverted repeat, which forms a double stranded hairpin Unlike RNAi the miRNAs do not have to perfectly match the target RNA RNA translation is blocked but it is not degraded |
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MOM and POP signaling
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MS descendants contribute to the pharynx
E gives rise to the gut MOM-2/MOM-5 signaling downregulates POP-1 in the posterior of EMS There is less POP-1 in E compared to MS MOM-2: More of MS (Wnt) MOM-2/MOM-5 are part of a non-canonical Wnt signaling pathway |
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Noncanonical Wnt Signaling
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POP-1/TCF acts as a repressor in both canonical and noncanonical Wnt signaling
B-catenin converts TCF into an activator in canonical signaling B-catenin is involved in the removal of POP-1 from the nucleus in C. elegans noncano. Wnt signaling MOM signaling removes POP-1 from the E cell nucleus Gut genes are then transcribed. |
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miRNAs
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lin-4 was the first miRNA identified
let-7 the second one identified in C. elegans; conserved in many species let-7 is required for the transition from the late larval to adult stage in C. elegans Hundreds of human miRNAs identified Involved in cell differentiation, proliferation and cell death |
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Development of C. elegans Vulva
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the vulva is an opening that connects the uterus to the outside
3 initial cells involved in creating the 22 cell vulva consists of 7 rings resulting from a series of cell fusions and cell shape changes |
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Development of the c. elegans vulva (II)
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The gonadal anchor cell is the inducing cell and 3 ectodermal cells take on a primary or secondary fate to form the vulva
Primary or secondary fate determined by distance to the anchor cell 6 ectodermal cells called vulval precursor cells (VPCs) have the potential to form the vulva |
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LIN-3 (EGF) Signals from the Anchor Cell
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LIN-3 (Epidermal Growth Factor) is the ligand
LET-23 is the receptor (EGFR) on the VPCs Primary cells also induce secondary fates via the LIN-12 (Notch) signaling pathway |
