Pattern formation slide 3
Terms in this set (12)
o Transition from loose type of commitment to irreversible determination.
o This transition is mediated by segmentation genes.
Lack large regions of the body (several continuous segments).
pair rule mutants
Lack portions of every other segment.
segment polarity mutants
Show defects (deletions, duplications, polarity reversals) in every segment.
Divide the embryo into a repeating series of segmental primordial along the anterior-posterior axis.
SGs mediate the transition from loose type of commitment to irreversible determination
o A transegmental unit in drosophila that includes the posterior compartment of one segment and the anterior compartment of the immediately posterior segment
o Separated by mesodermal thickening and ectodermal grooves.
o A fundamental unit of embryonic gene expression.
o Expression patterns in the early embryo are delineated by parasegmental boundaries, not by the boundaries of the segments.
what is the purpose of parasegments and segments
The cells of one compartment do not mix with the cells of the neighboring compartments, and parasegments and segments are out of phase by one compartment.
whats the the function of gap genes?
• Activated or repressed by the maternal effect genes, and are expressed in one or two broad domains along the anterior-posterior
• Anterior hunchback genes are truncated by posterior gap genes.
o This cut-off establishes distinct boundaries.
• Gap genes provide a boundary for pair-rule genes.
o Example: even-skipped.
• The transcription patterns of the anterior gap genes are initiated by the different concentrations of the hunchback and bicoid proteins.
• Gap genes control transcription activation and repression at the gene level.
• Repression interactions result in the creation of a precise system of overlapping mRNA expression patterns.
o Each domain serves as a source for diffusion of gap proteins into adjacent embryonic regions.
what is the function of pair rule genes?
• Pair-rule gene expression is the first indication of segmentation in the fly embryo.
• The transcription patterns of these genes divide the embryo into regions that are precursors of the segmental body plan.
• One band of vertical nuclei express a pair-rule gene, the next band does not express it, and the next band expresses it again, resulting in a "zebra stripe" pattern along the anterior-posterior axis.
• Primary Pair-Rule Genes:
o Controlled by maternal products.
• Secondary Pair-Rule Genes:
o Controlled by primary gene products.
o Paired, even-skip, odd-skip, barrel, sloppy-paired, hairy, runt, odd-pair, fTz
• In fTz, mutant has absence of odd # segments; ftz gene expressed in PS 2,4,6...
• Each row of nuclei within a parasegment has its own array of pair-rule products that distinguishes it from every row.
o These products activate the next level of segmentation genes, the segment polarity genes.
what is the function of segment polarity genes?
• Zygotic genes activated by the proteins encoded by the pair-rule genes whose mRNA and protein products divide the embryo into 14 segment-wide units, establishing the periodicity of the embryo.
• Segmentation genes are transcription factors that use gradients of early cleavage embryos to transform embryo to periodic parasegmental structure
• Accomplish two important tasks:
o Reinforce the periodicity established by earlier transcription factors.
o Through cell-to-cell signaling, cell fates are established in each parasegment.
Segment polarity gene-engrailed
Normal development pattern relies on the fact that only one row of each parasegment is permitted to express the hedgehog protein, and only one row of cells in each parasegment is permitted to express the wingless protein.
o As a result, the engrailed protein is found in 14 stripes across the anterior-posterior axis of the embryo.
o These stripes of engrailed transcription mark the anterior compartment of each parasegment.
o Maintain repeat structures within each segment
o In mutant, normal # of segments, have mirror image of component posterior/anterior, lose diversity
• Within each segment, a particular region of the posterior component has been lost and replaced by a mirror image of the anterior compartment
The engrailed gene is activated in cells that have high levels of what?
The key to this protein is the activation of the engrailed gene in those cells that are going to express Hedgehog.
The engrailed gene is activated in cells that have high levels of even-skipped, fushi tarazu, or paired transcription factors. Engrailed is repressed in those cells with high levels of odd-skipped, runt, or sloppy-paired proteins.
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