Drosophila Body Plan: Gap Genes And Pair-rule Genes Flashcards
See all page 77 ‘maternal effect genes’ ‘Vickie mutant experiments
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Explain how translation of vicious mRNA forms a concentration gradient in Bicoid protein
. mRNA is translated after fertilisation to form an anterior-posterior Bicoid protein gradient
. Bicoid protein is a ‘morphogen’- an antibody has been raised against the Bicoid protein and it has been labelled with a fluorescent tag so we can see how it changes through development. Cells in with the high concentration of the Bicoid protein are getting information that they are in the anterior end of the embryo
. Specifies the anterior part of the anterior-posterior
How is a hunchback or caudal concentration gradients established?
. Nanos (highest concentration is at the posterior of the embryo) suppresses/ inhibits translation of maternal hunchback mRNA in posterior
. Bicoid promotes production of embryos own (not from the mother) Hunchback protein in anterior
. Bicoid inhibits translation of Caudal protein in anterior forming the posterior-anterior gradient
How does maternal mRNAs become distributed?
. Gurken mRNA (from mother) is translated in the posterior (highest concentration here) and protein binds to Torpedo receptor in posterior follicle cells (somatic) this causes changes in the structure of the microfilaments that are associated with the cytoskeleton- provide structure of cells and also are like ‘tramways to move cells’. So this binding gets the posterior cells to stimulate the movement of these RNAs along the microtubules
- gurken is originally translated in the posterior but in the later in development it is actually stunted up, along with the nucleus to the dorsal part of the embryo
. Causes posterior follicle cells to produce a signal that rearranges microtubules to their locations
. Posterior localisation of maternal oskar RNA needed for nanos mRNA in posterior
. Movement of gurken mRNA associated with movement of nucleus to establish dorsal axis
Describe a gene expression cascade
. Gradients established by maternal effect genes (transcribed in mother and translated in the embryo)
. Gradients control the expression of the embryos own ‘zygotic’ genes (it is the action of the maternal gene products that actually switches on the embryos own genes)
. Developmental genes act in a strict temporal sequence, a heirarchy of gene activity- one phase of gene activity leaves on to another
. Results in the embryo being patterned I stops
. Broad regional differences are gradually refined into smaller domains of organisation
Put these in order: segmentation genes, pair-rule genes, maternal genes, selector genes, gap genes
Maternal genes-> gap genes -> pair-rule genes-> segmentation genes-> selector genes
Describe how maternal genes can end in the production of proteins for selectors genes
The maternal genes switch on the gap genes and head form a distinct gradient in the embryo and their function is to switch on another category of genes called the pair-rule genes, the protein products from those genes then active another category of genes called the segmentation genes and the protein products of these influence the production of proteins for selector genes
Describe gap genes
. Named because mutations commonly cause segment deletions, leaving gaps in the embryo form, broad zones
. The first zygotic genes to be expressed along AP axis, code for transcription factors
. Pattern of gene expression broadly similar to region affected in mutants
. Hunchback is a gap gene, activates by Bicoid protein (but also a maternal mRNA)
. Hunchback protein activates and represses other gap genes such as knifes, kruppel, and giant
. Gap genes and their products interact to define boundaries of expression
. AP axis becomes divided into unique regions (domains), each containing different combinations of transcription factors
Look at pictures on lecture 15 pages
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Describe pair-rule genes
Earliest segments are 14 parasegments in drosophila. As the zones of organisation become diner each one of the parasegments is divided into a front and back half so we can distinguish between the anterior and posterior of the parasegments. Final segments formed from posterior region of one parasegment, and anterior of the next (that join together) this causes a shift
Describe the alternate parasegments in the pair-rule genes
. Even-slipped (blue)- odd parasegments
. Fushi tarazu (brown)- even parasegments
. Transcription factors in the nucleus
. Each stripe a few cells wide
. Alternating stripes that add to 14. So each stripe corresponds to a single stripe
What is engrailed?
Is a segment polarity gene that is expressed in the anterior region of each parasegment. (It is also a selector gene that confers particular identity on a region or regions by controlling the activity of other genes)
Describe the expression of engrailed and why it is like this?
Engrailed gene expression is confined to a few cells in the anterior region of the parasegment and is due to activation of the paid-rule genes (even skipped and fushi tarazu)
How are parasegment boundaries defined?
Engrailed expression helps to define parasegment boundaries, confining cells and their descendants into ‘compartments’.
Engrailed-anterior of all parasegments (will eventually mark the posterior part of the final segment)
What happened if fushi tarazu is not expressed?
Engrailed expression is lost in even-numbered parasegments. Results in mutant with too few segments. Due to a loss of instructions from the pair-rule genes
