Gametogenesis to mid-blastula transition
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steps of fertilisation
fusion of gametes, creation of first zygotic cell, restores diploid chromosomes, initiates cell cycle
steps of gametogenesis
germ cell vs somatic cell, halving of genetic content, homologous recombination
what is the outcome of germ cell development
a male gamete or female gamete
what to germ cells give rise to
all cells in an organism - only cells to undergo meiosis
why is there a reduction division in formation of the germ line
if there was no reduction then the no of chromosomes in somatic cells would double with each generation
what are the precursor germ cells
primordial germ cells - formed in locations that seem to protect them from inductive signals that determine the fate of somatic cells and shut down the somatic development programme
in drosophila how is movement of transposons prevented
by an rna silencing pathway
what is a germplasm
a special cytoplasm that specifies primordial germ cells
what is an example of a germplasm? (drosophila)
primordial germ cells (pole cells) become distinct at the posterior pole of the egg 90 mins after fertilisation, the cytoplasm at the posterior end is the pole plasm which is distinguished by large organelles polar granules contain proteins and rnas
what happens when the posterior end of a drosophila egg is irridated by UV light
it destroys pole plasm activity so no germ cells develop, somatic cells in this region still develop
what happens if the pole plasm from an egg is transferred to another embryos anterior pole?
nuclei in the area are specified as germ cell nuclei - if this is then transplanted to a 3rd embryos posterior end the cells develop as fucntional germ cells
what does the oskar gene play a role in?
organisation and assembly of the pole plasm, only gene involved in pole plasm formation that has mRNA localised at the posterior pole the signal for this localisation is in the 3’ untranslated region of mRNA
what protein is required for oskars localisation
staufen protein, it links mRNA to the microtubule system polarised along the antero-posterior axis
what happens if oskars genes for 3’ localisation are replaced by bicoid 3’ localisation?
oskar mRNA will be localised at the anteriors of the egg - this is manipulation of DNA
when is germline lineage set up in the nematode?
at the end of the 4th cleavage division with all germ cells derived from the p4 blastomere - p4 cells are derived from 3 stem cell like divisions of p1 cells
what happens at each division of the nematode
1 daughter produces somatic cells and the other divides again to produce a somatic cell progenitor (P cell)
where does the nematode egg produce P granules and how are they distributed
produces p granules in its cytoplasm then asymmetrically distributed before the first cleavage division - then subsequently confined to the p cell lineage
what are p granules suggested to have a role in
due to their association with germ cell formation they may have a role in germ cell specification
what is pg 1-1 gene needed for?
germ cell development it is a p granule component
in the fly and nematode what is transcription repression needed for?
specification of germ cell fate
when are p granules localised
after fertilisation - to the posterior end of the cell
when is diploid chromosome reinstated
after fertilisation
when does meiosis occur
when the germ cells reach the gonad
when are chromosomes replicated in meiosis
1st division not 2nd