Lecture 2 - Jan 19.docx

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Cell and Systems Biology
Dorothea Godt

Lecture 2 – Jan 19 CSB429 Why do sperm stay connected? We don’t know! - Might be to maintain synchronization, so the y are all at same stage of development - Another model: during Meiosis, DNA is replicated and split, and then splits again. So one pair has one copy of gene X and the other pair have a different copy of gene X (from different parents respectively) o So what if one of the copies was mutated? In this case, remaining connected during first meiotic division has an advantage because the protein product of gene X will still be created even if one copy of gene X is not functional o If they were to separate, 2/4 of the sperm will die because they have a bad copy of gene X o So this is why they remain connected during meiotic divisions, although why they remain connected in mitotic is still a mystery This applies to the sex chromosomes, the Y copy of the gene is pretty small, by remaining connected, the X copy will still be abl to make gene product so the sperm can survive (if they split, you’d have 2 daughter cells (of a total 4) that have only Y Formation of Germline Cells in Drosophila Early Drosophila Embryo Parent sperm/egg come together and zygote forms and divides, until it becomes a blastoderm and then later undergoes gastrulation - So in this embryo, the primordial germ cells are the first to appear and later, all the remaining nuclei become enveloped by a membrane and we have a multi-cellular embryo These cells are determined to become germ cells (this is the only fate they will have, even if you move them to a new location, they will still develop into germ cells – their fate can not change [unless they die by apoptosis] So we have a cell time that only wants to take on the germ cell fate, and these are the cells that will create the entire next generation So these cells are predetermined to become germ cells, which will establish the next generation when they will be capable of becoming any time of cell These primordial germ cells always form at the posterior pole of the embryo The cytoplasm at this end of the embryo is special and we designate it the “pole plasm” or more generally for other organisms, the germ plasm The cells that come out of the pole plasm will become the primordial germ cells (PGC) If you use UV radiation on the pole plasm, no pole cells form! If you irradiate other parts of the embryo, the pole plasm forms fine So something about this pole must be important for development of PGCsSo people tried to rescue the function of UV irradiated pole plasms by transforming poleplasm from a normal un-irradiated pole plasm into the irradiated embryo. - Is able to rescue the function If you take cytoplasm NOT from the poleplasm of an unirridiated embryo and transport it to the irradiated pole plasm of one, Another experiment: They took the posterior pole plasm and transported it to the anterior end of another embryo and interestingly, pole cels form at both ends of the egg Tells us this poleplasm is important for inducing PGCs but also that it was sufficient In drosophila, PGCs = pole cells However, this still doesn’t tell us if these pole cells actually develop into germ cells (even if they pinch off like pole cells and what not, but doesn’t mean they will become germ cells) So to test if these anterior transported PGCs can develop into normal germ cells, you take the anterior PGCs, and transfer it to the posterior end of another host - As long as this new host or the original donor is labelled with GFP, it doesn’t matter but we can now follow the PGCs throughout development So again, Posterior of donor > anterior of first host > take PGCs from anterior pole cells > transplant them into posterior region of a new host (either with the new host labelled or the original donor labelled to figure out which cells are the donated PGCs) Then you can follow the development of the PGCs into adulthood (since you can’t do that on an embryo with PGCs in anterior and posterior pole) Turns out these transplanted PGCs can develop into germ cells, meaning they all carry the necessary information to develop into germ cells What makes the germ plasm so special? It contains polar granules on the posterior end and cells that have this polar granule incorporated will become pole cells How was all of this discovered? With the grandchildless mutants Mother fly with the mutant gene can lay eggs The F1 of these mother flies will develop into normal flies, however, they are completely sterile Problem is the eggs/embryos of the F1 do not make any PGCs Call this the maternal effect mutation, so the mother carries the mutation but the eggs are the one that have the phenotype (of no PGCs)Polar granules have many proteins involved, so mutations in these would affect PGC development Three mutants: tudor, vasa and staufen Upon investigation, of these mutants, not all had the same phenotype: grandchild-less mutants Class 1 – no polar granules, no PGC formation - Some major players are oskar (our focus) and vasa and tudor Class 2 – polar granule formation, no PGC formation - Germ cell-less (gcl) and 16S-mtlr RNA (ribosomal RNA – what do ribosomes have to do with makng germ cels? We’lll look at this next week) Class 3 – polar granule formation, PGC formation, loss of PGCs – so the PGCs are not being maintained - Nanos (nos), polar granule component (pgc, yes this is confusing ) Vasa = antibodies for vasa find it only in germ plasm (posterior) and PGCs The mutant mother is what causes the F1 generation to have no progeny, so the problem is in the P- generationm likely during oogenesis During oogenesis, vasa, gcl, etc. Are all being produced during oogenesis, this is why the presence of germplasm depends on the mother, because teh factors are all produced during oogenesis Example: Tudor Tudor expressed in nurse cells and are transported into the oocyte during oogenesis, and are later localized to posterior region of egg - In the follicle, the oocyte produces nothing, no transcription, everything needed for oocyte is produced by nurse cells (recall: all connected by cytoplasmic bridges) Stepwise assembly of polar granules during oogenesis: everything starts with O
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