|MadSci Network: Development|
You have asked a really great question. We all know that sperm and eggs are haploid, so where does the other genome come from? Actually, eggs of mammals and many other types of animals do not complete the meiotic divisions until late in their development. In Drosophila (fruit fly), the two meiotic divisions occur only after fertilization. Mammalian eggs initiate meiosis while the female embryo is still developing, then arrest in prophase of meiosis I. In humans, they can remain in this state for forty years! Near the time of ovulation, primary oocytes complete meiosis I, ejecting half of their genetic material as the first polar body. In most vertebrate species, meiosis II is not completed until after fertilization. Please see: http://www.ncbi.nlm.nih.gov:80/books/bv.fcgi?call=bv.View..ShowSection&rid= cell.figgrp.d1e100659 So the answer to your question is that you can essentially trick the secondary oocyte into using the genetic material that would have been ejected in the second polar body as the equivalent of a sperm nucleus. Meiosis produces four haploid products (easier to see in sperm development), so a regular embryo gets one haploid genome from the sperm and one from the egg. A parthenote gets two haploid genomes by getting (essentially) one from what would have been the egg pronucleus and one from what would have been the second polar body. Here is a news account of parthenogenesis in monkey cells: http://news.bbc.co.uk/hi/english/sci/tech/newsid_1794000/1794661.stm The original article is Science: Science 295: 779-780 (2002) [news and comment] and Science 295: 819 (2002) The treatments that induce parthenogenesis include electroporation and inhibition of protein synthesis or phosphorylation using drugs. Eggs can also be treated with MPF (maturation promoting factor) inhibitors. Please see: Biol Reprod 2001 Jul;65(1):253-9 You may also find this Glossary useful: http://www.informatics.jax.org/userdocs/glossary.shtml Yours, Paul Szauter Mouse Genome Informatics
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