chiaravi@silver.bacs.indiana.edu (Lucius Chiaraviglio) (04/21/89)
In article <8243@boulder.Colorado.EDU> pell@boulder.Colorado.EDU (Anthony Pelletier) writes: >In article <1390@rpi.edu> fargo@pawl.rpi.edu (Irwin M. Fargo) writes: >>> >>Pathway 2 is not only very improbable, but it's very impossible in humans. >> >>Normal human cells are diploid (They contain twice the necessary amount of >>genetic material). Sex cells are monoploid (only the necessary amount of >>genetic material). But, sex cells have their genetic material separated >>in such a way that they contain (follow this carefully) twice the amount of >>half the genetic makeup. (Read a book on the differences between meiosis >>and mitosis if this is still unclear) That is only true in eggs which do not complete the equational (second in most organisms) division of meiosis until after fertilization. >Anyway, there was alot of misinformation in you posting. >The term you want is "haploid." Monoploid is not incorrect, since it means >"1n." But haploid is the term used for gametes such as the 1n gametes >of a 2n organism (or 3n gametes of a 6n organism, for that matter). That isn't the way I have always heard it -- the way I have heard it, if an organism is tetraploid and makes gametes by standard meiosis, you are obliged to refer to the gametes as diploid, etc. >Finally, since ALL FOUR gametic nucleii (the egg nucleus and 3 polar bodies) >remain in the ovum until fertilization, something like what tim diescribed >really is not outside of the realm of possible. That is only true of some organisms. In others, the polar bodies become separate cells after each division of meiosis; these cells would most likely not contain enough materials to form an embryo successfully even if fertilized. > Very occasionally, polar >bodies do get fertilized. If the polar body that separated from the >egg nucleus at meiosis II was fertilized, as well as the egg nucleus, >the resulting fraternal twins would be exactly what tim suggested--that >is, they would have one set (the maternal) of chromosomes that was identical. Not likely, unless an aberrantly large polar body had been formed. I have seen scanning electron micrographs of eggs with polar bodies attached, and the polar bodies have < 1/64 of the volume of the egg. The smallest fraction of an egg (formed by mitotic division of the zygote followed by separation of the resulting cells) that I have heard of being able to form a viable embryo in mammals is 1/16 (and that only in some mammals), so a polar body of the size shown in these electron micrographs would die, possibly after a few divisions. -- | Lucius Chiaraviglio | ARPA: chiaravi@silver.bacs.indiana.edu BITNET: chiaravi@IUBACS.BITNET (IUBACS hoses From: fields; INCLUDE RET ADDR) ARPA-gatewayed BITNET: chiaravi%IUBACS.BITNET@vm.cc.purdue.edu Alt ARPA-gatewayed BITNET: chiaravi%IUBACS.BITNET@cunyvm.cuny.edu
pell@boulder.Colorado.EDU (Anthony Pelletier) (04/21/89)
In article <3791@silver.bacs.indiana.edu> chiaravi@silver.UUCP (Lucius Chiaraviglio) writes: >In article I wrote: >>In article <1390@rpi.edu> fargo@pawl.rpi.edu (Irwin M. Fargo) writes: >>>> >>>Pathway 2 is not only very improbable, but it's very impossible in humans. >>> > AP: >>Finally, since ALL FOUR gametic nucleii (the egg nucleus and 3 polar bodies) >>remain in the ovum until fertilization, something like what tim diescribed >>really is not outside of the realm of the possible. > LC: > That is only true of some organisms. In others, the polar bodies > >> Very occasionally, polar >>bodies do get fertilized. If the polar body that separated from the >>egg nucleus at meiosis II was fertilized, as well as the egg nucleus, >>the resulting fraternal twins would be exactly what tim suggested--that >>is, they would have one set (the maternal) of chromosomes that was identical. > > Not likely, unless an aberrantly large polar body had been formed. I > (expands on why it is not likely) >-- >| Lucius Chiaraviglio | ARPA: chiaravi@silver.bacs.indiana.edu Lucius, I think you are being a bit unfair in your reading of what I wrote. You point out, for example, that the possible scenario of a polar-body geting fertilized is "not likely." I believe the term I used in my posting was "not outside the realm of the possible." Does that really sound to you that I was suggesting it was likely? As for the development of the embryo, the fellow expressly requested we keep it to humans, not other animal's. What I described was essentially true for mice and probably humans (it is a bit hard to do the experiments on humans). The situation is very different for flies. Basically, as you say, meiosis II is delayed until fertilization. Actually, it is arrested at metaphase II. So, I oversimplified when I said that all four nucleii were present--all the material is there, they are arrested just prior to separation. In mammals, the polar-bodies are not extruded until after fertilization (hence the name). As you said, it is not very likely, but "it is not outside the realm of the possible" and "very occasionally" (god, how many qualifiers do you want?!) a polar body can get fertilized. Accurding to the information available to me, if this happens, it happens BEFORE extrusion. It is therefore possible (read: "not very likely" if you prefer) for two nucleii to get fertilized and it is not unheard of (read: even less likely) for both to develop in mice. The reason I brought this up was to contradict the "very impossible" phrase used by Ethan/Irwin. It is common for beginers in biology to think we really know what is possible and what is not. This is a habit that needs to be broken. -tony
eddy@boulder.Colorado.EDU (Sean Eddy) (04/22/89)
<3791@silver.bacs.indiana.edu>chiaravi@silver.UUCP (Lucius Chiaraviglio) writes: > Not likely, unless an aberrantly large polar body had been formed. I >have seen scanning electron micrographs of eggs with polar bodies attached, >and the polar bodies have < 1/64 of the volume of the egg. The smallest >fraction of an egg (formed by mitotic division of the zygote followed by >separation of the resulting cells) that I have heard of being able to form a >viable embryo in mammals is 1/16 (and that only in some mammals), so a polar >body of the size shown in these electron micrographs would die, possibly after >a few divisions. Your argument is not a valid one. Separating cells at two-cell, four-cell (or 16-cell) stages of development and trying to get the separated cells to develop into a viable embryo addresses the question of when crucial developmental information in the embryo gets segregated into different cells. Such an experiment does not address the question of how much egg cytoplasm is required to get a viable embryo. - Sean Eddy - Molecular/Cellular/Developmental Biology; U. of Colorado at Boulder - eddy@boulder.colorado.EDU !{hao,nbires}!boulder!eddy
fargo@pawl.rpi.edu (Irwin M. Fargo) (04/25/89)
In article <8272@boulder.Colorado.EDU> pell@boulder.Colorado.EDU (Anthony Pelletier) writes: >I think you are being a bit unfair in your reading of what I wrote. >You point out, for example, that the possible scenario of a polar-body >geting fertilized is "not likely." I believe >the term I used in my posting was "not outside the realm of the possible." >Does that really sound to you that I was suggesting it was likely? This is sci.bio, not sci.philosophy.meta. Please keep semanticism in the right newsgroup. Most USENET readers don't need every word explicity de- fined. >separation. In mammals, the polar-bodies are not extruded until after >fertilization (hence the name). As you said, it is not very likely, but >"it is not outside the realm of the possible" and "very occasionally" >(god, how many qualifiers do you want?!) a polar body can get fertilized. The way I learned it ("Biology" by Helena Curtis), after meiosis II, the egg is the only cell to survive and ever pass into the fallopian (sp?) tube. To my knowledge, the polar bodies die and are later ingested by white blood cells. Plus, I tend to agree that even if a polar body was fertilized, there would not be enough cytoplasmic material for the zygote to survive long enough for implantation on the uterin lining. >The reason I brought this up was to contradict the "very impossible" phrase >used by Ethan/Irwin. It is common for beginers in biology to think we really >know what is possible and what is not. This is a habit that needs to be broken. I don't know how you rank beginners, but I did take my college level biology course, and thus, must draw on that as my body of knowledge. And it is YOUR semantics in taking my phrase "very impossible" too literally, not my presump- tion of biological knowledge. Anyways, I don't think the basic ideas in bi- ology are going to change according to how much you know, just as 2 + 2 will still equal 4 even if I know how to do double integrals! Thank you and happy hunting! Actually: Ethan M. Young ____ [> SB <] "Travel IS life" Internet: fargo@pawl.rpi.edu /__ -=>??<=- - Irwin M. Fargo Bitnet (??): usergac0@rpitsmts.bitnet / ARGO : 3000 years of regression from the year 4990
eddy@boulder.Colorado.EDU (Sean Eddy) (04/25/89)
In article <1683@rpi.edu> fargo@pawl.rpi.edu (Irwin M. Fargo) writes: [ - misunderstandings of human reproduction deleted - ] > >I don't know how you rank beginners, but I did take my college level biology >course, and thus, must draw on that as my body of knowledge. And it is YOUR >semantics in taking my phrase "very impossible" too literally, not my presump- >tion of biological knowledge. Anyways, I don't think the basic ideas in bi- >ology are going to change according to how much you know, just as 2 + 2 will >still equal 4 even if I know how to do double integrals! (Please suggest an *alternative* definition of "very impossible"!) Look, you can feel free to believe that biology is ordered and built upon rigorous first principles like physics or math are. That kind of attitude makes learning (and teaching!) biology much more comfortable. However, there are no basic ideas in biology which are carved in stone. The ideas that exist have been established by observation. They can be invalidated at any moment. For instance, one of biology's "basic ideas" at one time was that the genetic material was protein. DNA (a boring repeating polymer of ATGC, another basic idea) clearly lacked the necessary complexity. In this case of your argument with Tony, you should know that animal reproduction has few rules, and we don't know all there is to know. Tony happens to be right, so far as I can tell -- I was about to rip in to him for saying there were three polar bodies, but then checked my facts and found a source that confirmed what he said. - Sean Eddy - Molecular/Cellular/Developmental Biology; U. of Colorado at Boulder - eddy@boulder.colorado.EDU !{hao,nbires}!boulder!eddy - - "That such pygmies should cast such giant shadows only shows how - late in the day it has become." - - biochemist Erwin Chargaff, of Watson & Crick