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Development of multicellular organism

  1. Sep 24, 2003 #1

    Monique

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    I have always wondered how an multicellular organism develops into such a complex, but highly reproducible being.. now, I’ve been reading ‘Development of Multicellular Organisms’from the book: Molecular Biology of the Cell by Alberts et al. and it all seems so simple! (the basic ideas at least :D)

    The nematode worm C. elegans, for instance:

    The hermaphrodite exists of 1031 somatic cells and ~1000 germline cells, the male of 959 somatic cells and ~2000 germline cells.

    The entry point of the sperm will define the future posterior pole of the worm. Asymmetric divisions organize the cellular molecules in such a way, that at the 16-cell stage, the molecules needed for the germline cells are all located in a single cell (and remember that there is no transcription during these early stages of nematode development). Cross talk between cells creates new cell types, for instance: two cells types have developed next to eachother. One excretes “A” and the other excretes “B” the cells in the middle will be exposed to “AB” and thus start excreting “C” cell directly next to “C” now get an “AC” or an “CB” signal, and will again produce a different cell types like extraembryonic tissue, dorsal epidermis, neurogenic ectoderm, mesoderm.

    The HOX complex is interesting too, homeotic selector genes. It is a complex of genes on some chromosomes, and the genes are expressed sequentially according to their order in the complex. There are about 10 genes in one complex, the upstream genes code for the anterior of the animal, and the downstream genes for the posterior, nice graded in between.

    And then the mechanosensory bristle of the fruitfly (Drosophila), it is a complex structure of a neuron, covered with a sheet cells, in a shaft cell, held in place by a socket cell, where the mechanosensory bristle is an extension of the shaft cell. It turns out that these four cell types all originate from a single sensory mother cell.

    The division of that cell is assymetric, thus giving one cell the advantage over the other, that one will become the neuron. The neuron destined cell will inhibit the other cell to become neuron by lateral inhibition. Asymmetric cell division continues and the other cell types are born. So lateral inhibition forces cells to act in opposite ways.

    So basically a simple repetitave process that has different effects under different conditions creates all this complexity.
     
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  3. Sep 27, 2003 #2
    Pretty outragious, Monique! I have often wondered if, at times, a single celled orgaism would wiggle its way into the organization of a group of these differenciating cells and provide a unique feature that is later incorporated, either genetically, morphologically or chemically, into the species and its offspring.

    We see an example somewhat like this when we look at the mitochondria carrying its own genome in our own cells and other animal cells. It just looks like a smaller (organelle sized) single celled organism that has worked its way into the metabolic and reproductive phazes of the animal cell.

    Somehow, the whole chemical, genetic and morphological structure of these single celled, intruding organims is adopted into the genetic sequencing of its host. The presence of what once was an intruding organism becomes a part of the host's genetic expression during mitosis. Absolutely fascinating!
     
  4. Sep 28, 2003 #3
    Nice write up BTW, but curiosity, is any of what you have revealled got anything to do with 'introns' and 'exons' (SP?) and the difference(s) between the two?
    Are they perhaps active, in a nacent state, and later some become dormant as they are no longer required for the purpose of differentiation?
     
  5. Sep 28, 2003 #4

    Monique

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    Well, exons are DNA regions that code of proteins, introns are the piece of non-coding DNA that are inbetween exons. Higher eucaryotes will have more of the intervening sequences, after translation these non-coding regions will be spliced out leaving only a string of exons that will be transcribed to a protein.

    The importance of these seemingly unnecessary intervening sequences (introns) is that they mediate the differential translation of the same gene! One gene will code for many proteins since sometimes a piece of the exon is taken out - alternative splicing.

    So basically introns and exons are part of a single gene and don't really have much to do with the differentiation of a single organism.

    Maybe you were thinking about a promotor of a gene? That is a non-coding region upstream of a gene, which can be activated by proteins which will lead to the expression/inhibition of transcription.
     
  6. Sep 28, 2003 #5
    No, actually, I was just asking a question.
     
  7. Sep 30, 2003 #6
    Just to continue to attempt to "pick your brains" on this subject, are the introns/exons involved, in a sense of, pleiotropic effects?

    Pleiotropic (link For definition, for readers who might not know that word)
     
  8. Sep 30, 2003 #7

    Monique

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    A very sharp and interesting question, first, here is a link to a publication which discusses 'Seven types of pleiotropy' http://www.ijdb.ehu.es/fulltext.9803/ft501.pdf

    I don't really know all the details of pleiotropy, but I am very sure the alternative splicing of the introns out of the exons plays a major role in the phenomena.

    Humans 'only' have about 30.000 genes, while the plant genome can be 5 times as large (I am having trouble with finding the estimated number of genes on the NCBI website.. if someone can help me out). This means that the organization of the human genome is much more efficient, and it is well known that a single gene has the ability to code for multiple proteins. Multiple proteins means multiple functions, thus pleiotropy.
     
  9. Sep 30, 2003 #8
    Looks neat, Thanks, will read it all if/when I can find the time, and chance.
     
  10. Sep 30, 2003 #9

    iansmith

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    Arabidopsis thaliana has 35 00 genes but its genome is quite small for a plant (5 chromosome for 125-megabase genome)
    http://arabidopsis.org/info/agilinks.jsp
    Oryza sativa L. ssp. indica (Rice) has 46,022 to 55,615 genes for 466 megabases and 12 chromosomes.
     
  11. Sep 30, 2003 #10

    Monique

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    NCBI used to give a tally with updates on these things, where did it go?
     
  12. Sep 30, 2003 #11

    selfAdjoint

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    About the introns and exons, I just saw a news story (sorry, I don't have the link) about the sequencing of the dog genome. And it turns out that dogs have introns and exons that closely match human ones. So these noncoding regions have been preserved across a long span of mammalian evolution, whereas if they were truly nonproductive, they would have been affected by random variation. So they may look like junk, but apparently they are not junk.
     
  13. Oct 1, 2003 #12

    Monique

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    hmm.. I'd have to see a publication which actually shows the similarity of the orthologous genes in humans and dogs. I'd doubt that the introns would be completely homologous.

    An important thing to keep in mind: how big is the genome, which fraction is coding, and which fraction of that is intervening sequence? I think about 2% of the human genome is coding, so let's say that 1% is made up of introns..

    So the chance a mutation would affect an intron is not that great.. so is the sequence of an intron conserved because it serves a purpose, or another mechanism?
     
  14. Oct 1, 2003 #13

    selfAdjoint

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    The paper is in Science. Here is a description (9/25/03) from Venter's Institute of Genomic Research.
     
  15. Oct 1, 2003 #14

    Njorl

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    It's a bit of a topic shift but I'll ask anyway.

    I seem to recall that in Earth's history, simple life evolved relatively quickly, but multicelled life took a relatively long time to evolve. First, am I recalling correctly? Second, was the capacity to make multi-use cells from a single parent cell the breakthrough that allowed complex life?

    If so, is this suprising to anyone else? To me, it seems like a bigger leap from no life to single celled life than from single celled life to multifunction cells. One guess I have is that the capacity could have occurred but the multipurpose cells were hindrances rather than benefits until there were lucky mutations. I suppose the original life had no competitors, so any self-replicating thing was "a winner" evolutionary speaking.

    Njorl
     
  16. Oct 1, 2003 #15

    Monique

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    Well, first: exactly where does the information come from the simple life evolved quickly and multicellular life took much longer? There are absolutely no records preserved to indicate these things.

    Second, I guess it is the fact that for a multicellular organism to evolve a cell has to change its tactics.

    For example: a very simple eucaryote, the yeast, is a single-celled organism. When the DNA is the yeast gets damaged, the cell will keep on dividing. Growth is more important than DNA integrity.

    For a multicellular organism to survive, a cell needs to sacrifice itself for the benifit of the other cells. If DNA damage occurs, programmed cell-death will have to be initiated otherwise the organism would die.

    Such a mechanism has to evolve and that will take time. Then there is the need of cell-signalling, intensive communication between cell to decide what to do, how to differentiate.

    A single-celled organism is self-governing, a multicellular organism will eventually have to specialize different cell types.

    Ofcourse there is a transition state, the sponge is a very good example, which is an aggregate of single cell, but I wouldn't say a multicellular organism..
     
  17. Oct 1, 2003 #16

    Njorl

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    Now that I've had some time to think about it, I'm pretty sure I'm remembering stuff from the booklet that came with "SIM-Earth" . Not the most reliable source I'm sure.

    Njorl
     
  18. Oct 1, 2003 #17

    Monique

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    ha, funny :P

    Yes, just think about it.. what traces did the first single cells leave behind? None. How about the first multicellular organisms? None either.. The only thing one can do is speculate how much time different processes would require to evolve.

    A definition of a single cell can be very simple, nucleic acids encapsulated by a lipid layer.

    The definition of a multicellular organism requires cell signaling, otherwise there is just a aggregate of cells, and is thus immediately immensely more complex.
     
  19. Oct 1, 2003 #18
    ingestion, infection, symbiosis

    read "MICROCOSMOS" by Lynn Margullis and Dorion Sagan. All spelled out.
     
  20. Oct 1, 2003 #19

    Monique

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    Could you elaborate on what is discussed in the book?
     
  21. Oct 1, 2003 #20
    Lynn is Dorion's mom. They wrote the theory of Symbiosis, and MICROCOSMOS takes it from precambrian to modern development. An excellent read....very interesting.
     
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