| MadSci Network: Cell Biology |
Hi Ashwin Unicellular organisms carry out all the functions of life. Unicellular species are those whose members consist of a single cell throughout their life cycle. This qualification is significant since most multicellular organisms consist of a single cell at the beginning of their life cycles. Usually, unicellular organisms contain only a single copy of their genome (except when undergoing cell division), although some organisms have multiple cell nuclei. The first organisms that existed were undoubtedly unicellular. How organisms then became multicellular is a big step in evolutionary terms and is under much debate. Ashwin, Try reding this section, it might help solving your dilemma : The evolution of multicellular organisms could have happened in three main ways: 1. Symbiotic Theory: This theory suggests that the first multicellular organisms occurred from symbiosis or cooperation of different species of protozoa, each having different tasks. Eventually these many symbiotic unicellular organisms formed a single multicellular organism, each organism becoming a differentiated cell. The problem of this theory is that it is still not known how each organisms DNA would be incorparated into one genome, therefore making replication of whole organism very difficult. 2. Cellularisation (Syncytial) Theory: This theory states that a protist could have developed internal membrane partitions around each of its nuclei. Many protists such as the ciliates or amoeba can have several nuclei and so this could be a valid theory. However, this has never been known to occur in any organism and so is generally disbelieved to have happened in the past either. However, a multicellular organism would be called multicellular and not a single celled protozoa that has undergone compartmentalisation, making this route hard to prove anyway due to our current definitions of organisms. 3. The Colonial Theory The third, final, and most convincing explanation of multicellularisation is the Colonial Theory which was proposed by Haeckel in 1874. This posits the symbiosis of many organisms of the same species (unlike symbiosis theory, which suggests the symbiosis of different species). This is most likely as it has been seen to occur independently many times (in 16 different protoctistan phyla). For instance, Dictyostelium is an amoebae which groups together during times of food shortage, forming a colony that moves as one to a new location. some of these amoeba then become slightly differentiated from each other. Other examples of colonial organisation in protozoa are Eudorina and volvox (the latter of which consisting of around 10,000 cells, only around 25-35 of which reproduce - 8 asexually and around 15-25 sexually). It can often be hard to tell however what is a colonial protist and what is a multicellular organism in its own right. Most scientists therefore believe that is by the Colonial theory that Multicellular organisms evolved. So have you got the DIFFRENCES IN MULTICELLULAR ORGANISMS AND UNICELLULAR ONES ??? Hope you got your answer. Mail me back with your iota of views. Regards Tarun Gupta B.Sc (Hons) Bioinformatics GGDSD College of Post Graduation (Panjab University) Sector-32/C Chandigarh-India Email: Hotbacteria@rediffmail.com Page: http://www.geocit ies.com/hotbacteria/aboutme.html
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