Is it theoreticaly possible to change the genotype (and sort of 'reset' the phenotype) of a multicellular organism ? Perhaps not in all of the cells ? If yes, is it practically possible at the moment ? Or how far does the science have to advance until it is (just a general idea, not a timetable) ?
You can read up on Agrobacterium tumefaciens transformation of plants. Basically, this bacteria inserts its own DNA into the host cells and changes their genotype to make them produce a more suitable environment for the bacteria to grow. Scientists now use this technology to insert their favorite piece of DNA into all types of plants. The transformed cells (that now contain foreign DNA and a gene for resistance to an antibiotic) can be regenerated into whole plants. Alternatively, one can bombard the DNA into cells using a biolistic system and regenerate the transformed cells. The DNA in this case inserts into the genome on its own. As you can guess, the efficiency isn't that great, but you only need one cell to regenerate a whole plant.
yes it is possible to reset the phenotype without changing the phenotype. C. Elegans, the nematode worm, will go into a dauerlarvea stage when the conditions are poor. It changes its shape completely and can live in this form for much longer than it normally would (it postpones death, which is already quite weird if you start thinking about it). And when conditions are returned to normal again it makes a proper larva again. But of course I didn't answer your question, but I would just like to point out that reseting of the phenotype is already possible without altering the genotypel.
In the case of individual cells the genetic code itself can be changed by use of a virus. (I always understood that retroviruses would not kill the cells that they infected, but whether or not this is useful is unknown to me.) Theoretically a transform virus could infect every cell of a multicellular organism and change the desired genes in every case. One hundred percent conversion might be a little much to ask for in practical terms. However, when it comes to rebuilding the organism to adopt the phenotypic expression that the implanted alleles are intended to represent, things become pretty shaky. If you're talking about converting all of an organism's cells to produce a new protein (if you wanted to change a frog's cells to begin producing GFP, for instance) this might be possible. If you're talking about changing Martin Brundle into a fly guy or something like that, the structure of the organism becomes more important; mutating the genetic code of a person's tissues might enable them to grow some kind of new creature within themselves, but hoping that the new growth would take on the functions of the old organism might be a bit much. Giving yourself an extra pair of arms is probably more easily accomplished by surgery than by transform viruses.
I know Idle... I should have clarified that statement... A retrovirus is an RNA virus as opposed to a DNA virus. The difference between RNA and DNA is that the Thymine in DNA is replaced with Uracil in RNA, because Thymine (and therefore DNA in general) is too large to move through the nuclear membrane. When a DNA virus uses a cell's nuclear DNA to replicate itself, its DNA cannot escape the nuclear membrane. So, the nucleus fills up with viral DNA and eventually ruptures. In this manner any cell infected with a DNA virus will die. When a retrovirus (that is, an RNA virus) uses a cell to replicate itself, the replica RNA can pass freely about the cell in a manner that DNA cannot. Consequently, whereas a retrovirus may kill the cell, it is not a foregone conclusion of the replication process the way it is with DNA viruses. Hence, if you used a DNA virus to try to implant new code into your own genome, it wouldn't work because all of the cells infected with the virus would eventually rupture and die. A retrovirus might be able to effect a genetic transformation on a multicellular organism, however. That's what I meant; sorry I wasn't clear.
Correct. True, but chemically a thymine is just a uracil with a methyl group attached to it...not much of a difference in terms of overall size (both are pyrimidine bases remember, and in order for them to be interchangeable, they need to have almost identical chemistry). If this is the case, how does the viral DNA enter the nucleus in the first place? Okay, okay nevermind. I'm dodging the issue. What I meant by saying that HIV is a retrovirus, is that they certainly do kill the cells. They are very efficient at it. One cell infected with HIV can cause up to 50 cells to lyse at once...that is quite a destructive power. They do it by causing the infected cells to stick to other cells, in what are called syncytia (which is a large multinucleated cell in the end). These syncytia can be formed by up to 50 cells at a time.
I understand, I'm saying that it could be possible to develop a retrovirus that would propagate without killing the cell - if one were looking for a way to change the genetic code of every cell in an organism, for instance - whereas most viruses kill the cell just by reproducing themselves, without any other operation at all.
Both DNA and RNA viruses usually replicate within the nucleus. RNA viruses have to transcribed into DNA before entering the nucleus anyway, as the reverse transcriptase would have trouble getting in there. Some DNA viruses can replicate outside the nucleus but only if they encode polymerases, I'm not sure if any RNA viruses do. HIV link
HIV uses reverese transcriptase to make double-stranded DNA molecules, which can insert into the host organisms DNA. It can remain here, seen by the host's cell as a piece of its own DNA, for up to 10 years before taking over the cell. During this period, anytime the cell divides, the piece of HIV DNA is passed onto new cells. In this way it can quickly move through out a bunch of host cells. The HIV virus usually infects lymphocytes (usually helper T cells and such). This is one theory why there are so many people in Africa that are dying of AIDS. There are a lot of pathogens there, which means that genereally their immune systems have to work harder than ours, which means there are more lymphocytes being produced, and if one of them becomes infected with the HIV virus, it can be spread rapidly to any of that cell's progeny. This may be why the HIV virus which is found in Africa is more virulent than that found here.
