In order to try to prevent mitochondrial DNA defects which affect 1 in 5,000 people and are usually mild, researchers in Britain hope to create human embryos that have the nuclear DNA from their parents but healthy mitochondrial DNA from an egg donor.
Times Online describes the technique: "First, an egg from a woman carrying mitochondrial defects will be fertilised in vitro using her partner's sperm. At the point of fertilisation, two "pronuclei" containing genetic material from the mother and father will be removed, and injected into an unfertilised egg from which the nucleus has been removed.
This donated egg will contain healthy mitochondria, but none of the nuclear DNA that makes up most of the human genetic code."
Doesn't that sound eerily familiar to somatic cell nuclear transfer, aka cloning. Instead of a somatic cell being taken from an adult patient, two pro-nuclei are taken from what sounds like a human zygote.
The Times Online article says it's not cloning because "the embryo that develops as a result will have a full complement of genes in its nucleus from a mother and a father. Clones created by nuclear transfer are genetically identical to the adult from which they are cloned, with the exception of their mitochondrial DNA."
I'm not buying this since cloning is cloning regardless of whether a human embryo is being cloned from a human adult or from a human embryo.
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The Times Online article says it's not cloning because "the embryo that develops as a result will have a full complement of genes in its nucleus from a mother and a father. Clones created by nuclear transfer are genetically identical to the adult from which they are cloned, with the exception of their mitochondrial DNA."
ReplyDeleteI'm not buying this since cloning is cloning regardless of whether a human embryo is being cloned from a human adult or from a human embryo.
The Times's explanation is correct. A "clone" is an exact genetic duplicate of a given organism. Clones can be created in various ways (planting a cutting from a plant to create a new plant is a form of cloning, since the new plant has the same DNA as the old one; multiple identical cultures of cells in laboratory dishes are also called "clones"), but being a "clone" has nothing to do with how the clone is created, it has to do with the genetic relationship between one organism and another. The best-known clones are identical twins, but the procedure for creating them is simple random accident.
In this case, the most significant technique involved in creating these genetic hybrids is nuclear transfer, essentially identically to that in the "somatic-cell nuclear transfer" used in the cloning that makes headlines, but the fact that they are made in a way similar to that of clones does not make them clones. The nuclear genetic makeup of these embryos comes from the IVF fertilization of an egg cell by a sperm, with mixing of the DNA of the mother and father in the standard way. The resulting organism is not genetically identical to any organism on the planet - it is not a "clone" of anything, which is to say it is not a clone at all.
It does have mitochondrial DNA from an egg donor which does not match the mitochondrial DNA of its "mother" - that's the whole point of the procedure - and it is created in a way similar to that of clones, but neither of those has anything to do with it being a clone of either one its mother or its father. The fact that it has DNA from both means it cannot be a clone of either.
In saying "cloned from . . . a human embryo", you seem to be suggesting that the resulting embryo is a clone because its nucleus has been transferred from one fertilized egg cell to another. But that means that it would be a "clone" of . . . itself. And, further, since the "donor" consisted only of one fertilized egg cell, and the "clone" is one egg cell with the nucleus of the "donor" inserted, the "clone" not only has DNA with the same sequence as the "donor" - it has the same DNA, exactly the same molecules themselves, now residing in a new location while there is nothing left in the old location.
When you walk from one room to another, do you become a "clone" of yourself? If not, how does a nucleus, removed from one egg and inserted into another, become a clone of itself?
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ReplyDeleteHi Kevin,
ReplyDeleteThe Times explanation seems to imply that clones can only be created from adults which is wholly incorrect. Organisms of any stage of development can be cloned. My main point against the article was the seeming implications that only adults could be cloned.
The article seems to indicate that the genetic material comes from an embryo or zygote. I may be incorrect but that how the article reads to me.
If a scientist takes cells from an embryo and then inserts them into a nucleus-free egg and zaps them - isn't the resultant embryo a clone?
Just because the embryos whose cells were removed no longer exists/is dead in no way determines whether the created entity is a clone or not.
Taking a cell from one organism and placing it in a nucleus free egg is much different than a mere change in location.
Do you think the survival rate for these embryos will be high? If the success rate is low, wouldn that indicate that maybe something more than a mere change in location is taking place?
I guess its a question of whether you believe that an organism is a bodily entity or just their DNA.
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