|MadSci Network: Molecular Biology|
To understand what cloning is and what led to it, I’ll have to give you a little bit of the background on embryology. As you might know, embryology studies the processes that develop a whole organism from a single cell: the egg. In other words, embryologist are interested in understanding how can a one single cell develop into multiple different cell types (differentiation) that constitute a healthy adult organism. Each cell type (differentiated cell) has its own set of proteins that are necessary for its function. For example, the red blood cells have hemoglobin, which other cells don’t. However, all the different cells in a single organism have the same nucleus, with the same genetic information. Thus, for the differentiated cells to have different proteins some genes in the nucleus are turned on or off, this is a process known in embryology as “restriction of nuclear potency”. As you can imagine, it is important to understand how can a single egg with one nucleus give rise to so many different cells that derive different gene expression patterns from the same original nucleus. Sometimes, the changes that a differentiated cell exerts on a nucleus can be difficult to reverse or change. A good way of thinking about it is in terms of a “specialized nucleus”. As the embryo develops into an adult organism, the nucleus of each cell becomes more and more specialized. Cloning is a process where the nucleus from one individual is transferred into another cell and generates an individual that is genetically identical to the nucleus donor. The initial cloning experiments were done by transferring nucleus from one embryo to an enucleated activated egg (nuclear transfer technique). The activated egg is obtained by pricking an egg with a clean glass needle, stimulating it to undergo all the changes that occur after fertilization, without actually providing it with a sperm. After its activation, the chromosomes can be removed from the egg, and replaced with the nucleus of the donor cell. But, for a successful cloning to occur, it is important to start with a donor nucleus that is not irreversibly specialized, or it wouldn’t be able to give rise to all the different cells that constitute an organism. Therefore, when a differentiated cell donates the nucleus, its “specialization” must be reversed. In other words, the specialized nucleus must be “reset” to its original naďve state. The factors important for “reseting”the nucleus in the recipient egg cytoplasm are still unknown. In the early cloning experiments, the problem of resetting a nucleus was avoided by using early embryos as donors, thus using nuclei that had not undergone extensive specialization. Recent advances now allow scientist to transfer nuclei from differentiated cells (more specialized nuclei). These advances include knowing which stage of the cell cycle at which the donor nucleus must be obtained. Also, using ovulated oocytes increased the frequency of successful clones. Both of these modifications increase the chances of the differentiated nucleus to be reset. In summary, the present cloning technology involved a series of discoveries that were culminated in the generation of Doll, the lamb, in 1998. My private opinion is that the breakthrough discovery that allowed cloning was done by Robert Briggs and Thomas King in the 1950s with the first nuclear transfer experiment, done in frogs. All the posterior advances are all built on the concept that nuclear transfer can generate clones. You can read more about cloning at: http://zygote.swarthmore.edu /gene4.html
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