|MadSci Network: Medicine|
Hi Greg, Very interesting question, and one about a subject that's been in the news over the past couple of years. The very short answer is "yes." Now, for a more detailed answer. When we speak of "cloning" a mammal such as a human being, we are talking about extracting the DNA from the cell nucleus of an adult individual, then injecting into another cell, and growing that single cell up into another whole animal. That other cell is what is called an "embryonic stem cell," a kind of cell that you can think of as being just like a brand-new, fertilized egg. When you take the nucleus out of that cell and replace with the nucleus from the adult's cell, the new DNA gets read just as if it were there in the stem cell to begin with. As you may know, DNA is a long, string-like molecule made of 4 different chemical subunits, and the exact sequence of those subunits is translated by the cell into RNA and proteins that carry out the work of the cell - making copies of itself, making the biochemicals it needs for all its life functions, and so on. Between individuals, there are some small differences. Out of the 3 billion chemical subunits (nucleotides) in one copy of your DNA, about 99.9% of them are completely identical to everyone else's. That leaves about 3 million that are different between different individuals, on average. Some of the differences have absolutely no consequences, and some do. Some are easy to spot, for example, genes that control eye color or hair curliness and so on. Some differences can cause diseases, such as a single change in the gene for the oxygen-carrying protein hemoglobin that can cause it to polymerize under low oxygen conditions, resulting in sickle cell anemia. All right, clearly some traits are known to be regulated by genes, so shouldn't a clone be physically identical? The answer is that it almost is, just as identical twins are almost identical physically, but not exactly. Biologists have had a long running discussion going on that is called the "nature/nuture" debate. Are the characteristics of an organism due to its inherent nature (DNA sequence), or to the series of environments and experiences it encounters on its way through life? The answer, of course, is BOTH. Some examples: if you grow a clone of a certain kind of plant at low altitudes, it may reach a certain height. At a higher altitude, it may grow to a greater height, and at a still higher altitude, it may grow to a lower height, even though they are all clones. Or, identical twins have slightly different experiences. I have identical twin friends who I can tell apart now (although at first I couldn't). One plays the trumpet and one the saxophone. Although they are incredibly similar in many ways, they've been to different lessons and played in different sections of the band. One has developed a little callus on his lip from the trumpet mouthpiece. And then there's illness or other exposures to microorganisms that can affect how your body grows, how your immune system develops, and so on. And that doesn't count brain differences. We now understand better brain cells grow and develop, and how some make connections to other cells, and other ones don't, and so they die. Those physical connections are reflected in the thoughts and emotions of the individual. And then there is a question about how the DNA starts out in the first place. If you take DNA from an adult, it may have accumulated a little damage. Every cell in the clone will start out having that damage, which was not present in the fertilized egg that gave rise to the adult animal that the DNA came from in the first place. So the answer is that clones will usually be very close to identical looking, although they may be quite different looking depending on what they have encountered in their environment. And there will be many, many small differences that are much harder to see, but that will make them distinct in body and mind, no matter how similar they seem.
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