MadSci Network: Medicine

Re: is there a physical differnce between the human and the clone?

Date: Thu Feb 14 17:15:31 2002
Posted By: Paul Odgren, Assistant Professor, Dept. of Cell Biology
Area of science: Medicine
ID: 1013445241.Me

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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