DNA (fwd)

[Brown, R Ken]

Mathew James Gering wrote:

>Monozygotic [maternal] twins do have the same DNA. Hence they
>are identical. Genetically identical that is, there are non-genetic
>variations. Dizygotic [fraternal] twins do not. Fraternal twins can 
>be visually nearly identical, but they are not genetically identical.

That's right. "Monozygotic". I'd forgotten the word.

>Clones on the otherhand and have identical nuclear DNA, but
>are notnecessarily identical because some early development
>processes rely on maternal genome material and not nuclear
>DNA, and depending on the cloning process they may have
>different mitochondrial DNA (mtDNA).

Yep - if you mean Dolly-style clones derived from nuclear transplants
into eggs. there was an interesting article in this week's "New
Scientist" magazine
(http://www.newscientist.com/ns/981010/contents.html) about the changing
popular meaning of the word "clone". It was originally used for a group
of plants produced vegetatively, like by cuttings or runners. Then it
was extended to other organisms that can reproduce by division, like
so-called protozoa  Then to DNA replicated from one ancestor. And now to
artificially cloned things like Dolly. 

If you got a Dolly-style clone made from you he or she wouldn't be your
child, but your sibling and wouldn't be identical to you for all the
reasons Matt pointed out. And of course because of different
education/experience/upbringing et.c  nto to mention the knowledge that
they had been got in such a different way, which must do *something* to
the psyche.  (Anyone read "Cyteen" by CJ Cherryh?)

Identical twins of course are a clone in the original sense, just as
much as a variety of apples or a colony of bacteria are a clone.  Really
genetically identical, right down to the maternal contribution, because
they drive from division at an early embryo stage (Sea-urchins can do it
as well but insects can't...)  If it were possible to re-potentiate
adult cells and transform them into embryos (it works in plants - but
then it has to because they can't move cells from one organ to another
so any shoot tip has to be able to produce any kind of plant tissue)
then we could make "real" twin-like clones of humans.  If the search for
the mysterious foetal stem cells pays off that might be possible (though
why anyone would want to clone a human foetus is beyond me. Assume all
the crude jokes about it being more fun to make another one using the
traditional method).


Ken Brown