pbs Digest, Vol 18, Issue 7

ConroeJoe@aol.com ConroeJoe@aol.com
Tue, 06 Jul 2004 17:01:55 PDT
In a message dated 7/6/2004 7:33:09 AM Central Daylight Time, 
pbs-request@lists.ibiblio.org writes:

> >In the scientific sense a clone is two or more individuals that are
> >genetically identical.


I cannot say for sure what clone means for all disciplines, or what the 
original word meant.  But, I can quite confidently state that in biology, 
especially in genetics, "clone" is meant to refer to 2 or more genetically identical 

The actual reason they are genetically identical is because the "cloning" 
process is actual asexual reproduction.  Thus, by definition (mitosis) the 2 or 
more progeny are genetically identical.  

However, it is clear that some mutations occur during mitosis and organisms 
propagated through "division" or other methods of "cloning" can sometimes 
accumulate significant mutations.  As near as I can tell, such mutations are very 
rare (less than 1 in a million or 1 in a billion cell divisions, on average) 
for most organisms.

Thus, while sharp-eyed people can find sports (mutations), and some mutations 
are propagated accidentally, most descendants of clonal propagation will, on 
average, be genetically identical to the progenitor--no matter if it existed 
200 or 500 years ago.  

Humans play the role of "natural selection" in this process--and can choose 
to select apples or bulbs, or whatever, that have pleasing qualities.  Thus, 
rare DNA replication mistakes can lead to variation.  Nonetheless, by 
definition, the modern genetic definition of cloning refers to individuals that are 
genetically identical, but with enough "fudge factor" to allow for minor 
variations each generation.  

Thus, it is clear that cloning leads to production of genetically identical 
individuals, but also that, sometimes through DNA replication errors, non 
clones are produced.  These "nonclones" may seem identical but may vary in tiny DNA 
errors or they may have major errors.  

Another wrinkle to the definition of cloning is that, sometimes, DNA 
sequences may be faithfully replicated but slight chemical modifications of the DNA 
(e.g., methylation) may happen (or fail to happen).  In this scenario the DNA is 
the same, and the progeny are genetically identical, but some genes may be 
silenced (or activated) relative to other genes.  Such condition may persist or 
may be lost in time (e.g., paternal vs. maternal inheritance) and the original 
"clone" may reappear visually (in terms of what you and I perceive).  

LINK:  one genetical definition of clone

LINK:  another definition of clone

LINK:  DNA replication, fidelity errors


LINK:   DNA clone, or "molecular clone"


LINK:  Epigenetic inheritance


LINK:  Organellar inheritance

LINK:  Maternal inheritance


Conroe, TX; where the Crinum are in full flower and leaf, and the mosquitos 
are as big as crows

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