Sarkiss writes:

>> Would it be fair to say then the cell, like a fractal, is self-similar
>> and independent of scale? 
>
>No, at least not in this planet. Cells might grow like self similar
>colonies but human body does not resemble cell and is not self
>similar.

That is of course the correct answer. Self-similarity is the critical feature
of fractal geometries and there is nothing greatly self-similar about a cell
and the complex multicellular organism that its cells create.



>I have another question regarding Genotype. Is it rigorously proven
>that all (e.g. human) cells have the same DNA? In other words, what
>are the experimental observations supporting that all biological
>tissues have the same DNA?
>
>I'd appreciate very much if someone can answer this question.

If by "rigorous," do you mean has anyone ever examined all 6 x
10^13 cells in
the human body to insure that they contain the same DNA, then the answer is
quite clearly "no." The task would would probably require several
billion years
to complete. But you aren't philosophically constrained to approach the problem
from that point of view either. Every cell in human body is a daughter cell, a
direct descendent of the original zygote, the first diploid cell that was
formed through the original union of a spermatozoon and an egg. There's simply
no where else for the DNA in those 6 x 10^13 cells to come from -- with the
rare exception of plasmid infections, a process that I will mention in a
moment.

Nonetheless, the genetic constitution of all of the cells in a human body is
not uniform. Indeed, there are cells in a human that do not contain any DNA at
all. Red blood cells in mammals -- but not in other vertebrates -- are
anucleate. Shortly after their development, the nucleus is squeezed out of the
cell in order to make more room for hemoglobin storage and transport. These
cells are  thus "dead-enders." They have no genetic future, even within the
lifetime of the individual they serve. They are are simply resorbed following a
short, tough life of a great deal of wear and tear.

Similarly, at least to a first degree of similarity, gametes (ova and
spermatozoa) contain only half of the chromosomes that the original zygote did.
In a normal somatic cell, two paired chromosomes are present for each
chromosome type, and thus the cells are said to be "diploid," but
in a gametic
cell, that number is halved by a complex cell-reproductive process,
"meiosis."
Under meiosis, the reduction in chromosomal count is generally randomly
accomplished (although not always; we now know of a number of instances where
the paternal genome is preferentially excluded during gametogenesis). Most
normally however, in a process termed "Mendelian segregation," either the
paternally or maternally derived chromosome of each chromosomal pair is
randomly chosen for inclusion into the a gamete. In humans, with their 23
separate chromosomes, this allows for 2^23 uniquely chromsomally complemented
"haploid" cells types, no two of which are necessarily alike.

However, the process is even more complicated than just this simple
segregation. During the early phases of meiosis, the chromosomes (technically,
now called "chromatids") quite frequently stick together, forming
chiasmata.
When these overlapping, sticky chromatids are pulled apart, as they will
eventually be by the forces governing cellular division, the chromatids often
tear into pieces and form new chromosomal arrangements. The consequence of this
process is that the resulting haploid gametes often contain chromosomal
constitutions that are unlike any other existing somatic cell in the
individual's body.

Recombination is most normally only a property of the highly explicit process
of "meiosis" (gametogenesis), but it does also occur at very low
frequencies
during mitosis as well. "Mitosis" is the process by which somatic
(non-germline) cells clonally reproduce. Ideally, each somatic daughter cell
would be a perfect genetic duplicate of its mother. But much worse, mitotic
reproductions are heir to all of the slings and arrows of Boltzmannian
thermodynamics, where a little bit of error creeps into each replication, just
as it does with an indefinitely xeroxed piece of text

Eventually, after approximately 60 to 90 clonal (mitotic) replications,
sufficient error will accumulate in the various lineages of somatic cell tissue
that almost all of the cells downstream will eventually either senesence (die)
or become neoplastic (cancerous). Indeed, this is the reason that we all
eventually die as individuals. At this terminal point in life, probably very
few of the 6 x 10^13 cells will contain precisely the same genetic code that
they inherited from the zygote. Most will have one to many novel mutations
infused into their code.

Otherwise, the only other major source of DNA differentiations between the
cells of an individual's body are infections. Plasmids and retroviruses allow
for "horizontal" gene transfer into the genomes of individual cells. If the
"infection" occurs in somatic tissue, the virally-infused DNA dies with the
host; it simply has no where to go. But many stretches of the human genome are
believed to consist of endogenous retroviruses, which are the result of ancient
retrovirus DNA that has became incorporated into the germline without causing
any adverse effects. 

Every individual multicellular organism begins life as a single cell, its DNA
constitution fixed as the result of a union of two gametes, but just as
assuredly, because all life exists in a positively entropic universe and swims
in a sea of thermodynamic noise, by the time the individual dies, probably no
two cells in its body contain precisely the same DNA.

Wirt Atmar
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