This reply is a whitewash. We do not know how genes control any step of
development, and the reverse may be true anyway. For nematodes, it is one
thing to observe the cell divisions and differentiations, another to
explain them. Sorry to be so blunt, Tom. The paradigm you give, though
widely believed, lacks sufficient explanatory power.
Best regards, -Dick Gordon, U.Manitoba[Jun23,95]
---------- Forwarded message ----------
Date: Fri, 16 Jun 95 13:41:29 GMT
From: Tom Thatcher <ttha at uhura.cc.rochester.edu>
To: molecular-evolution at net.bio.net
Subject: Re: Evolution? Can anyone really believe this?
In <Pine.SUN.3.91.950615220253.16209C-100000 at luna> "Vernon Lovic (REH)" <lovic at luna.cas.usf.edu> writes:
>What I am completely at a loss for, and I believe everyone in the
>scientific community is at a loss for is describing how the cell uses
>this extremely complex sequence of nucleotides to build an organism as
>complex as a human being--or even a plant! I must assume that the cell
>is able to decode this sequence (which only recently have scientists
>themselves with some of the most sensitive instruments available to man
>been able to accomplish in only simple organisms so far) and to
>specialize each cell into that which is required.
The basics of this process are well understood. Basically, every gene
is controlled by sequences which either turn it on or off in response
to various signals, such as position in the organism, gravity, concentration
of other proteins, extracellular chemical messages, etc. We do not know
every control signal of every gene in every cell in every organism,
but we know enough of the building blocks to be able to understand the
For example, when a fruit fly makes an egg, the egg is surrounded by
other cells (nurse cells) which fill the egg with its contents. One
of these is a specific message RNA that encodes a protein (which I
forget the name of). Anyway, this RNA is physically attached to one
end of the egg, which is sort of football shaped. When the RNA is
transcribed into protein, there is a natural concentration gradient of
the protein; highest at the end of the egg where the RNA is attached.
This protein is a transcription factor--a protein responsible for
turning on genes which have certain signals. This transcription factor
turns on a lot of genes which carry the instructions for making the
fly's head. At the end with the RNA attached, the head genes are turned
on. The other end of the embryo becomes the tail.
>I'm trying hard to make this clear, but I guess my primary argument is
>that SOMETHING is in control of the entire process of cell
>differentiation within the developing organism. To say that the DNA is
>the master code that determines an organisms characteristics is not
>telling us anything that reveals the process behind the decoding of the
>DNA resulting in a functioning organism! Is every cell within our body
>capable of decoding it's DNA and assigning itself to a position within
>the development of the organism depending on that code?
Essentially, yes. There is a group of researchers who study a tiny
worm C. elegans. By watching with microscopes, they have determined
exactly how one zygote developes into an adult worm. They have mapped
every cell division and know exactly which cells will form which structures
in the adult worm. They can go to a developing worm and zap a single
cell with a micro-laser; the adult worm grows up missing whatever
structure that cell is destined to give rise to. They are in the process
of identifying the various gene switches and signals that are responsible
for this process.
There are many more examples of control genes and sequences.
>If someone can explain to me how that happens I have yet more questions!
I think I answered most of this already. I want to correcxt a
misconception, though, it that cells do not move to their location,
as you seem to suggest, rather, location determines fate (mostly).
A newly divided cell in the embryo's tail doesn't think to itself,
"I'm a brain cell, better get a move on."
>No, I am not a creationist, I am just a disbeliever in evolution
>according to a lot of very basic ideas. Feel free to shed some light if
Don't know what this has to do with developmental biology. Maybe
you've got another agenda?
Tom Thatcher | You can give a PC to a Homo habilis,
University of Rochester Cancer Center | and he'll use it, but he'll use it
ttha at uhura.cc.rochester.edu | to crack nuts.