> Some of the simpler animals have a few cells, and they "branch" or divide
> to reproduce. So, do their cells, like people's, have the same DNA but
> are differentiated? An illustration:
> A A
> A A
> If A has a given function, and B has a given function, but they both have
> the same DNA, then my hypothesis is that they could not have evolved. The
> common text-book idea of the origin of multicellularity is that various
> cells developed inter-dependence. Then, they somehow got together. But
> if A & B have the same DNA but a different environment, making a different
> expression of the DNA, then their whole scenario brakes down, I believe.
> Could someone tell me if indeed A & B have the same DNA in some of the
> simple multi-cellular creatures?
> David Harrington
Hi. I haven't been following this thread, but think I can provide some
answers to the questions in this letter.
As this is my first posting, let me introduce
myself. I am a postdoctoral fellow in Biochemistry at Rice University.
My expertise is in protein biophysical chemistry, but I believe I have
been exposed to enough molecular biology to address this question. I was
fortunate to grow up in an environment where both science and religion
were assumed to be complimentary. This was also the atmosphere at Baylor
University, where I completed my Bachelor's degree. I began grappling
with how to explain their complimentarity when I met lots of
non-believers in grad school at the University of Iowa, and then when I
moved back to Texas and encountered several Christians who have trouble
accepting science. Personally, I don't need the smallest details of how
the two can be reconciled, but since so many people are turned against
one discipline or the other, I think I ought to make an effort to
understand the arguments of both sides and try to develop answers for
both. Thus I read the contributions to this list with great interest.
I am not clear exactly what you are asking, but here is my
guess. I think you may have mixed up 2 issues: reproduction
versus differentiation. One question for you - which organisms do you
have in mind?
I would respond that the organism evolved
when whatever change(s) occurred that allowed more than one cell to function
as a unit. This is a different situation from that of asexual
reproduction, which produces clones of the same organism.
Re-stating: Evolution might occur
when the DNA of an asexual reproducer changes so that the two cells do not
separate but instead stay together and function as a unit. These cells
may or may not be differentiated ( i.e. have different functions). The
evolutionary step allowing differential environmental response might come
at a later step.
Some species, like slime mold, are able to switch back and forth between
acting as individuals and as a unit. I don't know if this might considered an
evoluationary intermediate or just a unique brach of the "tree".
I can't tell from your question whether or not you know that
human cells reproduce within an individual by simple division. As you
state, environmental differences cause differences in DNA expression
patterns and thus different function. In fact, the fundamental
environmental difference experienced in sexual reproduction is which half
of the egg the cell came from!
Department of Biochemistry and Cell Biology
Houston, TX 77251