 |
 |
Good Syndrome
Name: Angela
Status: other
Grade: other
Location: CA
Country: USA
Date: N/A
Question:
How do cells limit gene expression?
Replies:
I am not completely sure what you intend by your
question, and it could refer to so very many aspects
of genetic controls that are in play with regard to
development as well as any other cellular activity
at any point in the development of a fertilized egg
to an adult of whatever species of plant, animal, or
microbe. I thought I would provide an insight on gene
expression provided years ago by one of the great
molecular biologists: Nobel Laureate and Professor
Sydney Brenner of Cambridge University, UK. Prof.
Brenner did his Nobel prize-winning work on unraveling
the genetic code using the bacterium E. coli, and
discovered how it was that particular triplets of
nucleotides in the DNA of an organism's genome are
translated into the individual amino acids of the
organism's proteins. Later, Prof. Brenner chose to
work on a small animal that eats the very microbe on
which he did his earlier work, the nematode worm C.
elegans. He wanted to understand what occurs in the
development of a fertilized egg on its way to becoming
an adult, and C. elegans having about 1000 cells as an
adult, easily grown in the lab, easily crossed between
individual worms having differing phenotypes, able to
be raised at different temperatures, and more made it
a great system of study for learning principles of
development.
Among many questions of development that were open
questions was how it was that the fertilized cell
gives rise to all of the differentiated cells ofthe
adult - at what point does the fertilized cell's
totipotency (I.e., having ability to form any cell type,
accessing any of its genes) become of narrower capability,
to omnipotency capable of giving rise to a subset of the
animal's cells by its daughter cells, to only giving rise
to a terminally differentiated cell whose daughter cells
will only be that one cell type. It was believed then as
it is now that these variant capabilities are determined
by the genes that are active and not active at differing
points in the organism's development. But what signaled
what genes are on or off, or as we now know, what form is
the gene or its messenger taking to express the category
of the gene itself - this really is complicated stuff.
Well, at a lecture in 1976 in Berkeley, Prof. Brenner
revealed one of those aphorisms I've not forgotten: What
genes are enlisted in development, indeed development itself,
includes two basic mechanisms by which the totipotent early
stage cells divide and provide generations of daughter cells
that form an adult. One mechanism was called the British plan
where the cell's development, the sets of genes in play, were
determined based on the parentage of each set of daughter cells.
Clearly, that is not sufficient because you'd never have the
differentiated cells arise, and of course they do. Basing the
next mechanism's descriptors on ablation studies where individual
cells in early stage worm embryos are specifically killed to see
how the remaining cells recover or not (an experimental approach
first shown in the early 20th C by Spemann and others), Prof.
Brenner described the American plan that is key to development
and gene expression. In the American plan, different sets of
genes are expressed, and development ensues, as a function of
what other cells or signals, chemicals that is, are in the
neighborhood.
There you have it: gene expression giving rise to all the
differentiated cells of an adult based on who was the earlier
cell's daddy and who and what were the neighbors!
Don Silvert
Angela,
Gene expression can be limited by miRNA (microRNA), also called
siRNA (small interfering RNA), which are complementary sequences
that bind to targeted mRNA which result in Gene Silencing.
-Alex Viray
Click here to return to the Molecular Biology Archives
| |
Update: June 2012
|
|
