Biology Reference
In-Depth Information
Now, almost by consensus, biologists believe that it is the spatiotemporal patterns
of gene expression, rather than the number or nature of genes, that determine differ-
ences among species and higher taxa. Differential expression of genes is also respon-
sible for differentiation of distinct types of cells that have the same genes. Biologists
need to know why differentiated cells of various types express different genes and
have different temporal and spatial patterns of gene expression.
Expression of nonhousekeeping genes, which the cell expresses for the organ-
ism's benefit, rather than its own, is induced by signals of extracellular origin. So,
where is the ultimate source of signals or instructions for their expression?
The most plausible answer from the genecentric view, that the instructions come
from the cell's genome, is rejected simply because these instructions come from
outside rather than from within the cell. The remaining alternative answer from that
viewpoint is that some cells have information or send instructions to other cells on
what genes they have to express for the benefit of the organism. But there are a num-
ber of problems that render this hypothesis implausible and/or nonverifiable.
First, the hypothetical cell(s) that would know what other cells must do are not
known as of yet.
Second, the hypothetical cells need to know the cells that must express which
genes.
Third, the hypothetical cells must know what chemical to secrete to induce
expression of relevant genes in other cells.
Fourth, given the inducer is a nonhousekeeping gene, the instructing cell's deci-
sion to secrete the inducer has to be implemented by secretion of another specific
inducer by a third type of cell. Obviously, cells of this third type will need another
type of cell to secrete the relevant inducer. Thus, the second alternative genecentric
hypothesis leads us to a logical dead end of endless tautological rounds of reasoning.
Each of the above arguments rejects the alternative gene-centered explanation of the
mechanism of the expression of nonhousekeeping genes if the three seemingly insur-
mountable difficulties did not exist.
There are two ways to approach the problem of the origin of instructions for the
expression of nonhousekeeping genes in metazoans. First, starting from the reliable
premise that nonhousekeeping genes are induced by extracellular signals, it is impor-
tant to trace the signal cascade back upstream to the ultimate source of information
for expression of nonhousekeeping genes and visualize the flow of information for
their expression.
The majority of the known inducers consist of hormones, growth factors, secreted
proteins, neurotransmitters, neuropeptides/neurohormones, and so on. It has been
known for at least more than half a century that secretion of hormones of the tar-
get endocrine glands (i.e., thyroid, adrenals, pineal, pancreas, and gonads) is stimu-
lated by pituitary hormones and that for this reason they are known as “stimulating
hormones.” Each of these pituitary hormones is induced by a specific neurohormone
secreted by the hypothalamus, one of the evolutionarily earliest structures of the ver-
tebrate brain. In turn, hypothalamic nuclei seem to secrete their neurohormones by
processing information from other brain areas. The secretion of growth factors and
Search WWH ::




Custom Search