Epigenetics of Reproduction in Animals - Building the Most Complex Structure on Earth - page 106

all of its body parts. So these genes are used in every animal, but the choreography is
what differs” ( Carroll, 2006 ).
We know that it is not genes, but the different patterns of their expression, that
provide the living world with the enormous combinatorial potential embodied in the
astonishing diversity of living forms. But choreography is an art, and the choreogra-
pher instructs. If one expands this metaphor, the question would be: What tells cells
throughout the animal body when and where to express which genes?
This question is a formidable challenge to any biologist. The idea that a material
entity, within an animal itself, may control the expression of thousands of genes in
virtually an infinite number of cells of widely different types seems next to impos-
sible. This is a question about a big “unknown”; hence, it would be wise to break it
down into smaller pieces (or partial questions) by going stepwise from the known to
the unknown.
We know about the classical Jacob-Monod model and a few other types of genetic
feedback systems of regulation in unicellulars. We also know that a unicellular can
adaptively respond to internal and external stimuli via epigenetic mechanisms (see
the section “The Control System in Unicellulars” in Chapter 1) that regulate gene
expression and DNA replication and determine cell structure and functions.
Transition to multicellularity raises the intricate issue of coordination of functions
of billions to trillions of cells of different types, each type with a different function
and structure. Even if one would agree with the simplicist Virchowian concept of the
organism as a “republic of cells,” a coordination of their activity at a supracellular
and systemic level is indispensable to prevent the otherwise inescapable fall of the
system into chaos.
This supracellular and systemic control of gene expression is now tangible in
eumetazoans. Each cell in these organisms expresses a variable number of genes
that are necessary for the cell's subsistence and normal functioning; they are called
housekeeping genes . The rest (i.e., thousands of nonhousekeeping genes in cells
of multicellular organisms) are expressed for the sake of the organism, to meet its
actual requirements. They are fees that the cell pays for membership in the organis-
mic community of cells.
Obviously, no cell knows what the organism needs at any moment in time. Even
theoretically, cells in multicellulars need instructions on when , where , and how long
they have to express each gene. These instructions have to reach the cell from extra-
cellular sources in a genetically intelligible form via chemical signals; different sig-
nals must be sent to different cells at different times during their lives.
Theory aside, let us consider what we know about these nongenetic extracellular
signals that the organism sends to all its cells. Most extracellular signals, such as
hormones, growth factors, secreted proteins, and neurotransmitter neuromodulators,
use signal transduction pathways (see the section “Epigenetic Information and Signal
Cascades” in this chapter) to transmit their messages to genes to induce their expres-
sion. The first step to the activation of a specific signal transduction pathway is the
binding of a ligand (e.g., hormone and growth factor) to its specific cell membrane
receptor. The binding of the ligand to its receptor activates a particular signal trans-
duction pathway by inducing a chain of phosphorylations, enabling the expression of