Biology Reference
In-Depth Information
Fig. 15.2 A diagrammatic
representation of the “triadic
control principle” (TCP), also
called “triadic control
hypothesis” (TCH), or the
“triadic control mechanism”
(TCM). Please note that
Processes 1, 2, and 3 are
internal to the system
(whence the notion of triadic
control) and X is the effector
unit . Process 4 is
environmental input,
and Process 5 is the
environmental message-
triggered function (or output)
of the system immediately
caused or effectuated by X
Environmental Stimuli
(4)
Control Unit
( 3 )
X
Production
Degradation
( 2 )
( 1 )
System Boundary
(5)
Function
that the same observation may be the result of a decreased rate of transcript
degradation in the anterior region of the embryo relative to the posterior region
without any gradient in the transcription rate of the associated gene between these
two regions. Adopting the first interpretation and inferring the associated gene to be
responsible for the observed mRNA gradient thus can lead to a false-positive (or
Type I) error if the alternative interpretation turns out to be true.
The structural elements forming the anterior-posterior axis in Drosophila are
already put in place during egg formation (or oogenesis) before fertilization.
Developing oocyte is polarized by mRNA molecules differentially bound to cyto-
skeletal elements (if the local mRNA concentration is determined predominantly by
transcription and not by transcript degradation; see the Triadic Control Principle
explained in Fig. 15.2 ). The genes coding for these mRNA molecules are called
maternal effect genes . The differentially bound mRNA molecules get translated
upon fertilization to form concentration gradients of the resulting proteins across
the egg cytoplasm. Bicoid and hunchback are two maternal effect genes playing the
most important roles in patterning anterior parts (head and thorax) of the Drosoph-
ila embryo, and Nanos and Caudal are maternal effect genes that are important in
determining posterior abdominal segments of the embryo. Maternally synthesized
bicoid mRNA molecules preferentially accumulate in the anterior end of develop-
ing Drosophila eggs, and Nanos mRNA accumulate at the posterior end of the eggs,
resulting in oppositely directed gradients of bicoid and nanos mRNA molecules.
When these mRNA molecules are translated into proteins, Bicoid and Nanos
gradient are formed along the anterior-posterior axis. Hunchback and caudal
mRNA molecules are evenly distributed throughout the interior of egg cells, but
their protein products are distributed unevenly because (1) Bicoid protein inhibits
the translation of hunchback mRNA and Nanos protein inhibits the translation of
 
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