Messenger Instability
The information for the sequence of amino acids in
a protein is carried from the DNA to ribosomes in the messenger RNA. Once the
necessary proteins have been synthesized from a messenger, it is necessary that
the translation of the messenger cease. In principle, exponentially growing
bacterial cells can eliminate unneeded mRNA merely by dilution due to growth
of the cells. Cells that have ceased overall growth, such as eukaryotic cells
in a fully grown organism, cannot use this approach.
Figure
7.17 Random decay of messenger from a
pool characterized by a rateconstant K
gives rise to an exponential decrease in the amount of messenger in the pool if
it is not replenished by new synthesis. Left: Kinetics if plotted on
rectilinear coordinates. Right: Kinetics if plotted on semilogarithmic
coordinates.
Complicated mechanisms can be imagined for
destruction of mes-senger once it has been used a fixed number of times. Cells
appear not to use strict bookkeeping on translation, however. Once a messenger
RNA has been synthesized, it has a fixed probability per unit time of being
degraded by nucleases (Fig. 7.17). This is a random decay process. The
population of such molecules then can be characterized as having a half-life
and will show an exponential decay in levels if synthesis stops. Some molecules
in the population will survive for long times while others will be degraded
soon after their synthesis.
Cells that must adapt to a changing environment
must vary their enzyme synthesis and consequently possess many messengers with
relatively short lifetimes. Most bacterial messengers have half-lives of about
two minutes, although some messengers have half-lives of over ten minutes. Some
messengers in eukaryotic cells have half-lives of several hours and other
eukaryotic messengers have half-lives of several weeks or even longer in stored
forms.
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