Biology Reference
In-Depth Information
Protein
mRNA
FIGURE 11-5.
Flow of genetic information in a cell. Information encoded in the DNA is copied into mRNA in
the nucleus. The mRNA then leaves the nucleus and goes to the cytoplasm, where it binds to
a ribosome. The information in the mRNA is used in the synthesis of a protein by the ribosome,
and the protein can then perform its intended function.
Proteins may be altered, post-translationally, through the addition of
chemical groups, such as sugars or lipids, or through the removal of
part of the protein by cleavage with a protein-cutting enzyme. Proteins
may be activated or inactivated by phosphorylation, the process of
attaching phosphate groups. They may also be permanently inactivated
by complete degradation. Many of these processes play a role in the
circadian clock mechanisms.
D. Identification of Circadian Genes
Adult Drosophila (fruit flies) undergo eclosion (the process of emerging
from their pupal cases) in the morning, when the high relative humidity
allows their wings to inflate slowly and properly. In the 1960s, the
timing of the process of eclosion in Drosophila was shown to vary with
the genetic constitution of the flies. Under conditions of constant
darkness (DD), different strains of flies gave different period lengths.
The clear implication of this observation was that specific genes were
responsible for the expression of this whole-organism behavior. In 1971,
Konopka and Benzer reported the identification of long, short, and
arrhythmic mutants of the period gene referred to, respectively, as per
L
,
per
S
, and per
0
(see Konopka and Benzer [1971]; Panda et al. [2002]).
Subsequently, cloning of the period gene permitted controlled studies
that demonstrated that using genetic engineering to insert a wild-type
per gene into per
0
flies restored circadian rhythmicity. Also, increasing
the expression of wild-type per was shown to result in shorter period
length. Both per mRNA and protein were found to oscillate rhythmically
in a manner consistent with the associated behavioral phenotype.
