ROLE OF mRNA IN EMBRYONIC DEVELOPMENT
135
are in good agreement when mRNA is assumed to have a half-life of
2 hr (Denis, 1967). The half-life of mRNA in early embryos must, therefore, be of this magnitude. The situation is more complex in the experiment performed with differentiated tadpoles. It was not possible to
account for the observed decay curve by adopting a definite half-life for
mRNA. The only way to explain the results is to assume the existence
of at least two classes of mRNA, with different decay rates (Denis,
1967). One class of mRNA would turn over with a half-period of a few
hours; the rest of the mRNA population would be much more stable. The
differentiated embryo would then synthesize both short-lived and longlived mRNA, whereas the early embryo would synthesize only unstable
mRNA.
B. Saturation Experiments with
RNA Labeled for Various Periods of Time
The saturation experiments (Fig. 10) carried out with RNA labeled
for various lengths of time confirm the indications given by the pulsechase experiments (Figs. 12 and 13). Ribonucleic acid from gastrulae
labeled for 1 hr saturates only half of the DNA sites that are accessible
to
3 2
P-RNA from the same stage (Fig. 10b). This means that only a
part of the mRNA population becomes labeled after a 1-hr pulse. Ribonucleic acid labeled with
1 4
C for 11 hr saturates the same percentage of
DNA (2.4%) as
3 2
P-RNA. It can be concluded that the whole mRNA
population of the gastrula becomes labeled after 11 hours of exposure to
1 4
C. On the other hand, RNA from later embryos labeled for 11 hr does not
saturate as high a percentage of DNA as
3 2
P-RNA from the same stage
(Fig. 10c and d). This indicates that a 11-hr pulse is not sufficient to
label all the mRNA molecules present in tail-bud embryos and in differentiated tadpoles.
The results of the saturation experiments suggest that in late embryos
a part of the mRNA population turns over rapidly, whereas the rest of
the mRNA population is stable. The unstable fraction can be labeled
either by a continuous exposure to
3 2
P or by a short pulse with
1 4
C0 2 . The
stable fraction can be labeled with
3 2
P but not with
1 4
C0 2 because the
DNA sites responsible for its synthesis, functional in earlier stages, are
no longer active at the time of the
1 4
C0 2 pulse. The experiments described
in Fig. 14 confirm the interpretation. Embryos of three different stages
exposed to
3 2
P from the beginning of development were given a 1-hr
pulse in
1 4
C0 2 . Double-labeled RNA extracted from these embryos was
hybridized with a fixed amount of DNA and increasing amounts of non-
135
are in good agreement when mRNA is assumed to have a half-life of
2 hr (Denis, 1967). The half-life of mRNA in early embryos must, therefore, be of this magnitude. The situation is more complex in the experiment performed with differentiated tadpoles. It was not possible to
account for the observed decay curve by adopting a definite half-life for
mRNA. The only way to explain the results is to assume the existence
of at least two classes of mRNA, with different decay rates (Denis,
1967). One class of mRNA would turn over with a half-period of a few
hours; the rest of the mRNA population would be much more stable. The
differentiated embryo would then synthesize both short-lived and longlived mRNA, whereas the early embryo would synthesize only unstable
mRNA.
B. Saturation Experiments with
RNA Labeled for Various Periods of Time
The saturation experiments (Fig. 10) carried out with RNA labeled
for various lengths of time confirm the indications given by the pulsechase experiments (Figs. 12 and 13). Ribonucleic acid from gastrulae
labeled for 1 hr saturates only half of the DNA sites that are accessible
to
3 2
P-RNA from the same stage (Fig. 10b). This means that only a
part of the mRNA population becomes labeled after a 1-hr pulse. Ribonucleic acid labeled with
1 4
C for 11 hr saturates the same percentage of
DNA (2.4%) as
3 2
P-RNA. It can be concluded that the whole mRNA
population of the gastrula becomes labeled after 11 hours of exposure to
1 4
C. On the other hand, RNA from later embryos labeled for 11 hr does not
saturate as high a percentage of DNA as
3 2
P-RNA from the same stage
(Fig. 10c and d). This indicates that a 11-hr pulse is not sufficient to
label all the mRNA molecules present in tail-bud embryos and in differentiated tadpoles.
The results of the saturation experiments suggest that in late embryos
a part of the mRNA population turns over rapidly, whereas the rest of
the mRNA population is stable. The unstable fraction can be labeled
either by a continuous exposure to
3 2
P or by a short pulse with
1 4
C0 2 . The
stable fraction can be labeled with
3 2
P but not with
1 4
C0 2 because the
DNA sites responsible for its synthesis, functional in earlier stages, are
no longer active at the time of the
1 4
C0 2 pulse. The experiments described
in Fig. 14 confirm the interpretation. Embryos of three different stages
exposed to
3 2
P from the beginning of development were given a 1-hr
pulse in
1 4
C0 2 . Double-labeled RNA extracted from these embryos was
hybridized with a fixed amount of DNA and increasing amounts of non-