ROLE OF mRNA IN EMBRYONIC DEVELOPMENT
121
after fertilization, the eggs do not contain more labeled RNA than do unfertilized eggs (Fig. 1). Figure 2 analyzes the RNA synthesized from
fertilization up till various stages of development. The RNA labeled
during cleavage is mostly soluble RNA. There is also a small amount of
RNA that sediments heterogeneously (Fig. 2a and b). From gastrulation onward, peaks of radioactivity corresponding to the 18 S and 28 S
components begin to appear. They become more apparent later on (Fig.
2d-f). As development proceeds, a large amount of radioactive material
accumulates in the superficial region of the gradients. At all stages of
development, labeled molecules can also be found in the lowest part of
the gradients; the sedimentation constant of these molecules is thus
higher than 28 S.
Several conclusions can be drawn from the analyses outlined above.
1. The labeling of sRNA starts at the beginning of cleavage and continues throughout development. During cleavage, the labeling of sRNA
is not the result of a real synthesis, but results from a turnover of the
terminal bases (pCpCpA; Brown and Littna, 1966b). A net synthesis of
sRNA can be detected after the onset of gastrulation. As sRNA is
metabolically stable, it accumulates during development.
2. The synthesis of rRNA only starts at the beginning of gastrulation.
This type of RNA also accumulates during development so that the
rRNA content of the embryo is approximately doubled by the feeding
stage.
3. A third type of RNA is synthesized from the first cleavage on. This
RNA sediments in a very wide region of the sucrose gradients (4 S to
more than 28 S). It is thus formed of very heterogeneous molecules. The
base composition of this RNA is close to that of DNA. The existence of
heterogeneous RNA is better demonstrated when the RNA is extracted
from ribosomes rather than from whole embryos. The RNA purified in
this way contains, besides the nonlabeled ribosomal components (18 S
and 28 S), a strongly labeled fraction that sediments in the 8-16-S region
(Fig. 3). This fraction has a base composition similar to that of DNA
(42% G + C; Dawid, 1965). The DNA-like RNA (dRNA) fraction is
present at all stages examined. It would correspond to the heterogeneous
RNA observed in the extracts of whole embryos (Fig. 2). In the living
cell, the heterogeneous RNA would thus be associated with the ribosomes.
It would undergo a partial degradation during the isolation procedure
and this probably explains the lower sedimentation coefficient of the
heterogeneous RNA purified from ribosomes. Brown and Littna consider the heterogeneous RNA to be mRNA. The synthesis of mRNA
121
after fertilization, the eggs do not contain more labeled RNA than do unfertilized eggs (Fig. 1). Figure 2 analyzes the RNA synthesized from
fertilization up till various stages of development. The RNA labeled
during cleavage is mostly soluble RNA. There is also a small amount of
RNA that sediments heterogeneously (Fig. 2a and b). From gastrulation onward, peaks of radioactivity corresponding to the 18 S and 28 S
components begin to appear. They become more apparent later on (Fig.
2d-f). As development proceeds, a large amount of radioactive material
accumulates in the superficial region of the gradients. At all stages of
development, labeled molecules can also be found in the lowest part of
the gradients; the sedimentation constant of these molecules is thus
higher than 28 S.
Several conclusions can be drawn from the analyses outlined above.
1. The labeling of sRNA starts at the beginning of cleavage and continues throughout development. During cleavage, the labeling of sRNA
is not the result of a real synthesis, but results from a turnover of the
terminal bases (pCpCpA; Brown and Littna, 1966b). A net synthesis of
sRNA can be detected after the onset of gastrulation. As sRNA is
metabolically stable, it accumulates during development.
2. The synthesis of rRNA only starts at the beginning of gastrulation.
This type of RNA also accumulates during development so that the
rRNA content of the embryo is approximately doubled by the feeding
stage.
3. A third type of RNA is synthesized from the first cleavage on. This
RNA sediments in a very wide region of the sucrose gradients (4 S to
more than 28 S). It is thus formed of very heterogeneous molecules. The
base composition of this RNA is close to that of DNA. The existence of
heterogeneous RNA is better demonstrated when the RNA is extracted
from ribosomes rather than from whole embryos. The RNA purified in
this way contains, besides the nonlabeled ribosomal components (18 S
and 28 S), a strongly labeled fraction that sediments in the 8-16-S region
(Fig. 3). This fraction has a base composition similar to that of DNA
(42% G + C; Dawid, 1965). The DNA-like RNA (dRNA) fraction is
present at all stages examined. It would correspond to the heterogeneous
RNA observed in the extracts of whole embryos (Fig. 2). In the living
cell, the heterogeneous RNA would thus be associated with the ribosomes.
It would undergo a partial degradation during the isolation procedure
and this probably explains the lower sedimentation coefficient of the
heterogeneous RNA purified from ribosomes. Brown and Littna consider the heterogeneous RNA to be mRNA. The synthesis of mRNA