ROLE OF mRNA IN EMBRYONIC DEVELOPMENT
139
VI. Amount of Messenger RNA Present at
Various Stages of Development
A. Amount of Messenger RNA Present in
the Unfertilized Egg of Xenopus
laevis
The mRNA present in the fertilized egg of Xenopus has no, or at least
very few, nucleotide sequences in common with the mRNA synthesized
by gastrulae and by later embryos (Figs. 5 to 8). This does not
necessarily mean that the egg does not contain any mRNA. Some DNA
sites different from those that function after fertilization might be active
during oogenesis and synthesize mRNA which would be stored in stable
form and persist until ovulation. According to Davidson et al. (1966) the
mature egg of Xenopus would contain 47 m/xg of mRNA. This amount
of mRNA represents more than 1% of the RNA content of the egg.
Davidson et al. assume that all the egg mRNA has been synthesized
long before ovulation, and this must be so since the amount of mRNA
produced in the last 24 hr of oogenesis does not exceed 1 m^g (Brown and
Littna, 1966a). The RNA that can be labeled the day before spawning
does not hybridize with DNA to a measurable extent (Fig. 10a). According to Davidson et al., the mRNA synthesized during oogenesis and
present in the mature egg is complementary to a considerable length of
genome (1.5%). The amount of mRNA found by Davidson et al. in the
Xenopus egg seems to be somewhat overestimated and should be determined again by other methods. If the mature egg really contains a
considerable amount of mRNA, it is, nevertheless, certain that this
mRNA has nothing or very little in common with the mRNA synthesized
by the developing embryo. The mRNA accumulated in the egg is likely
to be used during cleavage to direct the synthesis of protein. This is
probably the reason why protein synthesis can occur in the cleaving egg
in the absence of newly formed mRNA (Denis, 1964b).
It seems established that the unfertilized egg of the sea urchin contains
a large amount of mRNA (5% of total RNA; Slater and Spiegelman,
1966). In contrast with what was observed in Xenopus,
the mRNA
present in sea urchin eggs is partially homologous with the mRNA synthesized by gastrulae and plutei (Whiteley et al, 1966; Glisin et al.,
1966). In sea urchins, some genes would thus be active both during
oogenesis and during embryonic development. Nothing similar has so
far been observed in amphibians.
139
VI. Amount of Messenger RNA Present at
Various Stages of Development
A. Amount of Messenger RNA Present in
the Unfertilized Egg of Xenopus
laevis
The mRNA present in the fertilized egg of Xenopus has no, or at least
very few, nucleotide sequences in common with the mRNA synthesized
by gastrulae and by later embryos (Figs. 5 to 8). This does not
necessarily mean that the egg does not contain any mRNA. Some DNA
sites different from those that function after fertilization might be active
during oogenesis and synthesize mRNA which would be stored in stable
form and persist until ovulation. According to Davidson et al. (1966) the
mature egg of Xenopus would contain 47 m/xg of mRNA. This amount
of mRNA represents more than 1% of the RNA content of the egg.
Davidson et al. assume that all the egg mRNA has been synthesized
long before ovulation, and this must be so since the amount of mRNA
produced in the last 24 hr of oogenesis does not exceed 1 m^g (Brown and
Littna, 1966a). The RNA that can be labeled the day before spawning
does not hybridize with DNA to a measurable extent (Fig. 10a). According to Davidson et al., the mRNA synthesized during oogenesis and
present in the mature egg is complementary to a considerable length of
genome (1.5%). The amount of mRNA found by Davidson et al. in the
Xenopus egg seems to be somewhat overestimated and should be determined again by other methods. If the mature egg really contains a
considerable amount of mRNA, it is, nevertheless, certain that this
mRNA has nothing or very little in common with the mRNA synthesized
by the developing embryo. The mRNA accumulated in the egg is likely
to be used during cleavage to direct the synthesis of protein. This is
probably the reason why protein synthesis can occur in the cleaving egg
in the absence of newly formed mRNA (Denis, 1964b).
It seems established that the unfertilized egg of the sea urchin contains
a large amount of mRNA (5% of total RNA; Slater and Spiegelman,
1966). In contrast with what was observed in Xenopus,
the mRNA
present in sea urchin eggs is partially homologous with the mRNA synthesized by gastrulae and plutei (Whiteley et al, 1966; Glisin et al.,
1966). In sea urchins, some genes would thus be active both during
oogenesis and during embryonic development. Nothing similar has so
far been observed in amphibians.