ROLE OF mRNA IN EMBRYONIC DEVELOPMENT
129
from gastrulae (Fig. 9). A weaker competition is obtained with RNA
extracted from two different adult tissues (heart and liver). The same
observation was made by Whiteley et al. (1966) in similar experiments
carried out on sea urchins.
It must be concluded from these experiments that a large part of the
mRNA synthesized by early embryos is also present in adult cells. The
strong competition brought about by RNA from two different adult
tissues further indicates that the mRNA common to embryos and to
adult cells is widely distributed in adult tissues.
G. Conclusions
Messenger RNA present in unfertilized or cleaving eggs has no detectable nucleotide sequences in common with the mRNA of later embryos.
Most of the mRNA types synthesized in gastrulae are also present in
later stages, although a small portion of the mRNA produced by gastrulae
and neurulae is no longer present in later embryos. All the types of
mRNA synthesized at tail-bud stage also exist in differentiated tadpoles.
IV. Release of the Genetic Information
during Embryonic Development
The competition experiments described above suggest that RNA
present in growing embryos is complementary to an increasing number of
DNA sites. To confirm this conclusion, a series of saturation experiments
was carried out in which a small amount of DNA was incubated with
increasing amounts of RNA, until all the DNA sites complementary to
RNA were occupied. From the amount of RNA bound to DNA, one
can deduct the percentage of DNA transcribed in the form of complementary RNA, i.e., the number of genes expressed. In this type of experiment, it is necessary to convert the amount of radioactivity bound
to DNA into micrograms of RNA hybridized. This conversion can only
be made if a measurement of the specific activity of complementary RNA
is available. Such a measurement would be easy if all types of RNA
(rRNA, sRNA, mRNA) could be uniformly labeled. Unfortunately, a
uniform labeling of RNA cannot be achieved in amphibian embryos. The
unfertilized egg of Xenopus
contains 4 jug of RNA (mostly rRNA)
which cannot be labeled and which persists throughout development
without being degraded (Brown and Littna, 1964b). Between fertilization and feeding stage, the amount of rRNA present in one Xenopus
embryo is only doubled, whereas the sRNA and mRNA content is
multiplied by a much higher factor (Brown and Littna, 1964a, 1966a,b).
129
from gastrulae (Fig. 9). A weaker competition is obtained with RNA
extracted from two different adult tissues (heart and liver). The same
observation was made by Whiteley et al. (1966) in similar experiments
carried out on sea urchins.
It must be concluded from these experiments that a large part of the
mRNA synthesized by early embryos is also present in adult cells. The
strong competition brought about by RNA from two different adult
tissues further indicates that the mRNA common to embryos and to
adult cells is widely distributed in adult tissues.
G. Conclusions
Messenger RNA present in unfertilized or cleaving eggs has no detectable nucleotide sequences in common with the mRNA of later embryos.
Most of the mRNA types synthesized in gastrulae are also present in
later stages, although a small portion of the mRNA produced by gastrulae
and neurulae is no longer present in later embryos. All the types of
mRNA synthesized at tail-bud stage also exist in differentiated tadpoles.
IV. Release of the Genetic Information
during Embryonic Development
The competition experiments described above suggest that RNA
present in growing embryos is complementary to an increasing number of
DNA sites. To confirm this conclusion, a series of saturation experiments
was carried out in which a small amount of DNA was incubated with
increasing amounts of RNA, until all the DNA sites complementary to
RNA were occupied. From the amount of RNA bound to DNA, one
can deduct the percentage of DNA transcribed in the form of complementary RNA, i.e., the number of genes expressed. In this type of experiment, it is necessary to convert the amount of radioactivity bound
to DNA into micrograms of RNA hybridized. This conversion can only
be made if a measurement of the specific activity of complementary RNA
is available. Such a measurement would be easy if all types of RNA
(rRNA, sRNA, mRNA) could be uniformly labeled. Unfortunately, a
uniform labeling of RNA cannot be achieved in amphibian embryos. The
unfertilized egg of Xenopus
contains 4 jug of RNA (mostly rRNA)
which cannot be labeled and which persists throughout development
without being degraded (Brown and Littna, 1964b). Between fertilization and feeding stage, the amount of rRNA present in one Xenopus
embryo is only doubled, whereas the sRNA and mRNA content is
multiplied by a much higher factor (Brown and Littna, 1964a, 1966a,b).