ROLE OF mRNA IN EMBRYONIC DEVELOPMENT
143
mRNA is continuously degraded and resynthesized, sRNA and rRNA
accumulate in growing embryos. Between fertilization and feeding stages,
the rRNA content of the embryo doubles (Brown and Littna, 1964a),
whereas the sRNA content increases by a factor of 30 (Brown and
Littna, 1966b).
B. Synthesis of Messenger RNA during Embryonic Development
Two different types of experiment based on the hybridization technique
(Figs. 5-7 and 10) have consistently shown that RNA present in developing embryos is complementary to an increasing number of DNA sites.
This means that during embryonic development an increasingly large
proportion of the genetic information is transcribed into mRNA.
All the information obtained from the hybridization experiments
(Figs. 5-8, 10, 14, 18c, and Table I) can be summarized in a simple
diagram (Fig. 19). This diagram is based directly on the experimental
data. The procedure followed to establish it is described in detail elsewhere (Denis, 1966b, 1967). We shall simply state that the mRNA
present at three stages of development is represented by blocks of
various dimensions. The length of the blocks is proportional to the
percentage of DNA which is complementary to the RNA considered
(Fig. 10). The height of the blocks is proportional to the number of
copies of each class of mRNA. At each stage the mRNA labeled
with
3 2
P from the beginning of development is presumed to represent
the whole mRNA population of the embryo, both in number of copies
and in variety of nucleotide sequences. The RNA labeled with
1 4
C0 2 for
11 hr is considered unstable, whereas the RNA labeled with
3 2
P but not
with
1 4
C is considered stable. For reasons that cannot be exposed here,
neither the height nor the length of the block representing the mRNA
of each stage should be taken as measurements of absolute value. These
graphical representations may only be used for a stage-to-stage comparison.
The diagram given in Fig. 19 shows that mainly three types of genes
are active during embryonic development. These genes are involved only
in the synthesis of messenger RNA and not in that of soluble and ribosomal RNA.
1. Some DNA sites are active in early embryos (gastrulae and neurulae), but their products are no longer present in later embryos. The
activity of these genes would thus be restricted to the early stages of
development. The RNA synthesized by these genes is represented by the
separate block placed at the top left of the diagram. The existence of
143
mRNA is continuously degraded and resynthesized, sRNA and rRNA
accumulate in growing embryos. Between fertilization and feeding stages,
the rRNA content of the embryo doubles (Brown and Littna, 1964a),
whereas the sRNA content increases by a factor of 30 (Brown and
Littna, 1966b).
B. Synthesis of Messenger RNA during Embryonic Development
Two different types of experiment based on the hybridization technique
(Figs. 5-7 and 10) have consistently shown that RNA present in developing embryos is complementary to an increasing number of DNA sites.
This means that during embryonic development an increasingly large
proportion of the genetic information is transcribed into mRNA.
All the information obtained from the hybridization experiments
(Figs. 5-8, 10, 14, 18c, and Table I) can be summarized in a simple
diagram (Fig. 19). This diagram is based directly on the experimental
data. The procedure followed to establish it is described in detail elsewhere (Denis, 1966b, 1967). We shall simply state that the mRNA
present at three stages of development is represented by blocks of
various dimensions. The length of the blocks is proportional to the
percentage of DNA which is complementary to the RNA considered
(Fig. 10). The height of the blocks is proportional to the number of
copies of each class of mRNA. At each stage the mRNA labeled
with
3 2
P from the beginning of development is presumed to represent
the whole mRNA population of the embryo, both in number of copies
and in variety of nucleotide sequences. The RNA labeled with
1 4
C0 2 for
11 hr is considered unstable, whereas the RNA labeled with
3 2
P but not
with
1 4
C is considered stable. For reasons that cannot be exposed here,
neither the height nor the length of the block representing the mRNA
of each stage should be taken as measurements of absolute value. These
graphical representations may only be used for a stage-to-stage comparison.
The diagram given in Fig. 19 shows that mainly three types of genes
are active during embryonic development. These genes are involved only
in the synthesis of messenger RNA and not in that of soluble and ribosomal RNA.
1. Some DNA sites are active in early embryos (gastrulae and neurulae), but their products are no longer present in later embryos. The
activity of these genes would thus be restricted to the early stages of
development. The RNA synthesized by these genes is represented by the
separate block placed at the top left of the diagram. The existence of