118
HERMAN DENIS
sRNA, ribosomes, and energy (Nirenberg and Matthaei, 1962). Ribonucleic acids other than mRNA have, however, been shown to stimulate
protein synthesis in an acellular system derived from Escherichia
coli
(Holland et al, 1966; Drach and Lingrel, 1966). The stimulation of
protein synthesis would, therefore, not be an exclusive property of
mRNA.
5. Messenger RNA is complementary to a considerable length of the
genome, so that it easily forms hybrid molecules with DNA. In these
hybrid structures, RNA and DNA are associated by base-pairing as are
the strands of native DNA (Watson and Crick, 1953). Soluble and ribosomal RNA are also complementary to a given region of DNA and are
able to hybridize with it (Yankofsky and Spiegelman, 1962; Goodman
and Rich, 1962). But the portion of DNA on which sRNA and rRNA are
copied is small, and represents only 0.01-0.3% of the total length of the
genome (Giacomoni and Spiegelman, 1962; MacFarlane and Fraser,
1964; MacConkey and Hopkins, 1964; Ritossa and Spiegelman, 1965;
Vermeulen and Atwood, 1965; Attardi et al., 1965; Wallace and Birnstiel,
1966); therefore, mRNA hybridizes much more easily with DNA than
sRNA and rRNA.
Several of the properties described above have been used to determine
the amount and the type of mRNA synthesized during embryonic development. The present review will be exclusively devoted to experiments
carried out on the South African toad, Xenopus laevis. This species has
many advantages from the experimental point of view. At the present
time, it can be considered as the best material for biochemical research
on amphibian development.
II. General Pattern of RNA Synthesis in Embryonic Development
Brown and Littna (1964a) studied the synthesis of RNA during
embryonic development by the technique of centrifugation in sucrose
gradients. The RNA synthesized during a given period of development
was detected by exposing the embryos to labeled precursors of the
nucleic acids. Two different methods of labeling were used. The first one
consisted in injecting
3 2
P as phosphate into the female just before ovulation (Kutsky, 1950). Under these conditions, a large amount of radioactive phosphate enters the inorganic pool of the oocyte and remains
present in the embryos until the end of development (Brown and Littna,
1964a). All nucleic acids synthesized from fertilization up to the feeding
stage are labeled by this procedure. The second method consisted in exposing the embryos to
1 4
C0 2 in a confined medium (Cohen, 1954). In
HERMAN DENIS
sRNA, ribosomes, and energy (Nirenberg and Matthaei, 1962). Ribonucleic acids other than mRNA have, however, been shown to stimulate
protein synthesis in an acellular system derived from Escherichia
coli
(Holland et al, 1966; Drach and Lingrel, 1966). The stimulation of
protein synthesis would, therefore, not be an exclusive property of
mRNA.
5. Messenger RNA is complementary to a considerable length of the
genome, so that it easily forms hybrid molecules with DNA. In these
hybrid structures, RNA and DNA are associated by base-pairing as are
the strands of native DNA (Watson and Crick, 1953). Soluble and ribosomal RNA are also complementary to a given region of DNA and are
able to hybridize with it (Yankofsky and Spiegelman, 1962; Goodman
and Rich, 1962). But the portion of DNA on which sRNA and rRNA are
copied is small, and represents only 0.01-0.3% of the total length of the
genome (Giacomoni and Spiegelman, 1962; MacFarlane and Fraser,
1964; MacConkey and Hopkins, 1964; Ritossa and Spiegelman, 1965;
Vermeulen and Atwood, 1965; Attardi et al., 1965; Wallace and Birnstiel,
1966); therefore, mRNA hybridizes much more easily with DNA than
sRNA and rRNA.
Several of the properties described above have been used to determine
the amount and the type of mRNA synthesized during embryonic development. The present review will be exclusively devoted to experiments
carried out on the South African toad, Xenopus laevis. This species has
many advantages from the experimental point of view. At the present
time, it can be considered as the best material for biochemical research
on amphibian development.
II. General Pattern of RNA Synthesis in Embryonic Development
Brown and Littna (1964a) studied the synthesis of RNA during
embryonic development by the technique of centrifugation in sucrose
gradients. The RNA synthesized during a given period of development
was detected by exposing the embryos to labeled precursors of the
nucleic acids. Two different methods of labeling were used. The first one
consisted in injecting
3 2
P as phosphate into the female just before ovulation (Kutsky, 1950). Under these conditions, a large amount of radioactive phosphate enters the inorganic pool of the oocyte and remains
present in the embryos until the end of development (Brown and Littna,
1964a). All nucleic acids synthesized from fertilization up to the feeding
stage are labeled by this procedure. The second method consisted in exposing the embryos to
1 4
C0 2 in a confined medium (Cohen, 1954). In