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JEAN BRÄCHET
RNA's are believed to be synthesized in close contact with chromosomal
DNA and to represent an accurate copy of the genetic material itself.
Their function is to receive genetic information directly from DNA and
to transfer it to the cytoplasm, where the bulk of protein synthesis
occurs. The task of assembling the amino-acids in a precise sequence
(formation of polypeptide chains) belongs to the ribosomes. These small
granules are often attached to the membranes of the endoplasmic
reticulum ; but, in very early stages of embryonic development, most of
them are free in the cytoplasm. Ribosomes are very rich in ribosomal
RNA, which is chemically different from messenger RNA. The latter,
after its migration from the nucleus to the cytoplasm, apparently
becomes attached to the ribosomes: in this way, it adds the genetic
information which they lack and which is necessary to assemble the
amino-acids in the correct sequence. The integrity of both messenger and
ribosomal RNA's is apparently required for protein synthesis. Finally,
the amino-acids cannot be utilized without prior activation by enzymes
and adenosinetriphosphoric acid (ATP). The activated amino-acids
(which have now become amino acyl-adenosinemonophosphoric acids)
must attach themselves to a third kind of RNA : the soluble or transfer
RNA's, often called the S-RNA's. There is a specific S-RNA for almost
every one of the amino-acids; their task is to transfer the attached
activated amino-acid to the ribosomes. This is a very brief summary of
our present picture of the mechanisms of protein synthesis and transfer
of genetic information : things have obviously become more complicated,
but better understood, than twenty years ago, when Caspersson and the
author drew, independently, the conclusion that RNA must be directly
involved in protein synthesis.
There is no doubt that, in the near future, molecular biologists will
become more and more interested in the problem of cell differentiation
and, like the chemical embryologists of the old days, they will try to
explain it on a molecular basis. There is no doubt that DNA, the various
kinds of RNA, and the proteins will be at the heart of such an explanation. This is the main reason for the present review: it is important for
molecular geneticists as well as embryologists to know where we now
stand in a field which has a tremendously promising future.
This review must necessarily be limited in scope and length. Therefore,
we shall deal only with two types of biological materials, with which we
have long personal experience, early morphogenesis in amphibians and
regeneration in the unicellular alga Acetabularia
mediterranea.
The
reasons for the choice of these two materials is that, although they
represent relatively simple cases of morphogenesis, all the basic
problems of development are involved: in early amphibian embryogenesis, we shall meet problems related to the biochemical mechanisms
JEAN BRÄCHET
RNA's are believed to be synthesized in close contact with chromosomal
DNA and to represent an accurate copy of the genetic material itself.
Their function is to receive genetic information directly from DNA and
to transfer it to the cytoplasm, where the bulk of protein synthesis
occurs. The task of assembling the amino-acids in a precise sequence
(formation of polypeptide chains) belongs to the ribosomes. These small
granules are often attached to the membranes of the endoplasmic
reticulum ; but, in very early stages of embryonic development, most of
them are free in the cytoplasm. Ribosomes are very rich in ribosomal
RNA, which is chemically different from messenger RNA. The latter,
after its migration from the nucleus to the cytoplasm, apparently
becomes attached to the ribosomes: in this way, it adds the genetic
information which they lack and which is necessary to assemble the
amino-acids in the correct sequence. The integrity of both messenger and
ribosomal RNA's is apparently required for protein synthesis. Finally,
the amino-acids cannot be utilized without prior activation by enzymes
and adenosinetriphosphoric acid (ATP). The activated amino-acids
(which have now become amino acyl-adenosinemonophosphoric acids)
must attach themselves to a third kind of RNA : the soluble or transfer
RNA's, often called the S-RNA's. There is a specific S-RNA for almost
every one of the amino-acids; their task is to transfer the attached
activated amino-acid to the ribosomes. This is a very brief summary of
our present picture of the mechanisms of protein synthesis and transfer
of genetic information : things have obviously become more complicated,
but better understood, than twenty years ago, when Caspersson and the
author drew, independently, the conclusion that RNA must be directly
involved in protein synthesis.
There is no doubt that, in the near future, molecular biologists will
become more and more interested in the problem of cell differentiation
and, like the chemical embryologists of the old days, they will try to
explain it on a molecular basis. There is no doubt that DNA, the various
kinds of RNA, and the proteins will be at the heart of such an explanation. This is the main reason for the present review: it is important for
molecular geneticists as well as embryologists to know where we now
stand in a field which has a tremendously promising future.
This review must necessarily be limited in scope and length. Therefore,
we shall deal only with two types of biological materials, with which we
have long personal experience, early morphogenesis in amphibians and
regeneration in the unicellular alga Acetabularia
mediterranea.
The
reasons for the choice of these two materials is that, although they
represent relatively simple cases of morphogenesis, all the basic
problems of development are involved: in early amphibian embryogenesis, we shall meet problems related to the biochemical mechanisms