ROLE OF mRNA IN EMBRYONIC DEVELOPMENT
117
role of repressor could be played by some of the proteins that are associated with DNA in the chromosomes. The proteins most frequently
proposed as possible repressors are the histones (Stedman and Stedman,
1950; Bloch, 1962; Huang and Bonner, 1962, 1964; Barr and Butler,
1963).
3. At the level of the translation mechanism. In differentiated cells,
the whole genetic information enclosed in DNA would be transcribed.
The filtration of the information would occur at the level of messenger
RNA translation into proteins, i.e., at the ribosomal level. In this case
the ribosomes could have different properties in different tissues or even
in different regions of the fertilized egg.
Several authors have suggested that differentiated cells contain mRNA
that is never translated into proteins (Kruh et al., 1964, 1966). However,
it must be said that no evidence is so far available to justify the attribution of tissue specificity to the ribosomes (Hirsch, 1966).
The purpose of the present review is to examine which of the three
models mentioned above best fits the experimental data so far obtained.
The first step in the transmission of the genetic information is known
to consist in the transcription of one DNA strand into a complementary
copy called "messenger RNA" (Jacob and Monod, 1961). By studying
the synthesis of mRNA during embryonic development, we might obtain
some information on the mechanism that controls the release of the
genetic information during the process of cell differentiation.
Messenger RNA can be characterized by several physical and chemical
properties that enable us to distinguish it from the other types of RNA.
1. Messenger RNA differs from soluble (s)RNA and from ribosomal
(r)RNA by its overall base composition. The base composition of mRNA
is always close to that of DNA (about 40% of guanosine (G) + cytosine (C) in all animal species; Sueoka, 1961), whereas the base composition of sRNA and rRNA differs considerably from that of DNA. The
latter types of RNA have a much higher G + C content (60%) than
DNA.
2. In most cells, mRNA is a mixture of molecules of various sizes,
whereas sRNA and rRNA have a fixed molecular weight and, hence, a
definite sedimentation speed in the ultracentrifuge.
3. The lifetime of mRNA molecules is limited and usually shorter
than that of sRNA and rRNA.
4. Only mRNA should be able to direct the synthesis of proteins in
an artificial system in which all the elements necessary for the synthesis
of polypeptides are provided: amino acids and activating enzymes,
117
role of repressor could be played by some of the proteins that are associated with DNA in the chromosomes. The proteins most frequently
proposed as possible repressors are the histones (Stedman and Stedman,
1950; Bloch, 1962; Huang and Bonner, 1962, 1964; Barr and Butler,
1963).
3. At the level of the translation mechanism. In differentiated cells,
the whole genetic information enclosed in DNA would be transcribed.
The filtration of the information would occur at the level of messenger
RNA translation into proteins, i.e., at the ribosomal level. In this case
the ribosomes could have different properties in different tissues or even
in different regions of the fertilized egg.
Several authors have suggested that differentiated cells contain mRNA
that is never translated into proteins (Kruh et al., 1964, 1966). However,
it must be said that no evidence is so far available to justify the attribution of tissue specificity to the ribosomes (Hirsch, 1966).
The purpose of the present review is to examine which of the three
models mentioned above best fits the experimental data so far obtained.
The first step in the transmission of the genetic information is known
to consist in the transcription of one DNA strand into a complementary
copy called "messenger RNA" (Jacob and Monod, 1961). By studying
the synthesis of mRNA during embryonic development, we might obtain
some information on the mechanism that controls the release of the
genetic information during the process of cell differentiation.
Messenger RNA can be characterized by several physical and chemical
properties that enable us to distinguish it from the other types of RNA.
1. Messenger RNA differs from soluble (s)RNA and from ribosomal
(r)RNA by its overall base composition. The base composition of mRNA
is always close to that of DNA (about 40% of guanosine (G) + cytosine (C) in all animal species; Sueoka, 1961), whereas the base composition of sRNA and rRNA differs considerably from that of DNA. The
latter types of RNA have a much higher G + C content (60%) than
DNA.
2. In most cells, mRNA is a mixture of molecules of various sizes,
whereas sRNA and rRNA have a fixed molecular weight and, hence, a
definite sedimentation speed in the ultracentrifuge.
3. The lifetime of mRNA molecules is limited and usually shorter
than that of sRNA and rRNA.
4. Only mRNA should be able to direct the synthesis of proteins in
an artificial system in which all the elements necessary for the synthesis
of polypeptides are provided: amino acids and activating enzymes,