10. D N A A N D R N A S Y N T H E S I S
321
synthesis. By taking advantage of the high absorption by the nucleic
acids in the ultraviolet at 2,600 A, Caspersson used the u.v. microscope
to show that embryonic and other cells undergoing a rapid protein
synthesis had large nucleoli and cytoplasm rich in nucleic acids. Brachet
made similar observations using a pyronin-staining method for R N A .
With this procedure the RNA-rich cells stain intensely red; however,
it was found that sections that had previously been digested with the
enzyme RNAase failed to take up the pyronin. Subsequently these
observations were extended by autoradiographic methods (Brachet
and Ficq, 1956); the association between R N A and protein synthesis
was also demonstrated from biochemical studies on bacterial cultures
(Gale and Folkes, 1953a, b) and it was shown that digestion of the
R N A with RNAase inhibits uptake of amino acids into proteins (Gale
and Folkes, 1953c). (See the reviews by Brachet, 1960, and by Davidson,
1960.)
Current work favours the hypothesis that R N A acts as an intermediate in the transfer of information from D N A to protein: the biological properties of a protein are determined by the structural arrangement of its amino acids, and this structure is determined by the chromosomal DNA. The question as to the method in which this information
is "read out" from the D N A is at present the subject of much speculation and research, and evidence has accumulated that the configuration determining the position of a particular amino acid is a triplet
of nucleotides (Nirenberg and Matthaei, 1961; Crick, Barnett, Brenner
and Watts-Tobin, 1961; Martin, Matthaei, Jones and Nirenberg,
1961; Lengyel, Speyer and Ochoa, 1961; Speyer, Lengyel, Basilio
and Ochoa, 1962). A critical note has been sounded by Chargaff (1962)
concerning the apparent excess of uridine in the coding triplets proposed from the experiments involving incorporation of amino acids.
In recent experiments with E. coli a component of the R N A has been
isolated having a base composition similar to the D N A (U being
regarded as equivalent to T ) and having a very short half-life: the
role of a "messenger" carrying information from D N A to ribosomes
has been assigned to this fraction (Volkin, Astrachan and Countryman,
1958; Gros, Hiat, Gilbert, Kurland, Risebrough and Watson, 1961;
Brenner, Jacob and Meselson, 1961). The relevance of the concept
of such a short-lived "messenger R N A " to the cells of higher organisms
(e.g. the red cell) remains yet to be seen. (See the reviews by Jacob and
Monod, 1961, and by Yeas, 1962.)
I I . D N A
S Y N T H E S I S IN C E L L C U L T U R E S
With this general picture of nucleic-acid metabolism in mind, it will
M
321
synthesis. By taking advantage of the high absorption by the nucleic
acids in the ultraviolet at 2,600 A, Caspersson used the u.v. microscope
to show that embryonic and other cells undergoing a rapid protein
synthesis had large nucleoli and cytoplasm rich in nucleic acids. Brachet
made similar observations using a pyronin-staining method for R N A .
With this procedure the RNA-rich cells stain intensely red; however,
it was found that sections that had previously been digested with the
enzyme RNAase failed to take up the pyronin. Subsequently these
observations were extended by autoradiographic methods (Brachet
and Ficq, 1956); the association between R N A and protein synthesis
was also demonstrated from biochemical studies on bacterial cultures
(Gale and Folkes, 1953a, b) and it was shown that digestion of the
R N A with RNAase inhibits uptake of amino acids into proteins (Gale
and Folkes, 1953c). (See the reviews by Brachet, 1960, and by Davidson,
1960.)
Current work favours the hypothesis that R N A acts as an intermediate in the transfer of information from D N A to protein: the biological properties of a protein are determined by the structural arrangement of its amino acids, and this structure is determined by the chromosomal DNA. The question as to the method in which this information
is "read out" from the D N A is at present the subject of much speculation and research, and evidence has accumulated that the configuration determining the position of a particular amino acid is a triplet
of nucleotides (Nirenberg and Matthaei, 1961; Crick, Barnett, Brenner
and Watts-Tobin, 1961; Martin, Matthaei, Jones and Nirenberg,
1961; Lengyel, Speyer and Ochoa, 1961; Speyer, Lengyel, Basilio
and Ochoa, 1962). A critical note has been sounded by Chargaff (1962)
concerning the apparent excess of uridine in the coding triplets proposed from the experiments involving incorporation of amino acids.
In recent experiments with E. coli a component of the R N A has been
isolated having a base composition similar to the D N A (U being
regarded as equivalent to T ) and having a very short half-life: the
role of a "messenger" carrying information from D N A to ribosomes
has been assigned to this fraction (Volkin, Astrachan and Countryman,
1958; Gros, Hiat, Gilbert, Kurland, Risebrough and Watson, 1961;
Brenner, Jacob and Meselson, 1961). The relevance of the concept
of such a short-lived "messenger R N A " to the cells of higher organisms
(e.g. the red cell) remains yet to be seen. (See the reviews by Jacob and
Monod, 1961, and by Yeas, 1962.)
I I . D N A
S Y N T H E S I S IN C E L L C U L T U R E S
With this general picture of nucleic-acid metabolism in mind, it will
M