320
J . S E E D
E. coli for fourteen generations in
1 5
N medium, in order to label the
D N A heavily with
1 5
N . By ultra-centrifugation (in a CsCl densitygradient) of the D N A from the resultant bacterial population, they
were able to detect only one band corrrsponding to
1 5
N labelled D N A .
However, they found that after further growth in ordinary
1 4
N medium,
hybrid D N A molecules occurred containing both
1 5
N and
1 4
N in
approximately equal amounts, and at the end of one generation-time
these hybrid (half-labelled) molecules were the only ones present. At
later times, only hybrid and completely unlabelled D N A molecules were
found, the two types being present in equal amounts at the end of two
generation periods. This evidence thus provides general support for a
complementary scheme of replication, although the experiments do
not go as far as to show that the conserved D N A sub-units are in fact
the single helical strands of the Watson-Crick model.
The implication that new D N A molecules are in some way constructed on those already present has received considerable support from
the experiments of Kornberg and collaborators on the synthesis of
D N A in vitro (Kornberg, Lehman, Bessman and Simms, 1956; Lehman,
Bessman, Simms and Kornberg, 1958; Bessman, Lehman, Simms and
Kornberg, 1958). These workers found that the synthesis of D N A
in vitro required the presence of an enzyme, "polymerase", the four
nucleoside triphosphates, M g
2+ ions and a "primer" DNA. The ratio
of (A + T ) to (G + C) in the newly synthesized D N A corresponded
closely with that of the D N A primer for a variety of DNAs with
different A + T / G + C ratios, and an analysis of the frequencies of all
nearest-neighbour linkages between nucleotides showed that the base
composition of the product was similar to that of the primer (Josse,
Kaiser and Kornberg, 1961). An analogous system, using an enzyme
" R N A polymerase" has been found for the DNA-primed synthesis of
R N A from the four triphosphates (Weiss, 1960; Weiss and Nakomoto,
1961; Hurwitz, Bresler and Diringer, 1960; Huang, Maheshwari and
Bonner, 1960; Stevens, 1960, 1961; Burma, Kroger, Ochoa, Warner
and Weill, 1961; Ochoa, Burma, Kroger and Weill, 1961; Furth,
Hurwitz and Goldmann, 1961).
C. T H E R O L E OF R N A
While for most species the evidence favours D N A as the prime
genetic material, the role of R N A is less clear. In contrast with the
constancy of D N A per cell, the amount of R N A can vary widely
between different tissues of the same organism, and indeed it was this
variation that led Caspersson (1941) and Brachet (1942) independently
to the conclusion that there was a relation between R N A and protein
J . S E E D
E. coli for fourteen generations in
1 5
N medium, in order to label the
D N A heavily with
1 5
N . By ultra-centrifugation (in a CsCl densitygradient) of the D N A from the resultant bacterial population, they
were able to detect only one band corrrsponding to
1 5
N labelled D N A .
However, they found that after further growth in ordinary
1 4
N medium,
hybrid D N A molecules occurred containing both
1 5
N and
1 4
N in
approximately equal amounts, and at the end of one generation-time
these hybrid (half-labelled) molecules were the only ones present. At
later times, only hybrid and completely unlabelled D N A molecules were
found, the two types being present in equal amounts at the end of two
generation periods. This evidence thus provides general support for a
complementary scheme of replication, although the experiments do
not go as far as to show that the conserved D N A sub-units are in fact
the single helical strands of the Watson-Crick model.
The implication that new D N A molecules are in some way constructed on those already present has received considerable support from
the experiments of Kornberg and collaborators on the synthesis of
D N A in vitro (Kornberg, Lehman, Bessman and Simms, 1956; Lehman,
Bessman, Simms and Kornberg, 1958; Bessman, Lehman, Simms and
Kornberg, 1958). These workers found that the synthesis of D N A
in vitro required the presence of an enzyme, "polymerase", the four
nucleoside triphosphates, M g
2+ ions and a "primer" DNA. The ratio
of (A + T ) to (G + C) in the newly synthesized D N A corresponded
closely with that of the D N A primer for a variety of DNAs with
different A + T / G + C ratios, and an analysis of the frequencies of all
nearest-neighbour linkages between nucleotides showed that the base
composition of the product was similar to that of the primer (Josse,
Kaiser and Kornberg, 1961). An analogous system, using an enzyme
" R N A polymerase" has been found for the DNA-primed synthesis of
R N A from the four triphosphates (Weiss, 1960; Weiss and Nakomoto,
1961; Hurwitz, Bresler and Diringer, 1960; Huang, Maheshwari and
Bonner, 1960; Stevens, 1960, 1961; Burma, Kroger, Ochoa, Warner
and Weill, 1961; Ochoa, Burma, Kroger and Weill, 1961; Furth,
Hurwitz and Goldmann, 1961).
C. T H E R O L E OF R N A
While for most species the evidence favours D N A as the prime
genetic material, the role of R N A is less clear. In contrast with the
constancy of D N A per cell, the amount of R N A can vary widely
between different tissues of the same organism, and indeed it was this
variation that led Caspersson (1941) and Brachet (1942) independently
to the conclusion that there was a relation between R N A and protein