326
J. SEED
3-day experiment there was a reduction in the number of mitoses and
an increase over the controls in the percentage of cells with high D N A
values, and the authors suggested that the action of RNAase was to
inhibit development of mitosis rather than to inhibit D N A synthesis.
In an investigation ofthe effects of RNAase and DNAase on the uptake
of pyrimidines in HeLa cells, Feinendegen, Bond and Drew (1961a)
found that both enzymes reduced the uptake of thymidine and deoxyuridine, the greater effect being observed with RNAase. With cytidine
and uridine, RNAase produced a slight enhancement of uptake into
R N A , consistent with the work of Ledoux and Revell (1955) on ascites
tumour cells. No effect was observed with either DNAase or RNAase
on the uptake of cytidine or uridine into D N A .
I I I . R N A
S Y N T H E S I S IN C E L L C U L T U R E S
Because of the relative simplicity of the technique, many experiments
on R N A synthesis in cell cultures have utilized the uptake of radioactive
precursors into R N A to indicate synthesis. However, as Thomson and
Paul (1957), Thomson, Paul and Davidson (1956, 1958) and Graham
and Siminovitch (1957) have shown for L-strain cells, a part ofthe newly
synthesized R N A is subsequently broken down again and thus not all
the uptake may represent net synthesis. Partly on account of this R N A
turnover, the interpretation of some ofthe experiments with precursors
has been questioned (see below).
Studies with radioactive precursors have established that the uptake
of label into nuclear R N A in rat liver is much more rapid than that
into cytoplasmic R N A (Marshak, 1948; Jeener and Szafarz, 1950;
Smellie, M c l n d o e and Davidson, 1953). Other experiments with
starfish oocytes (Vincent, 1954) have shown that the rapid nuclear
incorporation occurs particularly into the nucleolus. From observations
of this nature, Marshak and Calvet (1949) and Jeener and Szafarz
(1950) suggested that nuclear R N A was a precursor of cytoplasmic
R N A .
More direct evidence for this suggestion was furnished by Goldstein
and Plaut (1955), who labelled Amoeba proteus by growing it in
3 2
P -
containing medium. The nucleus of a radioactive amoeba was then
removed and grafted into a normal unlabelled amoeba or into an
anucleate half. Subsequent autoradiography showed that the cytoplasm of the grafted amoeba became radioactive after a further 12 h
growth and that the radioactivity in both nucleus and cytoplasm
disappeared after RNAase extraction. In addition, the original unlabelled nucleus of a grafted amoeba (now a binucleate cell) did not
J. SEED
3-day experiment there was a reduction in the number of mitoses and
an increase over the controls in the percentage of cells with high D N A
values, and the authors suggested that the action of RNAase was to
inhibit development of mitosis rather than to inhibit D N A synthesis.
In an investigation ofthe effects of RNAase and DNAase on the uptake
of pyrimidines in HeLa cells, Feinendegen, Bond and Drew (1961a)
found that both enzymes reduced the uptake of thymidine and deoxyuridine, the greater effect being observed with RNAase. With cytidine
and uridine, RNAase produced a slight enhancement of uptake into
R N A , consistent with the work of Ledoux and Revell (1955) on ascites
tumour cells. No effect was observed with either DNAase or RNAase
on the uptake of cytidine or uridine into D N A .
I I I . R N A
S Y N T H E S I S IN C E L L C U L T U R E S
Because of the relative simplicity of the technique, many experiments
on R N A synthesis in cell cultures have utilized the uptake of radioactive
precursors into R N A to indicate synthesis. However, as Thomson and
Paul (1957), Thomson, Paul and Davidson (1956, 1958) and Graham
and Siminovitch (1957) have shown for L-strain cells, a part ofthe newly
synthesized R N A is subsequently broken down again and thus not all
the uptake may represent net synthesis. Partly on account of this R N A
turnover, the interpretation of some ofthe experiments with precursors
has been questioned (see below).
Studies with radioactive precursors have established that the uptake
of label into nuclear R N A in rat liver is much more rapid than that
into cytoplasmic R N A (Marshak, 1948; Jeener and Szafarz, 1950;
Smellie, M c l n d o e and Davidson, 1953). Other experiments with
starfish oocytes (Vincent, 1954) have shown that the rapid nuclear
incorporation occurs particularly into the nucleolus. From observations
of this nature, Marshak and Calvet (1949) and Jeener and Szafarz
(1950) suggested that nuclear R N A was a precursor of cytoplasmic
R N A .
More direct evidence for this suggestion was furnished by Goldstein
and Plaut (1955), who labelled Amoeba proteus by growing it in
3 2
P -
containing medium. The nucleus of a radioactive amoeba was then
removed and grafted into a normal unlabelled amoeba or into an
anucleate half. Subsequent autoradiography showed that the cytoplasm of the grafted amoeba became radioactive after a further 12 h
growth and that the radioactivity in both nucleus and cytoplasm
disappeared after RNAase extraction. In addition, the original unlabelled nucleus of a grafted amoeba (now a binucleate cell) did not