428
T. G. HSU
Exchanges taking place during the second synthesis period would
involve only one of the twin chromosomes in the second metaphase,
and are known as single exchanges.
Now, if the two half-chromatids are complementary, the proportion
between twin exchanges and single exchanges is calculated to be 1:3
respectively. If they are not complementary, exchanges can occur
between any two of the four subunits, and the proportion of single
exchanges would be greatly increased. A minimum proportion between
twins and singles would be 1:10 respectively. Analytical data support
the former assumption, viz. they are complementary. A word must
be added here that colchicine appears to reduce the number of exchanges (La Cour and Pelc, 1958). The mechanism of this inhibition
of exchanges is not known, but it is a problem worthy of further
investigation.
D. CELL SYNCHRONIZATION
It would be safe to say that tritiated compounds will continue to be
powerful tools to attack biological problems. If mammalian cell cultures
are to be used frequently as material for research in cell biology (and
there is no reason to suspect that they will not), one important task must
be accomplished, namely, to find a foolproof method to synchronize
mitosis. Needless to say, many problems can be solved with much
precision when synchronous cultures are used. With the techniques
of determining stages of D N A synthesis, one can correlate various
biochemical activities of cells with D N A synthesis, if the cells are
synchronized for mitosis.
Mitosis will be synchronized in a cell population if all the cells begin
to synthesize D N A at the same time. In microbiological research,
numerous methods have been devised to achieve cell synchrony
(cf. Prescott, 1961). One of them is the change of temperature. Chilling
to 4°C was the first method applied to mammalian cell cultures by
Gey, Bang and Gey (1954) and elaborated by Newton and Wildy (1959).
So far, no one seems to be able to produce consistent data, however.
Miura and Utakoji (1961) repeated the work of Newton and Wildy with
another human cell line and found no significant mitotic synchronization. The interplay among temperature deviation, nucleic acid synthesis, and other vital processes of the cells is certainly quite complex.
When one process is hampered or inhibited by such a general change
as temperature, many other enzymatic processes are likewise affected.
Therefore, when conditions are just right, one may occasionally achieve
some degree of synchronization with such a procedure, but not always.
But a partial synchrony of mitosis, even if it reaches something like
T. G. HSU
Exchanges taking place during the second synthesis period would
involve only one of the twin chromosomes in the second metaphase,
and are known as single exchanges.
Now, if the two half-chromatids are complementary, the proportion
between twin exchanges and single exchanges is calculated to be 1:3
respectively. If they are not complementary, exchanges can occur
between any two of the four subunits, and the proportion of single
exchanges would be greatly increased. A minimum proportion between
twins and singles would be 1:10 respectively. Analytical data support
the former assumption, viz. they are complementary. A word must
be added here that colchicine appears to reduce the number of exchanges (La Cour and Pelc, 1958). The mechanism of this inhibition
of exchanges is not known, but it is a problem worthy of further
investigation.
D. CELL SYNCHRONIZATION
It would be safe to say that tritiated compounds will continue to be
powerful tools to attack biological problems. If mammalian cell cultures
are to be used frequently as material for research in cell biology (and
there is no reason to suspect that they will not), one important task must
be accomplished, namely, to find a foolproof method to synchronize
mitosis. Needless to say, many problems can be solved with much
precision when synchronous cultures are used. With the techniques
of determining stages of D N A synthesis, one can correlate various
biochemical activities of cells with D N A synthesis, if the cells are
synchronized for mitosis.
Mitosis will be synchronized in a cell population if all the cells begin
to synthesize D N A at the same time. In microbiological research,
numerous methods have been devised to achieve cell synchrony
(cf. Prescott, 1961). One of them is the change of temperature. Chilling
to 4°C was the first method applied to mammalian cell cultures by
Gey, Bang and Gey (1954) and elaborated by Newton and Wildy (1959).
So far, no one seems to be able to produce consistent data, however.
Miura and Utakoji (1961) repeated the work of Newton and Wildy with
another human cell line and found no significant mitotic synchronization. The interplay among temperature deviation, nucleic acid synthesis, and other vital processes of the cells is certainly quite complex.
When one process is hampered or inhibited by such a general change
as temperature, many other enzymatic processes are likewise affected.
Therefore, when conditions are just right, one may occasionally achieve
some degree of synchronization with such a procedure, but not always.
But a partial synchrony of mitosis, even if it reaches something like