12. G E N E T I C C Y T O L O G Y
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chromosome bridges and fragments were present. However, Mendelsohn (1960b) showed that, in mouse mammary tumors, tritium incorporated into the D N A has no demonstrable effect on growth or
survival. In this respect, cell culture offers a more ideal system for
reducing such effects to a minimum because timing can be controlled
more precisely. In so-called pulse labeling, the dose is applied for as
little as 5 or 10 min after which time the unused labeled compound,
if any, is washed away and the growth medium is supplemented with
an excess of nonlabeled compound (chasing) to further dilute the free
labeled agent. Cell populations can be fixed for autoradiography at
various intervals after labeling. In our studies on the cell cycle of a
cell line from the Chinese hamster (Hsu, Dewey and Humphrey, 1962),
metaphase chromosome analysis revealed that H 3- T D R (0-5 /xc/ml)
caused no more breakage than was seen in the unlabeled populations,
up to 16 h after a 10 min labeling.
It is probably also appropriate to mention briefly the techniques used
5. Autoradiograph showing a Chinese hamster cell in metaphase, 6 h after pulseehng with tritiated thymidine.