RATES OF CHAIN GROWTH AND UNITS OF REPLICATION IN DNA OF MAMMALIAN CHROMOSOMES

The rate of DNA chain growth was measured in Chinese hamster cells in culture by the use of the density label [3H]bromodeoxyuridine and an inhibitor of thymidylate synthesis, fluorodeoxyuridine. After 5 to 15 minutes of growth in which the analog was substituted for thymidine, the DNA was extracted, sheared and banded in a CsCl gradient. On the basis of the ratio of label in fully substituted DNA fragments to that in partially substituted ones produced by breakage across junction points between segments of bromouracil hybrid and unsubstituted DNA, the rates of DNA chain growth were estimated. In cells growing under the best conditions, the chains appear to be growing at the rate of 1 to 2 μ per minute. For periods of growth that allow the substituted segments to become about twice the length of the fragments produced by shear, the bromouracil-substituted segments are broken nearly at random interstitially and across the junction points with unsubstituted DNA. However, after longer intervals, the indications are that breakage across junctions may be non-random in that some junctions are less susceptible to breakage by shear than regions between junctions. The suggestion is made that these may represent junctions between units of replication (replicons). Since larger segments of chromosomes or whole chromosome arms in some cases incorporate [3H]thymidine for only about three hours in a replication cycle, the maximum size of a replicon in such regions would be the amount of DNA which can grow at the measured rate of 1 to 2 μ per minute in that time. However, some of the gradient profiles reveal a high proportion of partially substituted DNA with bromouracil segments only 1 to 2 μ long. Therefore, the large continuous pieces of DNA in chromosomes may be assembled by the end-to-end joining of rather small subunits during replication. © 1968.