Replication of the kinetoplast DNA of Leishmania tarentolae and Crithidia fasciculata.

1. Replicating kinetoplast DNA networks from both Crithidia fasciculata and Leishmania tarentolae have an equilibrium density in ethidium bromide— CsQ which is less than that of covalently closed non-replicating networks. After a few hours of chase, these networks assume the covalently closed posi-. tion in the gradient. 2. Pulse-labeled minicircles isolated, from sonicated networks band in the upper position in ethidium bromide—CsCl equilibrium gradients. Both "free" and network minicircles incorporate [̂ H] thymidine at the same rate in a pulse. Labeled single stranded fragments of less than unit minicircle length are released from pulse-labeled minicircles in alkali. 3. Intact networks of Leishmania and Crithidia can be isolated in the process of replication: replication involves a doubling of the surface area of the networks as visualized by spreading.the kinetoplast DNA on glass slides, staining and examining in the light microscope. 4. DNA replication within the kinetoplast DNA networks of both Leishmania and Crithidia in all parts of the S phase is restricted to the periphery of the structures. In C. fasciculata the pulse-labeled DNA remains in position as the network enlarges by peripheral growth, and then becomes redistributed throughout the network sheet by an unknown mechanism after one cell generation. 5. In the case of L. tarentolae it was demonstrated by density transfer experiments that all network minicircles replicate by an apparent semi-conservative pattern in one cell generation. This implies that there is some type of