DNA Replication Pattern in Somatic Chromosomes of Crepis capillaris

Asynchronous replication of chromosomal DNA was demonstrated in root tip cells of Crepis capillaris by means of pulse labelling with tritiated thymidine and autoradiography. The results obtained may indicate that DNA replication starts almost synchronously in all chromosomes but terminates asynchronously. The replication term of the chromosome 3 is shorter than that of others. Time of initiation and termination of DNA replication differ in bulk as well as in certain regions. This trend is also true even in a single chromosome.As for sequential incorporation of 3H-thymidine, two distinct labelling patterns were noticed. Namely, in chromosomes 1 and 2, early replication was observed in subterminal regions, while late replication occurred in regions, both sides of the centromere and in the interstitial region of the long arm. These results may strongly suggest that the replication of chromosomal DNA does not proceed uniformely in longitudinal direction, either distally or proximally, along the chromosome arms. Further investigations were undertaken to make clear the relationships between DNA replication patterns and the chromosome structure. In cytologically, there were two types of DNA in the chromosomes, the one is the late replicating and heterochromatic and the other, the early replicanting one, euchromatic.