II. REPLICATION OF DNA IN CHROMOSOMES 89 



By the iis^c of i)ulse-labeling techniques with thymidine-H^ asyn- 

 chronous hibeling of sectors of chromosomes can be demonstrated in a 

 variety of cells with relatively large chromosomes. However, of perhaps 

 more significance is the observation that a large chi'omosome can be 

 labeled at many sites simultaneously. For example, the Y-chromosome 

 and the late replicating arm of an X-chromosome in the cells of Chinese 

 hamster can be labeled at points along the whole length in 10 minutes of 

 a total replication period of SYz hours for these particular chromosomes 

 (Taylor, 1960c). The late replicating X-chromosome of the human com- 

 plement can likewise be labeled along the whole length in a 10-minute 

 period although it, like the sex chromosomes of the hamster, can be 

 labeled in isolated sectors when supplied with the isotope very near the 

 end of the S period (Morishima et al., 1962). This rapid labeling of 

 DNA along the whole length of chromosomes was also noted in root 

 cells of Bellevalia (Taylor, 1959a). These observations must mean that 

 a large chromosome has several and probably a very large number of 

 sites where DNA replication may be occurring simultaneously. 



VI. Chromosome Reproduction in Relation to DNA 

 Replication and Protein Synthesis 



A. general comments 



Chromosome reproduction involves replication of DNA and synthesis 

 of proteins at least in higher organisms. Synthesis of RNA may be 

 involved directly in the process but this has not been demonstrated so 

 clearly as in the case of the other two polymers. DNA replication can be 

 dissociated from protein and RNA synthesis in bacteria. By using a 

 mutant of Escherichia coli that requires arginine, uracil, and thymine 

 for growth, Maal0e and Hanawalt (1961) showed that DNA would 

 increase by 40% or more without additional protein or RNA synthesis. 

 This would be the amount formed if all nuclei in synthesis should finish 

 but not begin another round of replication. By restoring arginine and 

 uracil to allow a short period of protein and RN'A synthesis, another 

 replication could then be initiated. Cohen (1962) has found a change in 

 state of the DNA after this transition period. If the DNA is isolated be- 

 fore protein synthesis is allowed to occur, in cells starved for arginine and 

 uracil, the isolated DNA can be precipitated with streptomycin. After 

 the period of protein synthesis and the initiation of a new cycle of 

 replication, the DNA isolated from these cells is no longer precipitated 

 by streptomycin. It is perhaps significant that the single-stranded DNA 

 of ^X174 is not precipitated by streptomycin. Cavalieri and Rosenberg 

 (1961a) also reported a change in state of E. coli DNA when cells were 



