Synchronisation by subculture

It is often difficult to avoid introducing some degree of synchronous growth in a culture simply as a results of routine operations. Thus Stubblefield et al. (1967) found that subculture of Don-C cells every 24 h exerted a selection pressure favouring cells with a 12 h generation time and highest mitotic frequencies at about 7 and 19 h. 

More frequently a cell culture is allowed to leave the exponential growth phase and enter a stationary phase before it is subcultured (see 4.2). When this happens the subsequent round of DNA synthesis and cell division is partially synchronised. 

It is found (Fig. 11.4) that few cells enter S-phase before 12h, but by 18 h about 70% of the cells will be making DNA. All the cells divide shortly after 24 h. This is somewhat longer than the normal cycle time and this has been interpreted in two different ways, viz.  

1) Subculturing provides a stimulus and, after a short lag the cells begin to traverse the whole of G1 before passing through S-phase to mitosis. 

2) There is a period of about 12 h while cells are growing, and only then is their transition probability increased sufficiently to allow them to pass from the A state to the B-phase and initiate DNA synthesis (see 10.4). 

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