Blockade of S-phase

Synchrony at the Gl/S-interphase has been accomplished by interfering with the synthesis of one or more deoxyribonucleoside triphosphate, which are required for DNA synthesis while allowing other cellular procedures such as synthesis of RNA and protein to proceed. 

1) Only 70% of the cells are in synchrony — the remaining 30% being up to 6 h ahead of the main group. 

2) Events other than those concerned directly with DNA synthesis may proceed in the presence of reagents which block DNA synthesis and certain aspects of the cell s rqetabolism may reflect G2 or even G1 activity while DNA synthesis related events are held at the Gl/S-phase boundary. In addition DNA strand breaks occur at high frequency in cells blocked in S-phase (Li and Kaminskas, 1984) and some replicons initiate repeatedly (Vassi-lev and Russev, 1984). 

This unbalanced growth rapidly leads to cell death if prolonged for more than a generation time (Ruekert and Mueller, 1960). Unbalanced growth will occur in any cells committed to division ( 10.4) yet blocked in one function including those maintained in colcemid for more than a few hours. Moreover, selective blocking of DNA synthesis may have effects which are not apparent until the synchronised cells are released and proceed to the next G1-phase (Firket and Mahieu, 1966 Cress and Gerner, 1977). Schindler et al. 

In practice synchronisation at the Gl/S boundary may be achieved by a number of reagents as well as thymidine, e.g. aminopterin, amethopterin, 5-fluorodeoxyuridine or hydroxyurea. 

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