Rate of DNA Replication

Rate of DNA Replication

Beyond any stretch of imagination the DNA synthesis happens to be an extremely rapid phenomenal process, which stands at approximately 1000 nucleotides. sec^{– 1} in E. coli at 37° C. Of course, initially it mostly appears that the prevailing ‘speed of DNA synthesis’ is not likely to happen, in view of the fact that ‘nucleotide substrates’ should be first synthesized adequately, and subsequently must undergo diffusion to the ‘replication fork’. Besides, there could be quite a few genuine attempts whereby the so called ‘wrong nucleotides’ to pair at each strategic position well before the correct bases do pair up actually. However, it is pertinent to state here that the ultimate speed, as well as specificity of DNA replication are virtually monitored and governed by almost the same fundamental principles which actually guide all chemical reactions.

Figure 6.2. illustrates the overall summary of various events that usually take place at the ‘replication fork’.

Explanation : The sequel of events at the ‘replication fork’ may be explained as under :

(1) Log-Phase Growth : Under certain experimental parameters viz., log-phase growth in a relatively rich nutrient culture medium the E. coli cells have been seen to grow exceedingly faster in comparison to the two ensuing ‘replication forks’ are able to complete the circular chromosomes.

(2) Furthermore, under such conditions, the E. coli cell eventually initiates the distinct ‘multiple replication forks’ strategically located at the very origin on the chromosome. Thus, an alto-gether ‘new pair of forks’ critically comes into being before the ‘last pair of forks’ has just finished.

(3) Follow up of above (2) evidently suggests that the overall ‘rate of DNA synthesis’, in fact, closely matches the ‘rate’ at which the E. coli cell undergoes division.

(4) Likewise, when the actual growth of the cell slows down noticeably, one may apparently observe a ‘delayed initiation of DNA synthesis’ occurring at the origin of replication.

(5) In a broader perspective, one may observe that the rate at which each and every replication fork moves, mostly remains constant. However, the careful modulation of the initiative procedure in replication enables the ‘cell’ to largely monitor and control its overall predomi-nant rate of DNA synthesis to closely match not only its rate of growth but also its cell-division.