Rapid growth Contradiction When conditions for growth are favorable. E coli can grow with a doubling time(生长倍增时间) of approximately 20 min. However, the time it takes to synthesize a complete copy of the E coli chromosome Is 40 min, under optimum conditions and segregation of the DNA and division takes another 20 min. Thus. the shortest cell cycle ar nd. therefore, doubling time for E coli should be 60 min. This is obviously not the case. For cells to divide faster than every 60. min, DNA replication must begin in one cycle and finish in another
Rapid growth When conditions for growth are favorable, E. coli can grow with a doubling time（生长倍增时间） of approximately 20 min. However, the time it takes to synthesize a complete copy of the E. coli chromosome is 40 min, under optimum conditions and segregation of the DNA and division takes another 20 min. Thus, the shortest cell cycle and, therefore, doubling time for E. coli should be 60 min. This is obviously not the case. For cells to divide faster than every 60. min, DNA replication must begin in one cycle and finish in another. Contradiction
Why? When cells are growing fast(doubling time <60 min) nitiation of replication occurs, as normal, producing two replication forks which move bidirectionally round the chromosome to the termination point. However, the origins on these new strands then initiate further rounds of replication before the previous round of DNA replication has finished Fig ). Thus, when cell division occurs the DNA in the daughter cells is already replicating. The faster the cell growth rate, the more replication forks are formed such that the DNA in new cells may have multiple replication forks
When cells are growing fast (doubling time <60 min), initiation of replication occurs, as normal, producing two replication forks which move bidirectionally round the chromosome to the termination point . However, the origins on these new strands then initiate further rounds of replication before the previous round of DNA replication has finished (Fig.). Thus, when cell division occurs the DNA in the daughter cells is already replicating. The faster the cell growth rate, the more replication forks are formed such that the DNA in new cells may have multiple replication forks. Why?
Indications That repeated DNA replication can occur without cell division indicates that the control of the two processes is not linked. This is in contrast to eukaryotic cells where the two processes are directly connected
That repeated DNA replication can occur without cell division indicates that the control of the two processes is not linked. This is in contrast to eukaryotic cells where the two processes are directly connected . Indications
Slow growth Fast growth origins eplication Initiation of forks DNA synthesis DNA synthesis o→ already nitrated in new cell Fig. 2. Coordination of DNA synthesis and cell division in slow and fast growing bacteria
Fig. 2. Coordination of DNA synthesis and cell division in slow and fast growing bacteria. Slow growth Fast growth origins Replication forks Initiation of DNA synthesis DNA synthesis already initiated in new cells
Typical growth curve of unicelled crone
Part B Typical growth curve of unicelled microbe