Multiforked chromosome

Under all nutrient conditions at a fixed temperature, the rate of replication-fork movement remains fairly constant. The overall rate of DNA replication is determined by the frequency of initiation at oriC. To accelerate replication, initiation at daughter origins occurs before the ongoing cycle of replication is completed. This results in the formation of a multiforked chromosome, as shown in Fig. 16-6. Note that a multiforked chromosome contains at least four copies of oriC. 

Fig. 16-6 A multiforked chromosome generated by reinitiation of replication at oriC.

What conditions give rise to multiforked chromosomes in bacteria ... 

Since the replication fork moves at -1000 bases s and there are 4.6 x 10 base pairs in the chromosome, it will take the two replication forks (that copy both strands simultaneously) 4,600,000/(1000 x 2 x 60) = -38 min to copy the whole E. coli genome. It will then take another -20 min to separate the cellular components. To double the cell number in less than 60 min means the cell must initiate the next round of replication before the previous one has finished. This results in multiforked chromosomes. 

About 33 min would be required to replicate the chromosome because you have a bidirectional mode of replication with two replication forks. If you had multifork replication (one round of replication starting before the other finished), a 20-min division time would be possible. 

A multiforked bacterial chromosome is one that contains more than two replication forks and results from reinitiation at the daughter origins within a replication bubble. In this situation, cycles of replication are completed at more frequent intervals, and this gives shorter generation times. It occurs under conditions of fast growth, induced by nutritionally rich media. 

Under what conditions do sections of eukaryotic chromosomes exist as multiforked structures ... 

Unlike the situation in bacteria, multiforked structures in eukaryotic chromosomes do not form in response to a demand for increased rate of DNA replication. But, multiforked DNA does appear over restricted regions of eukaryotic chromosomes this gives DNA (or gene) amplification and occurs in specific situations. 

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See also in sourсe #XX -- [ , , ]

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