Eukaryotes replication bubbles

Fig. 16-5 Tandemly arranged replicons in eukaryotic DNA generate a chain of replication bubbles. O denotes an origin of replication.

Eukaryotes tend to have much more DNAthan prokaryotes. Therefore, to speed the process of replication it occnrs in multiple replication bubbles (Fig. 63.3). Within each bubble, DNA replication occnrs at two replication forks. However, in prokaryotes there is only a single replication bubble (Fig. 63.4). In both prokaryotes and enkaryotes, the consensus sequence at an origin of replication varies but all have a high content of A and T, which facilitates separation of the strands. Remember that A=T bonds (two hydrogen bonds) are more easily separated than CsG bonds (three hydrogen bonds). 

In contrast to bacterial chromosomes (see section LA. of this chapter), eukaryotic chromosomes have multiple points of origin at which replication begins. Bubbles appear at these points on the chromosomes. At each end of a bubble, a replication fork forms thus, each bubble has two forks. DNA synthesis occurs at each of these forks, as illustrated in Figure 13.8. As the bubbles enlarge, they eventually merge, and replication is completed. Because eukaryotic chromosomes contain multiple points of origin of replication (and, thus, multiple replicons-units of replication), duplication of such large chromosomes can occur within a few hours. 

Replication of a eukaryotic chromosome. There are several origins of replication, and there are two replication forks for each origin. The bubbles that arise from each origin eventually coalesce. 

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