Biology 12

Chapter 7 Nucleic Acids: The Molecular Basis of Life • MHR 235

Figure 7.24 shows the pattern of replication
along a linear strand of eukaryotic DNA.
Replication is initiated at hundreds or even
thousands of replication origins at any one time.
Replication continues until all the replication
bubbles have met and the two new DNA molecules
separate from each other.
The packaging of chromatin means that individual
replication forks proceed much more slowly in
eukaryotic cells than in prokaryotes. In mammals,
for example, the rate of movement of replication

forks is less than one tenth the rate in E. coli.

Nevertheless, the presence of multiple replication

forks means that the whole process can be

accomplished very quickly. All the chromosomes

in a eukaryotic cell are replicated simultaneously

during the S1 phase of the cell cycle.

Elongation

Recall that the formation of a new DNA strand

relies on the action of DNA polymerase. This

enzyme has a very specific role in catalyzing the

elongation of DNA molecules — it attaches new

replication origin

Figure 7.23The movement of the replication

forks around the circular chromosome in a

prokaryote. Note that one replication bubble

incorporates two replication forks, and that

replication proceeds in both directions around

the circular chromosome.

Figure 7.24Replication bubbles open simultaneously at many sites along a linear
DNA strand. As replication proceeds along the strand, the bubbles grow until they
meet and the daughter strands separate from each other.

replication origins

fork movement direction

replication forks