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Before the After one switch round of to 14N replication

After two After three rounds of rounds of replication replication

Before the After one switch round of to 14N replication

After two After three rounds of rounds of replication replication

3. The E. coli chromosome contains 4.7 million base pairs of DNA. If synthesis at each replication fork occurs at a rate of 1000 nucleotides per second, how long will it take to completely replicate the E. coli chromosome with theta replication?

Bacterial chromosomes contain a single origin of replication, and theta replication usually employs two replication forks, which proceed around the chromosome in opposite directions. Thus, the overall rate of replication for the whole chromosome is 2000 nucleotides per second. With a total of 4.7 million base pairs of DNA, the entire chromosome will be replicated in:

4,700,000 bp X

1 second 2000 bp

(b) With conservative replication, the entire molecule serves as a template. After one round of replication, some molecules will consist entirely of 15N, and others will consist entirely of 14N; so

2350 seconds X

39.17 minutes

1 minute 60 seconds

At the beginning of this chapter it was stated that E. coli is capable of dividing every 20 minutes. How is this possible if it takes almost twice as long to replicate its genome? The answer is that a second round of replication begins before the first round has finished. Thus, when an E. coli cell divides, the chromosomes that are passed on to the daughter cells are already partially replicated. This is in contrast to eukaryotic cells, which replicate their entire genome once, and only once, during each cell cycle.

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