Explain why DNA replication cannot proceed to the regions of the chromosome labeled as I in Figure 1B unless topoisomerase binds ahead of each advancing replication fork in region II .

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Because replication only follows single strands, not double strands, of DNA.

Explanation:

As you can see in the figure below, the regions of the chromosome marked as I are on a double strand of DNA and replication proceeds only in regions of single strands of DNA.

Topoisomerase, is the enzyme responsible for binding the double strand of DNA and separating each strand, forming two single strands, before the replication bubble arrives, only then can replication continue.

For this reason, we can say that in the region of the chromosome marked as I, it will be necessary for the topoisomer to separate each strand of DNA for replication to proceed.

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