(b) 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

DNA replication is the process in which DNA makes a copy of itself during cell division. The separation of two single strands of DNA creates a Y-shape fork and various enzymes play a crucial role in DNA replication.

DNA replication is a vital process in which parental genetic information is copied in the daughter cells before cell division.

The double-helical structure of the DNA is opened with the help of the enzyme DNA helicase.

Due to the double-helical structure of the DNA, each strand is tightly wounded. The strands after the replication bind with the enzyme topoisomerase, to close the replication fork.

The diagram shows replication, which will not proceed in region II because the DNA molecule will be super coiled and unwind it before the replication starts.

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The explanation is found below

Explanation:

DNA replication is an important phenomenon that must occur prior to any cell division. Before DNA replication can take place, the double-helical structure of the DNA molecule must unwind. This separation into a Y-shaped replication fork at the origin of replication (where replication starts) is carried out by DNA HELICASE.

However, due to the double-helical nature of the DNA, each strand is so tightly wound to each other. To prevent the strands from getting too tightly wound to each other as the DNA is unwinded, an enzyme called TOPOISOMERASE binds at a binding site close to the replication fork.

Hence, in the diagram, replication will not proceed if TOPOISOMERASE does not bind ahead of each advancing replication fork in region II because the DNA molecule will become super coiled and difficult to separate/unwind in order to continue replication.

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