Final answer:
The linking number of a plasmid treated with type IB topoisomerase remains unchanged as the enzyme relieves supercoiling pressure without altering the overall linking number in covalently closed circular DNA.
Step-by-step explanation:
Effect of Type IB Topoisomerase on Linking Number
When a 600-bp segment of DNA in a relaxed circular-covalently-closed plasmid changes from B-DNA to Z-DNA and is then treated with an enzyme that has type IB topoisomerase activity, the value of the linking number remains unchanged (c) Remains unchanged). Type IB topoisomerases are known for their ability to relieve supercoiling pressure by breaking and rejoining one strand of the DNA duplex. This introduces or removes supercoils, thereby adjusting DNA topology to facilitate processes such as replication or transcription. However, this process does not alter the overall number of times one strand wraps around the other (the linking number) when the ends of the DNA are covalently closed, as is the case with plasmids.