In the R- tube, which statement is correct about plasmid configurations and gel migration?

In the R- tube, which statement is correct about plasmid configurations and gel migration?

• A. The plasmid can exist in all three configurations, and the supercoiled configuration migrates fastest.
• B. The plasmid is always supercoiled and migrates the slowest.
• C. The plasmid only exists as a nicked circle, which migrates the fastest.
• D. The plasmid forms a single linear fragment that dominates migration.

Answer: (A)

The key idea is how plasmid topology affects how DNA moves through a gel. Plasmids can exist in three common forms: supercoiled, nicked/open circular, and linear. The supercoiled form is highly compact, so it navigates the gel matrix more easily and moves the fastest. The nicked circular form is relaxed and bulkier, so it migrates more slowly. The linear form lies in between in mobility. Because a plasmid sample can include all three configurations, the statement that the plasmid can exist in all three configurations and that the supercoiled form migrates fastest is the best description of what typically happens in gel migration. The other options misstate either the variety of forms, or the relative speeds of the conformations.

The key idea is how plasmid topology affects how DNA moves through a gel. Plasmids can exist in three common forms: supercoiled, nicked/open circular, and linear. The supercoiled form is highly compact, so it navigates the gel matrix more easily and moves the fastest. The nicked circular form is relaxed and bulkier, so it migrates more slowly. The linear form lies in between in mobility. Because a plasmid sample can include all three configurations, the statement that the plasmid can exist in all three configurations and that the supercoiled form migrates fastest is the best description of what typically happens in gel migration. The other options misstate either the variety of forms, or the relative speeds of the conformations.