Final answer:
To always have the unit act as a composite transposon, one of the four inverted repeats must become dysfunctional. This can happen due to mutations that prevent the cut-and-paste transposition without the aid of an autonomous element.
Step-by-step explanation:
In biology, particularly genetics, a composite transposon, or Tn element, is a region of DNA that contains two insertion sequences (IS) that can mobilize the DNA found between them, including, for example, antibiotic resistance genes. If one of the four inverted repeats found at either end of such composite transposons becomes dysfunctional, the transposon would always act as a unit because it would no longer be able to carry out the cut-and-paste transposition on its own. Instead, it would require assistance from an autonomous element that provides the necessary enzymes for transposition. The loss of functionality in one of the inverted repeats could occur due to mutations or other mechanisms that might limit the mobility of the transposon within the genome.