Final answer:
In replicative transposition, a cointegrate is formed when the transposase holds the transposon ends together and catalyzes DNA cleavage at a new insertion site. The cointegrate is made through semi-conservative replication and is resolved through recombinational mechanisms.
Step-by-step explanation:
In replicative transposition, a cointegrate is formed when the transposase nicks and trims the DNA at the original insertion site, but does not excise the transposon completely. The transposase holds the transposon ends together while catalyzing a hydrolytic attack of DNA at a new insertion site. This results in the formation of a cointegrate structure where each transposon copy is made by semi-conservative replication. The cointegrate is resolved through recombinational mechanisms, leaving copies of the transposon at both the original site and the new insertion site.