Practice question

When two loxP sites reside on same DNA molecule in inverted orientation, Cre recombinase still forms tetrameric synapse but alignment is antiparallel, causing exchange to flip intervening segment rather than delete it. Cleavage in spacer region and rejoining in opposite direction inverts DNA between sites, retaining both loxP sites on chromosome with reversed orientation of internal sequence. This principle underlies generation of invertible gene traps, switching cassettes, and orientation-specific lineage tracing. Inversion is reversible because sites remain, allowing repeated flipping if Cre persists.