Practice question

CRISPR arrays consist of alternating direct repeat sequences and unique spacers derived from prior encounters with mobile genetic elements. Bioinformatic comparisons demonstrate spacer origins matching bacteriophage genomes, plasmids, and transposons containing protospacer adjacent motifs required for interference. These stored fragments constitute molecular chronology of infection history, with new spacers added proximal to leader sequence and older spacers retained distally perhaps losing function. Transcription of entire array and processing into crRNAs allows sequence-specific recognition of returning invaders, enabling targeted nucleolytic destruction. Thus spacers are not random DNA but records enabling adaptive recall of pathogenic sequences encountered throughout lineage evolutionary trajectory.