Which sequences in the genomes of some prokaryotes act as a genomic record of previous viral attacks?

The CRISPR sequences in the genomes of some prokaryotes serve an important function as a genomic record of previous viral attacks. CRISPR, which stands for Clustered Regularly Interspaced Short Palindromic Repeats, consists of short, repetitive DNA sequences that are found in the genomes of bacteria and archaea.

When a prokaryotic cell is invaded by a virus (bacteriophage), it can integrate segments of the viral DNA into its own CRISPR array. This incorporation allows the organism to "remember" the virus, as these sequences can serve as a genetic memory. In the event of a subsequent viral attack, the prokaryote can transcribe the relevant CRISPR sequences into RNA, which then guides an enzyme (usually Cas protein) to target and cut the viral DNA, effectively providing immunity against that specific virus.

This adaptive immune system is crucial for prokaryotes as it allows them to survive and defend against frequent viral infections, illustrating how CRISPR sequences function not only in defense but also as a historical record of past infections. Other options like plasmids, transposons, and operons play different roles in genetic variation, mobility, or regulation, but they