Figure 1.

Steps in CRISPR-Cas mediated acquired immunity.

Although there exist countless variations in the exact proteins involved between different prokaryotic organisms, this process involves 3 distinct phases. In the adaptation phase, phage DNA fragments (protospacers) are uptaken and integrated by Cas1-Cas2 complexes into CRISPR loci. Each CRISPR is an alternating series of repeats and fragments of DNA derived from past phage infections (spacers). In response to phage reinfection, these CRISPR-arrays are then expressed and the corresponding long RNA transcripts (pre-crRNA) cleaved into individual CRISPR RNAs (crRNAs). These then complex with interference complexes, which degrade the phage genome in a sequence specific manner dictated by the spacer-derived guide RNA (gRNA) segment of the crRNA.