CRISPR systems are diverse and several different types have been identified, but they have a number of common components.
The “library” of CRISPR systems is a DNA array that alternates between short repeat regions and “spacer” sequences that come from past invaders. The name CRISPR comes from an early description of these arrays: Clustered Regularly Interspaced Short Palindromic Repeats.
Genes that code for the proteins involved in CRISPR are called cas (CRISPR associated) genes, usually located near the CRISPR array.
The spacers in the CRISPR arrays are transcribed and processed into crRNAs that are each used to match a single spacer.
Invading genetic sequences are targeted and destroyed by CRISPR complexes. These complexes all contain at least one protein and a crRNA but are highly diverse, and different CRISPR systems are often distinguished by their complexes. In some systems the CRISPR complex has multiple protein subunits, while others have a single protein. Some also have additional components, such as the Type II CRISPR/Cas9 system’s tracrRNA.
