Figure 1 : Type II CRISPR-Cas System of Streptococcus pyogenes.

shows three important steps: acquisition, expression, and interference by the CRISPR-Cas system of Streptococcus pyogenes. The Cas operon, CRISPR array, and tracrRNA encoding bacterial genomic regions are shown. The tracrRNA is transcribed from the minus strand and CRISPR-array from the plus strand. After transcription, the processing of pre-crRNA involves the base pairing between a 25 nucleotide sequence of tracrRNA and the complementary repeat region of the pre-crRNA transcript which triggers the cleavage by the housekeeping ribonuclease RNAse III. Cas9 is required specifically in the production of mature crRNAs. Once mature crRNAs are formed, they direct Cas 9 for cleavage of target DNA. (Fig1 is adapted from ref 6 and 17).

Kawalpreet K Aneja Journal of Eye and Ophthalmology  2020 7:1DOI : 10.7243/2055-2408-7-1