Final answer:
The double-stranded endonuclease in the CRISPR system that mediates cutting of DNA at a site complementary to a sequence on the gRNA is Cas9.
Step-by-step explanation:
The correct answer is A) Cas9. Cas9 is a double-stranded endonuclease involved in the CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) system and is responsible for making a double-stranded break in DNA. This action is mediated by the Cas9 enzyme forming a complex with guide RNA (gRNA), which possesses a complementary 20-nucleotide sequence to a target sequence in the DNA. The gRNA directs the Cas9 to the correct location in the DNA sequence, where Cas9 cleaves the DNA.
CRISPR/Cas9 technology has enabled researchers to precisely edit genes by targeting specific DNA sequences. The process involves engineering a system consisting of Cas9 and a single guide RNA (sgRNA) that is complementary to a target region in genomic DNA. The precision of this method has revolutionized the field of genetic engineering, allowing for precise gene editing in a variety of organisms.