CRISPR 101: Cytosine and Adenine Base Editors

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A cartoon overlayed on several crystal structures showing the parts of the prime editor: the Cas9 nickase domain, the reverse transcriptase domain, and the pegRNA.
Alt text: A schematic illustration of the CRISPR/Cas9-based approaches mentioned above. The first illustration (A) shows a DSB made at a transcription start site upstream of an exon. The second illustration (B) shows a DSB made at splice sites. The third illustration (C) shows a DSB made upstream and downstream of an exon to fully remove a genomic fragment. The fourth illustration (D) shows a DSB made within an exon to insert a synthetic polyadenylation signal. 
Overview of the parts of CRISPR. The bacterial chromosome encodes a tracrRNA (in some systems including Cas9), Cas proteins, and a CRISPR array. The CRISPR array is composed of identical repeat sequences and variable spacer sequences. The array is transcribed and processed into crRNAs, each including one repeat and one spacer. In bacteria, these crRNAs are bound by Cas proteins (Cas9 shown here). The repeat sequence base pairs with the tracrRNA, and the spacer sequence is used to target complementary DNA sequences. In laboratory settings, an sgRNA includes the crRNA and tracrRNA sequences in a “single-guide RNA” that performs both functions. Cas9 cuts both the target and nontarget DNA strands upstream of the PAM site found in the nontarget strand.
Cartoon summary of Cas9 activity.
Person shooting at target with bow and arrows but missing

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