CRISPR 101: Any Base Transversion Editors

By Emily P. Bentley

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The nucleobases are shown with arrows describing conversions between them. In step one, a cytosine deaminase converts cytosine to uracil; this is catalyzed directly by the base editor. Next, base excision of uracil (also by the base editor) is repaired in two different ways by the cell, shown by an arrow that splits into two outcomes. Repair in E. coli leads to adenine, while repair in mammalian cells leads to guanine.
A schematic overview of the CRISPR classification system.
A cartoon depiction of cytidine base editing. A base editor, consisting of a cytidine deaminase fused to Cas9, is shown binding to DNA using its guide RNA. The guide RNA base pairs to target DNA, leaving the opposite strand of DNA free to be contacted by the cytidine deaminase, which converts a C to a U within this single-stranded sequence. This deamination yields DNA with a G:U mismatch without creating a double-strand break. Mismatch repair preserves the edit IF the modified strand is used as the template, converting the mismatched G to an A and yielding a single-base-pair edit.
Diagram of the Cascade complex with gRNA bound to target DNA, after recruitment of Cas3. Cas3 is about to nick the non-target strand.

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