Donor Template Design

Donor Template Design encompasses the principles and strategies for constructing the DNA molecules used as repair templates during homology-directed knock-in of desired sequences ¹.

How It Works

Donor templates provide the genetic information that cells incorporate at a nuclease-induced break site via HDR. The two main formats are single-stranded oligodeoxynucleotides (ssODNs) for small edits (point mutations, short tags) and double-stranded DNA plasmids for larger insertions (fluorescent reporters, selection cassettes, full gene cassettes).

Key design parameters include homology arm length, which must be sufficient for efficient strand invasion (typically 30-80 nt for ssODNs, 500-2000 bp for plasmids). Asymmetric arms, where the PAM-proximal arm is shorter, can improve efficiency by aligning with the strand first released after Cas9 cleavage. The donor should also be designed to prevent re-cutting after integration, either by mutating the PAM or introducing silent mutations in the guide target sequence.

Strand selection matters for ssODNs: using the non-target strand (complementary to the guide RNA) generally yields higher knock-in rates. Additional considerations include avoiding strong secondary structures in the donor and placing the desired edit close to the cut site, as HDR efficiency decreases with distance from the break ¹.

Computational Considerations

Automated design tools optimize donor architecture by evaluating homology arm thermodynamics, predicting secondary structures, and calculating cut-to-edit distances. These tools also verify that the designed donor eliminates the original guide target site to prevent re-cleavage after successful integration ².

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