Genome Engineering: Cut Site, Repair Choice and Editing Outcome
In Cas9 cutting-based editing, guide pairing and a compatible PAM position a DNA break. Repair pathways and available templates shape the resulting allele. Compare possible outcomes and judge what sequence and functional tests can establish.
What a learner can do afterwards
- Explains how guide pairing and a compatible PAM position Cas9 cutting, and distinguishes an indel, a frameshift and loss of gene function.
- Explains how cell-cycle state and donor design affect donor-directed repair without guaranteeing the intended allele.
- Assesses possible off-target cutting and limits conclusions to the sites tested and the assay's detection limit.
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You have seen how guide pairing and a compatible PAM help Cas9 select DNA. They position the break. Repair pathways and any template used during repair then shape the resulting allele, a version of the DNA sequence. Delivering the cutter does not specify that allele. This lesson concerns Cas9 cutting and repair, not every kind of CRISPR editor.
End joining can restore DNA exactly or leave insertions or deletions, called indels. In coding DNA, an indel shifts the reading frame when its net size is not a multiple of three. Deleting three bases preserves the frame, but may still affect the protein. A frameshift can disrupt a gene; a confirmed knockout also needs evidence of lost function. Larger DNA changes can occur too.
Homology-directed repair (HDR) can copy a chosen change from a matching donor. In many mammalian cells, HDR is favoured in S and G2, but end joining still competes. A supplied donor can be the template; the designed edit need not come from a sister chromatid. The donor needs matching flanks and the intended change. A change near the break can help. Suitable arm lengths depend on donor format and cell type. No donor guarantees a knock-in.
Similar DNA beside a compatible PAM may be cut off-target. For commonly used Cas9 systems, mismatches far from the PAM are often better tolerated than mismatches in the PAM-near seed region. This is a tendency, not a rule for every sequence. Longer Cas9 exposure can increase off-target cutting. Chromatin accessibility can affect access, but an accessibility map alone does not predict editing success.
A delivery-marker signal reports the marker, not the target allele. Check the intended DNA sequence and relevant gene function. A short sequencing assay may miss larger DNA changes. For off-target tests, report which sites were examined and the detection limit. A negative result cannot exclude rarer events or changes at untested sites. Sequencing alone does not establish overall safety.
Guide and PAM position the break. Repair and templates shape the allele. Tests support conclusions within their coverage and detection limits.
Lesson, questions, diagrams and flashcards written for this dataset. DNA examples are illustrative.
Readings and flashcards written for this dataset. Linked videos: How CRISPR lets you edit DNA, by TED-Ed; Introduction to genetic engineering, and Applications of DNA technologies, by Khan Academy.
More to read
Different alleles after repair
A Cas9 break does not specify one repaired allele. End joining can restore the original DNA or leave insertions and deletions. Larger changes can also occur. A short sequencing assay around the expected break may therefore miss some outcomes.
In coding DNA, an indel shifts the reading frame when its net size is not a multiple of three. Deleting one codon preserves the frame, yet can still affect the protein. A sequence change, a frameshift and a functional knockout are different claims. A sequence test addresses the DNA claim; a claim of lost function also needs relevant functional evidence.
Sequence recognition and DNA access
Guide pairing and a compatible PAM help Cas9 recognise a target. Similar sequences elsewhere beside compatible PAMs can raise concern about unintended cutting. For commonly used Cas9 systems, PAM-near mismatches are often less tolerated than PAM-distant mismatches. The effect still depends on the sequence and system.
Recognition is also affected by the experimental setting. Chromatin can affect access to DNA, and delivery affects which cells receive editing components. An accessibility map supplies context about the sampled cells. It is not a direct measurement of Cas9 cutting and cannot by itself establish editing efficiency or the resulting allele.
A DNA result and a treatment result
Gene addition can supply a functional gene without correcting the original allele. Cutting-based genome editing instead creates a break at a selected site and relies on repair to produce an outcome. Depending on the purpose, that outcome might disrupt a sequence or incorporate a donor-directed change.
A confirmed DNA change does not by itself demonstrate a treatment benefit. Which cells were reached, how they function and what unintended effects occurred are separate questions. Evidence from sampled cells also has limits. A result about an edited allele should not be presented as proof of lasting benefit or overall safety.
Donor information and test limits
A donor for homology-directed repair contains the intended change and matching sequence around the target. Keeping the change near the break can help incorporation. Suitable matching-arm lengths vary with donor format and cell type. Supplying a donor does not prevent competing repair or guarantee that every cell acquires the intended allele.
Verification should examine the target rather than rely on a delivery marker. The claim may also require tests for larger DNA changes or altered gene function. Off-target results need stated site coverage and detection limits. A negative result means that the test detected no event under those conditions, not that all possible changes were excluded.
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Where it sits
Learn first
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.