CRISPR/Cas9 is a revolutionary gene-editing technology that allows scientists to make precise changes to the DNA of living organisms. Originally discovered as a bacterial immune system, it has been adapted into a versatile tool for genetic research, medicine, and biotechnology.
How CRISPR/Cas9 Works
- Guide RNA Design
A short guide RNA (gRNA) is designed to be complementary to the target DNA sequence. The gRNA is approximately 20 nucleotides long and determines where the Cas9 enzyme will cut. The target sequence must be directly followed by a protospacer adjacent motif (PAM) sequence, typically NGG.
- Complex Formation
The gRNA is combined with the Cas9 protein, forming a ribonucleoprotein complex. The gRNA acts as a GPS, guiding Cas9 to the exact location in the genome that matches the gRNA sequence.
- DNA Binding and Cleavage
The Cas9-gRNA complex scans the DNA for the PAM sequence. Once found, Cas9 unwinds the DNA and checks if the adjacent sequence matches the gRNA. If it matches, Cas9 creates a double-strand break in the DNA.
- DNA Repair
The cell’s natural repair mechanisms take over. There are two main pathways:
- Non-homologous end joining (NHEJ): The broken ends are directly rejoined, often causing small insertions or deletions that disrupt the gene.
- Homology-directed repair (HDR): If a repair template is provided, the cell uses it to make precise edits or insert new genetic material.
- Verification
The edited cells or organisms are analyzed by PCR and sequencing to confirm the desired genetic modification was successful.
Practical CRISPR Experiment Design
Design the guide RNA using an online tool such as CRISPick (Broad Institute) or CHOPCHOP. Enter the target gene sequence and select the highest-ranked gRNA with minimal predicted off-target sites (low MIT score). Order the gRNA as two complementary oligonucleotides with overhangs for cloning into an expression vector (e.g., pX459 for SpCas9 and puromycin selection), or purchase an in vitro transcribed gRNA and recombinant Cas9 protein for RNP delivery. For RNP formation, mix 100 pmol of Cas9 protein with 120 pmol of gRNA in 10 µL of PBS, incubate at room temperature for 15 minutes. Deliver the RNP into 2 × 10^5 cells by electroporation using a Neon or Lonza system with the recommended program for the cell type. After 48 hours, extract genomic DNA and PCR-amplify the target region (300–600 bp centered on the cut site). Sanger sequence the PCR product and analyze the chromatogram using ICE (Inference of CRISPR Edits) or TIDE software — these tools decompose the mixed traces to quantify editing efficiency (indel percentage) and identify the most common edits. A typical successful experiment yields 30–80% editing efficiency in HEK293T cells. For HDR-mediated precise editing, co-deliver a single-stranded oligodeoxynucleotide (ssODN) repair template (100–200 nt, with 50 nt homology arms on each side) at 10 pmol per reaction.
Real-World Application
In knockout studies of the TP53 gene in HCT116 cells, a gRNA targeting exon 4 is designed via CRISPick. RNP electroporation achieves 65% editing efficiency by ICE analysis. Single-cell cloning isolates a homozygous knockout clone confirmed by Sanger sequencing showing a 2 bp deletion causing a frameshift and premature stop codon. Western blot confirms absence of p53 protein.
