CRISPR-Cas9 is a gene-editing technology that lets scientists target specific DNA sequences and change them with high precision. It matters in medicine because many diseases begin with errors in DNA, the instructions cells use to make proteins. By cutting DNA at a chosen location, CRISPR can help researchers disable harmful genes, repair mutations, or add useful genetic information.
This makes it a powerful tool for studying disease and developing new treatments.
Key Facts
- CRISPR-Cas9 uses a guide RNA to locate a matching DNA sequence.
- Cas9 is an enzyme that cuts both strands of DNA at the target site.
- A 20 base guide sequence can target a matching 20 base DNA region.
- DNA base pairing rules are A pairs with T, and C pairs with G.
- Edit efficiency = edited cells / total treated cells.
- After cutting, cells repair DNA by nonhomologous end joining or homology-directed repair.
Vocabulary
- CRISPR
- CRISPR is a gene-editing system adapted from bacterial defenses that can be programmed to target specific DNA sequences.
- Cas9
- Cas9 is a DNA-cutting enzyme that acts like molecular scissors when guided to a target sequence.
- Guide RNA
- Guide RNA is a short RNA molecule designed to match the DNA sequence that scientists want Cas9 to find.
- PAM sequence
- A PAM sequence is a short DNA pattern near the target that Cas9 must recognize before it can cut.
- Homology-directed repair
- Homology-directed repair is a DNA repair process that can copy a provided template to make a precise genetic change.
Common Mistakes to Avoid
- Thinking CRISPR automatically fixes every mutation, which is wrong because Cas9 mainly makes a cut and the cell repair process determines the final edit.
- Ignoring the PAM sequence, which is wrong because many Cas9 enzymes cannot bind or cut DNA unless the correct PAM is next to the target.
- Assuming guide RNA can be any length, which is wrong because target matching usually depends on a designed guide region of about 20 bases for Cas9 systems.
- Confusing gene editing with changing every cell in the body, which is wrong because medical editing usually reaches only treated cells or tissues.
Practice Questions
- 1 A CRISPR treatment is applied to 50,000 cells, and 12,500 cells show the intended edit. Calculate the edit efficiency.
- 2 A guide RNA target region is 20 bases long. If 18 bases match the DNA target and 2 bases do not match, what fraction of the guide matches the target?
- 3 Explain why a CRISPR-Cas9 tool needs both a guide RNA and a Cas9 protein to edit DNA accurately.