D1.3.9 Use of the CRISPR sequences and the enzyme Cas9 in gene editing (HL) Notes

How Does CRISPR-Cas9 Work?

1. Guide RNA (gRNA): A short RNA sequence guides Cas9 to the specific DNA target.
2. Cas9 Enzyme: Acts as molecular scissors, cutting the DNA at the target site.
3. DNA Repair: The cell repairs the break, allowing scientists to:
   1. Disable a gene (knockout).
   2. Insert a new gene.
   3. Correct a mutation.

Steps of CRISPR-Cas9 Gene Editing

1. Design the gRNA: Scientists create a gRNA complementary to the target DNA sequence.
2. Introduce the System: The gRNA and Cas9 are delivered into the cell.
3. Target Recognition and Cutting:
   1. The gRNA binds to the target DNA.
   2. Cas9 cuts both DNA strands at the specified location.
4. DNA Repair:
   1. Non-Homologous End Joining (NHEJ): Repairs the break but often introduces small errors, disabling the gene.
   2. Homology-Directed Repair (HDR): Uses a template to insert or correct DNA sequences.

Example: Treating Sickle Cell Disease

1. Sickle cell disease is caused by a single base mutation in the HBB gene, leading to abnormal hemoglobin.
2. CRISPR-Cas9 has been used to:
   1. Cut the mutated gene in stem cells from a patient’s bone marrow.
   2. Introduce a corrected version of the gene via HDR.
   3. Reintroduce the edited cells into the patient, producing healthy red blood cells.

Ethical Considerations of CRISPR

1. Human Germline Editing: Editing embryos could prevent genetic diseases but raises concerns about unintended consequences and "designer babies."
2. Biodiversity and Ecosystems: Using CRISPR to control pests (e.g., mosquitoes) might disrupt ecosystems.
3. Equity and Access: Will gene-editing therapies be accessible to all, or only to those who can afford them?

International Regulation

1. Diverse Regulations: Countries have varying laws on gene editing.
2. Need for Harmonization: Scientists advocate for global guidelines to ensure safe and ethical use of CRISPR.

Reflection and Future Directions

1. CRISPR-Cas9 has transformed biology, offering solutions to genetic diseases and agricultural challenges.
2. However, its use must be guided by ethical considerations and robust regulations.