Gene editing is a precise form of genetic engineering that allows scientists to make targeted changes to the DNA sequence of an organism. Unlike traditional genetic modification, which often inserts whole genes randomly, gene editing can:
Insert a specific DNA fragment at a predetermined locus.
Delete a defined segment of DNA.
Replace an existing sequence with an alternative sequence.
How Gene Editing Works
The process relies on molecular tools that recognise a specific DNA sequence and create a double‑strand break (DSB). The cell’s own repair mechanisms then repair the break, and the desired change is introduced during this repair.
Target recognition – a guide molecule (e.g., RNA or protein) binds to the target site.
Induction of a double‑strand break – nucleases such as Cas9, TALENs or ZFNs cut the DNA.
Repair pathway selection – the cell repairs the DSB by either non‑homologous end joining (NHEJ) or homology‑directed repair (HDR).
Insertion, deletion or replacement – the desired DNA change is introduced during HDR, or small insertions/deletions arise from NHEJ.
Key Repair Mechanisms
Repair Pathway
Typical Outcome
Relevance to Gene Editing
Non‑Homologous End Joining (NHEJ)
Random small insertions or deletions (indels)
Used to create gene knock‑outs by disrupting the reading frame.
Homology‑Directed Repair (HDR)
Precise insertion or replacement using a donor template
Enables targeted insertion of a new gene or correction of a mutation.
Common Gene‑Editing Tools
CRISPR‑Cas9 – RNA‑guided nuclease; highly versatile and easy to design.
TALENs (Transcription Activator‑Like Effector Nucleases) – protein‑based DNA binding domains fused to a nuclease.
ZFNs (Zinc‑Finger Nucleases) – zinc‑finger DNA‑binding domains linked to a nuclease.
Example: Correcting a Point Mutation
Consider a disease‑causing point mutation in the \$\beta\$‑globin gene (HBB). Using CRISPR‑Cas9:
Design a single‑guide RNA (sgRNA) that binds to the mutant allele.
Introduce Cas9‑sgRNA complex and a single‑stranded DNA donor carrying the correct base.
Cas9 creates a DSB at the target site.
During HDR, the cell uses the donor template to repair the break, replacing the mutant base with the normal one.
The net result is a precise replacement of the defective DNA segment without affecting other parts of the genome.
Advantages of Gene Editing over Conventional Genetic Engineering
Target specificity reduces off‑target effects.
Ability to make small, precise changes (single nucleotides) rather than inserting large transgenes.
Potential for therapeutic applications such as correcting genetic disorders.
Ethical and Safety Considerations
While gene editing offers powerful possibilities, it raises important questions:
Potential unintended off‑target mutations.
Germline editing and heritable changes.
Regulatory frameworks and public acceptance.
Suggested diagram: Schematic of CRISPR‑Cas9 mediated gene editing showing guide RNA, Cas9 cleavage, and HDR‑mediated insertion of a donor template.
Summary
Gene editing is a sophisticated form of genetic engineering that enables the insertion, deletion, or replacement of DNA at defined genomic locations. By harnessing cellular repair pathways, scientists can achieve precise modifications, opening avenues for research, agriculture, and medicine.