HDR stands for Homology-Directed Repair and is used to achieve what?

• A. Random mutations.
• B. Non-homologous end joining repair.
• C. Base editing.
• D. Precise edits using donor DNA guided by homologous sequences.

Answer: (D)

HDR is the process that uses a matching DNA template to repair a double-strand break with high fidelity, allowing precise edits. When a break is made, the cell can use a donor DNA fragment that has sequences matching the regions just upstream and downstream of the cut. These homology arms guide the repair so the exact sequence from the donor is copied into the genome, enabling precise insertions, replacements, or corrections. This is how scientists achieve targeted changes like inserting a tag, correcting a mutation, or swapping in a new sequence at a specific site.

In contrast, non-homologous end joining tends to join the broken ends directly and often introduces small insertions or deletions, leading to random or disruptive changes rather than precise edits. Base editing is a different approach that changes individual bases without relying on a donor template or creating a full double-strand break. HDR is especially used when precise, template-directed changes are needed.