Homology-directed repair (HDR)

Definition

Homology-directed repair (HDR) is a precise DNA repair mechanism that cells use to fix double-strand breaks in DNA by using a homologous sequence as a template. This process is vital for maintaining genomic integrity and allows for accurate repair, which is essential in genetic engineering applications like gene editing. HDR takes advantage of the natural cellular machinery to ensure that the repaired DNA matches the original sequence, minimizing errors that could lead to mutations.

5 Must Know Facts For Your Next Test

1. HDR is most efficient during the S and G2 phases of the cell cycle when sister chromatids are available as templates for repair.
2. This mechanism plays a crucial role in genetic engineering, allowing scientists to introduce specific mutations or insert genes with high precision.
3. HDR is less common than non-homologous end joining (NHEJ), which often leads to imprecise repairs and potential genomic instability.
4. The process of HDR requires several key proteins, including Rad51, which facilitates the search for homology and strand invasion.
5. Researchers can enhance HDR efficiency in cells through methods like using small molecules or optimizing conditions for gene editing.

Review Questions

• How does homology-directed repair compare to non-homologous end joining in terms of precision and efficiency?
  • Homology-directed repair (HDR) is more precise than non-homologous end joining (NHEJ) because it uses a homologous sequence as a template to ensure accurate restoration of the original DNA sequence. While HDR results in fewer mutations and maintains genomic integrity, it is less efficient and only occurs during specific phases of the cell cycle when sister chromatids are present. In contrast, NHEJ can occur at any time but often leads to insertions or deletions at the break site, making it less reliable for accurate DNA repair.
• Discuss the importance of template DNA in the homology-directed repair process and its implications for genetic engineering.
  • Template DNA is crucial in homology-directed repair as it provides the necessary sequence information required for accurate repair of double-strand breaks. In genetic engineering, researchers often supply synthetic template DNA with desired modifications alongside CRISPR-Cas9 or other cutting methods to facilitate precise gene editing. The successful incorporation of these changes relies on HDR using the provided template, allowing for targeted modifications that can produce desired traits or correct genetic disorders.
• Evaluate strategies that can be employed to enhance the efficiency of homology-directed repair in gene editing applications.
  • Enhancing the efficiency of homology-directed repair (HDR) in gene editing can be achieved through various strategies. One approach involves using small molecules that promote HDR over NHEJ by modulating specific cellular pathways. Additionally, optimizing transfection conditions or employing delivery systems can increase the availability of both the CRISPR-Cas9 components and template DNA. Researchers are also investigating gene editing tools that improve HDR rates by altering the design of guide RNAs or enhancing the activity of proteins like Rad51, which are pivotal in facilitating homologous recombination.

Related terms

CRISPR-Cas9:

A powerful tool for gene editing that uses a guide RNA and a Cas9 enzyme to create precise cuts in the DNA, which can then be repaired by mechanisms like HDR.

Non-homologous end joining (NHEJ): An alternative DNA repair pathway that joins broken DNA ends together without a template, often leading to insertions or deletions and is less precise than HDR.

Template DNA: A DNA strand that serves as a guide for the HDR process, providing the necessary information to accurately repair the broken double-strand DNA.