Non-homologous end joining and homology-directed repair
– These are two different ways that cells can repair double-strand breaks (DSBs) in their DNA, which can be caused by various factors such as radiation, chemicals, or CRISPR-Cas9 editing.
🔅 NHEJ
– Non-homologous end joining (NHEJ) is a fast and simple way of repairing DSBs, but it is also error-prone and can introduce mutations. NHEJ works by directly ligating the two broken ends of the DNA without the need for a homologous template.
– This means that the DNA sequence at the break site is not restored to its original state, but rather joined together in a random way.
– Sometimes, this can result in small insertions or deletions of nucleotides, which can alter the reading frame of the gene and cause a frameshift mutation. Other times, this can result in larger deletions or translocations of DNA segments, which can disrupt the function or regulation of genes.
– NHEJ is the dominant pathway for DSB repair in mammalian cells, especially in non-dividing cells where a homologous template is not available.
🔅 HDR
– Homology-directed repair (HDR) is a more precise and accurate way of repairing DSBs, but it is also less efficient and requires a homologous template.
– HDR works by using a template DNA molecule that has a similar sequence to the broken DNA to guide the repair process. This means that the DNA sequence at the break site is restored to its original state or modified according to the template.
– HDR can be used to insert, delete, or replace specific DNA sequences in a controlled manner, which makes it useful for gene editing applications. However, HDR is not readily accessible in non-dividing cells, as it requires the presence of a sister chromatid or an exogenous donor DNA molecule to provide the template.
– HDR also competes with NHEJ for DSB repair, and its efficiency depends on various factors such as cell cycle stage, DNA damage response, and delivery method of the template.
