Question

CRISPR/Cas9 technology is powerful but can produce unintended consequences beyond simply failing to produce a desired modification. Answer the following questions about unintended consequences.

a. What is one unintended consequence that may arise when allowing cells to repair Cas9 cuts by non-homologous end joining?

b.What is one unintended consequence that might arise when attempting to use a repair template to repair Cas9 cuts by homologous recombination?

c.What is one unintended consequence that might arise from careless use of gene drive technology?

Answer 1

CRISPR/Cas9 is a genome editing tool derived from adaptive immunity of bacteria against invading bacteriophages. Using the guide RNA in this system we can target any region of genome and the Cas9 enzyme brings about the cut in the target site. Then the damaged DNA is repaired by cell's own DNA repair mechanism. But this tool has its own limitations as follows.

a. When non-homologous end joining is used to repair the cut made by the Cas9 enzyme a certain length of DNA may become deleted. As in this case the two broken ends are simply joined to each other without replacing the lost nucleotides.

b. When homologous recombination is used to repair the cut made by the Cas9 enzyme, a DNA template is used to replace the lost nucleotide but complex modifications such as unexpected genomic rearrangements takes place around the target site

c. Most gene drive technologies suffer from one of the serious consequences called off-target effects i.e. modification in the non-desirable non-target sites. Such effects can be eliminated by the careful analysing of the target genome for any similar sequences to the target sequences.