Generally CRISPR/ Cas uses NHEJ for joining DNA fragments. In this method addition or deletion can appear. Is there any method to get desired addition or deletion in all the cells of T1 generation.
Short answer: No, unfortunately not.
However, you can increase your chances of getting the intended modification by HR in various ways. Mainly, you need to offer a repair template in molar excess. This is often an oligonucleotide or plasmid sequence.
Impossible until now, insertion or deletion is decided by cell’s endogenous repair mechanisms in random way. If you want to insertion desired sequence you can utilize HR as the first answer.
NHEJ is currently unpredictable, and it is not possible to exactly know what final genotype you are going to get after a double stranded break caused by CRISPR-Cas9.
However, as mentioned by the two other researchers, you can supply a repair template, containing your desired mutation. This is called Homology Directed Repair, or HDR. There are a large number of manuscripts on this, in many different organisms
Recently ,paper about increasing HR efficiency by Cas9-induced DSB and suppress NHEJ related protein expression .You can find the paper ,I believe you can learn more
It is very tough but not impossible, also not with NHEJ, repair mechanism will be HDR (Homologus Directed Repair) but even than it is not guaranteed to be 100% of expected. In this cases rather than knockout it will be like knock-in as briefly explained by Ryan above, you have to deliver an additional fragment (depicting your desired sequence) while delivering gRNA (probably two gRNAs in this case) and Cas9 into your target cells. Also, it will require high level of expertise to carry it out.
Will suggest you to read more on knock-in using CRISPR/Cas9 system.
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