Using Cas-sgRNA system, you will be able to produce cuts which will be probably joined by NHEJ. This may not result in the correction of the mutation. If you provide a template along with the cut, then specific homology directed repair mechanism will be initiated which may have a better chance of correcting the mutation.

Thanks for reply. I may not have been precise - I've meant Cas9 nickase which should cleave only one strand . This would still be corrected with NHEJ?

Single nick would probably result in base excision repair mechanism which will be precise and will repair based on the template i guess. Double nicks would stimulate NHEJ and therby indels will be formed right??

Marek, you probably think that the wild type copy can be used as a template for correcting the transversion on the other allele.  But unless your CRISPR can distinguish the wild type and mutant allele, both alleles will be cut. Even if you do make such a specific CRISPR, having extra copies of wild type sequence in the donor DNA should largely enhances the correction rate. 

Now I realize how silly that question was. And I guess I can't make CRISPR as specific to distinguish alleles differing by one basepair. Can you suggest any other approach? I need to correct a heterozygous mutation. It' sufficient it it works only in vitro - cells with corrected gene are more viable 

So you just design a crispr near the mutation site. The crispr will cut both alleles (wild type and mutant) and you just need to cotransfect a donor DNA with correct seqeuence (make silent mutations to destroy the crispr recognition site) or ssDNA oligos as the donor.

sorry this is not  the answer, I am  in trouble editing the gene and need help.  I am trying to do single cell mutation (knockin) using Addgene plasmid, pSpCas9 (BB)-2A-GFP.   I have zero knowledge in gene editing. could anybody there help me how I can go about it. The mutation is on the exon, do I have to use single gRNA or two. Using single gRNA, can I use single strand oligo as a donor template. I read in google that in such case, one may need to modify the donor dna sequence not ot have have the crispr site by silent mutaiotns, is it true? I dont want to manipulate the gene except changing one nucleotide.

You can use single gRNA and ssDNA oligos as the donor. Yes, you must destroy the crispr site in your donor sequence.

Thanks Wang, ignore my knowledge in this subject please.... does single gRNA cause double strand break right? Do you mind briefing me how this works.

Single gRNA can still make DSBs because the Cas9 cuts both strands. People like to use two gRNAs together with mutated Cas9 (only has nickase activity, that is cut single strand) to reduce the off-target effect. Addgen has a nice introduction page and collection of all kinds of Cas9 and gRNA plasmids, which is a good place for you to learn some details.

Thanks Wang, I will see that. I am trying to find out whether it is possible to introduce a single nucliotide change (point mutation) using WT Cas 9 and single stand oligo donor template.