This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: AIIMS 2019, Subject name: Biochemistry
In CRISPR-Cas 9 system, which repair mechanism is used for genome editing? 
Select and explain. 
A. Non homologous end repair, B. Homologous repair, C. Mismatch repair, D. Nucleotide excision repair
A: Non homologous end repair
Answer: A: Non homologous end repair 
Explanation: * CRISPR-Cas-9 endonuclease system introduces double stranded breaks in DNA at the specific target site. Now cell tries to repair this double stranded breaks by two mechanism: 1. Non-homologous end repair 2. Homologous repair * Non-homologous repair mechanism is always a possibility as it does not require homologous DNA. * The homologous repair will only work if the DNA homologous to the cut DNA is present but homologous DNA is not always present or available. So, if both options are given, best option is Non-homologous repair.