What is a genome editing? Why is there a need to edit a genome? Explain the methodology of editing using CRISPR-Cas9.
Genome editing is a set of techniques or technologies that can be used to change the DNA of an organism by adding, removing or altering the specific DNA sequence of the genome. It is a biotechnological adaptation of a naturally occurring genome editing system present in bacteria.
There is a need to edit a genome because knock-in and knock-out technology helps in the generation of animal/plant models to understand the molecular basis and development of new drugs for the disease. In nature, bacteria use this system to memorise the DNA signature of past viral attacks so that subsequent viral infections can be countered effectively.
The methodology of editing using CRISPR-Cas9 begins with designing and producing a guide RNA along with Cas9 endonuclease in the cell. The guide RNA is required for positioning the Cas9 endonuclease at the location where DNA needs to be edited. One part of guide RNA is complementary to the target DNA sequence, while the other part attracts and binds to Cas9 endonuclease.
Once the complex of guide RNA bound to target DNA and Cas9 is assembled, a double strand break is introduced in the DNA by the endonuclease. The broken DNA undergoes template-based homologous DNA repair where a modified DNA template with desired changes is provided. Consequently, the repaired genomic DNA possesses an edited DNA sequence. Gene knock-out can be created using deletion or insertion of DNA sequence, while gene editing introduces mutations at specific places through homologous recombination.
Explanation
This answer covers all three parts of the question as per NCERT context: (1) Definition of genome editing from the textbook, (2) Need for genome editing including generation of animal/plant models and drug development, and (3) Step-by-step methodology of CRISPR-Cas9 including guide RNA design, Cas9 positioning, double strand break, and homologous DNA repair. The answer uses key terms like 'knock-in', 'knock-out', 'guide RNA', 'Cas9 endonuclease', 'double strand break', and 'homologous DNA repair' which are explicitly mentioned in the textbook context.