Question

What is CRISPR? How does it work? How can it be used to insert a piece of DNA at a specific location in the genome?

Answer 1

Clustered regularly interspaced short palindromic repeats (CRISPR) is a segment of DNA containing short repetitions of Base sequences involved in the defence mechanism of prokaryotic organisms to viruses.

Palindromic repeat means sequence of nucleotides is same in both directions.

Working : These regions are composed of short DNA repeats and spacers. When virus infects bacterium, new spacer from virus is incorporated among existing spacers. CRISPR sequence is transcribed and processed to generate short CRISPR RNA molecules.

The CRISPR RNA associates and guides bacterial molecular machinery to a matching target sequence in invading virus.

Thus the molecular machinery destroys the invading viral genome.

CRISPR - Cas9 also called CRISPR system consists of two key molecules that introduce specific DNA sequence into the genome.

1. Enzyme called Cas9 which is a molecular scissor that can cut two strands of DNA at specific location in the genome, so that new gene can be added or removed.

2. Pieces of RNA called guide RNA(gRNA) which consists of small piece of pre-designed RNA sequence (about 20 Bases long) located within longer RNA scaffold.

The scaffold part binds to DNA and the pre-designed sequence guides Cas9 to right part of genome and hence desired sequence is inserted into genome.