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Old 12-03-2017, 09:04 PM
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https://ensia.com/notable/how-crispr-works/
"Less often emphasized is that CRISPR can also be used to add new genes or parts thereof. The key here is understanding what happens after Cas9 makes its cuts.

Genome editing with site-specific nucleases
A Cas9-caused break in DNA can be repaired in four different ways, two of which open the door to inserting a new gene of choice. Image courtesy of Elsevier


The cell’s DNA repair machinery typically takes over in one of two different modes. In the first mode (called “non-homologous end joining,” or NHEJ), it usually glues the two pieces back together, but imperfectly, deactivating the gene (see “a” above). Such “gene knockouts” don’t involve any foreign DNA but can eliminate traits that affect food quality, confer susceptibility to diseases or divert energy away from valuable end products such as grain or fruit. Occasionally, say researchers, this pathway may leave a DNA cut with “sticky ends,” enabling foreign genes of interest to be directly spliced in (b) — a double-stranded DNA insertion somewhat akin to “old-fashioned” genetic engineering.

A second kind of repair (called “homology-directed repair” or “homologous recombination” — HR) is much less common but far more accurate. In HR, the cut ends aren’t just jammed back together; the cell machinery copies a nearby piece of DNA to fix the damaged sequence. By providing a DNA snippet of their choice, scientists can induce the cell to fill in any desired sequence, from a small mutation (c) to a whole new gene (d). This HR pathway, says Fuguo Jiang, a postdoctoral fellow in Doudna’s lab, is not yet fully understood. But, as this illustration shows, it involves a meticulous process of one strand of donor DNA being stitched into the host gene, providing the template for cellular repair."

This is easier to understand
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