Discover 4

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IN THE LAB

IN NATURE

April 2018^ DISCOVER^47

TOP: WILLIAM ZUBACK/DISCOVER. BOTTOM: ALISON MACKEY/DISCOVER

the

basics

Together, they seek out the target DNA sequence, encoded in the guide RNA.

A CRISPR array can be

recognized by short bits

of repeating, partially

palindromic DNA

sequences. (A palindrome

is a string of letters that

reads the same backward

and forward, e.g. CAATAAC.)

Targeting tool + Editing tool

Gene is disabled New gene is inserted

DNA

Single guide RNA Cas9 enzyme

DNA sequence

Virus

Virus DNA

Bacteria

Sandwiched between each

repeat is a spacer containing

a sample of genetic code from

previous viral invaders. The next

time the same virus attacks, the

spacer will be used as a search-

and-destroy template for the

bacteria’s CRISPR system.

The CRISPR-Cas9 system has two main components:

The cut DNA can now be edited.

or

When the target is located, Cas9

unwinds the DNA. If it matches the

guide RNA, Cas9 cuts the DNA.

Certain bacteria have an adaptive immune

system. Each time a virus attacks one of

these types of bacteria, a snippet of the

invader’s genetic code is stored away in the

bacteria’s DNA for future reference in a

sequence called a CRISPR array.

CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats. It’s part of a natural bacterial defense

system that scientists are now using to cut DNA more precisely than any previous method of genetic engineering.

Here’s a simplified look at how it works.

Scientists realized they could

repurpose this system and

program it to seek out any

string of genetic code.

SOURCES: Innovative Genomics Institute, Harvard Medical School