Genome Editing with CRISPR-Cas9

contains a copy of our genome, over 20,000 genes, 3
billion letters of DNA. DNA consists of two strands,
twisted into a double helix and held together by
a simple pairing rule. A pairs with T, and G
pairs with C. Our genes shape who we are as
individuals and as a species. Genes also have profound
effects on health, and thanks to advances
in DNA sequencing, researchers have identified
thousands of genes that affect our risk of disease. To understand how
genes work, researchers need ways to control them. Changing genes in living
cells is not easy, but recently a new
method has been developed that promises to
dramatically improve our ability to edit the DNA of
any species, including humans. The CRISPR method is based on a
natural system used by bacteria to protect themselves
from infection by viruses. When the bacterium detects
the presence of virus DNA, it produces two types of
short RNA, one of which contains a sequence that matches
that of the invading virus. These two RNAs form a complex
with a protein called Cas9. Cas9 is a nuclease, a type
of enzyme that can cut DNA.

When the matching sequence,
known as a guide RNA, finds its target within
the viral genome, the Cas9 cuts the target
DNA, disabling the virus. Over the past few years,
researchers studying the system realize that it
could be engineered to cut not just viral DNA but
any DNA sequence at a precisely chosen location by changing the
guide RNA to match the target. And this can be done
not just in a test tube, but also within the
nucleus of a living cell. Once inside the nucleus,
the resulting complex will lock onto a short
sequence known as the PAM. The Cas9 will unzip the DNA
and match it to its target RNA. If the match is
complete, the Cas9 will use two tiny molecular
scissors to cut the DNA. When this happens, the cell
tries to repair the cut, but the repair process
is error prone, leading to mutations that
can disable the gene, allowing researchers to
understand its function. These mutations are random,
but sometimes researchers need to be more
precise, for example, by replacing a mutant
gene with a healthy copy.

This can be done by adding
another piece of DNA that carries the desired sequence. Once the CRISPR
system has made a cut, this DNA template can
pair up with the cut ends, recombining and replacing
the original sequence with the new version. All this can be done
in cultured cells, including stem cells
that can give rise to many different cell types. It can also be done in a
fertilized egg, allowing the creation of
transgenic animals with targeted mutations. And unlike previous
methods, CRISPR can be used to target
many genes at once, a big advantage for studying
complex human diseases that are caused not by
a single mutation, but by many genes
acting together.

These methods are being
improved rapidly and will have many applications
in basic research, in drug development,
in agriculture, and, perhaps eventually,
for treating human patients with genetic disease. [MUSIC PLAYING] .

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