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- Gene editing is making precise
and permanent changes to living things
at the level of the DNA-
the A's, T's, C's, and G's that are
the source code and the
building blocks of DNA.
There are several ways to
do targeted gene editing,
but the one everybody is talking about
right now, is CRISPR-Cas9.
We've taken the natural
bacterial immune system
and tweaked it to be a highly precise pair
of molecular scissors that can be directed
to any particular sequence
of A's, T's, C's, and G's.
CRISPR-Cas9 has a few ingredients:
a guide RNA, the Cas9 enzyme,
the DNA that we want to change,
and a short DNA that has the edit we want.
We synthesize a guide RNA that contains
the sequence that we want to edit.
For example, the DNA sequence that causes
a disease like sickle cell anemia.
We have to have the
DNA in the living thing
that we want to edit.
For example, a chromosome
in a human sperm or egg.
We mix these things with Cas9,
an enzyme that's shaped like a hand.
I just happen to have Cas9 on me.
This is Cas9.
Cas9 grabs the guide RNA,
and then starts searching
the chromosomal DNA for a match.
When the guide RNA
matches the DNA sequence,
Cas9 finds it and cuts
the chromosomal DNA.
So Cas9 is both the hand and the scissors,
and the guide RNA guides the hand
to the right place to cut.
If we want to knock out a gene-
make it not work anymore-
we can stop right there.
But if we want to edit the gene,
we add one more ingredient:
DNA that contains the
sequence we want to add in.
Adding that template DNA to the mix tricks
the cell into repairing its DNA
with the new sequence, so in this case,
the sequence that is healthy instead of
the old sequence, which we
said was sickle cell anemia.
Now the cell's chromosome
has the edited DNA sequence,
and will pass it on to every
other cell when it divides.
If you've edited a sperm or an egg,
then every cell in the embryo,
and eventually the person,
will carry the new DNA sequence.
So is this easy?
Not really, but it's a lot easier
than any other way we've
had to edit DNA so far.
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