Since the discovery of DNA's fundamental role in building
and sustaining life, scientists have dreamed of having the
ability to easily edit DNA in very precise ways. But why would
they want to do this? Well, making specific changes
to DNA sequences can help scientists better understand
the function of certain genes, produce specific disease models,
or even repair defective genes that cause diseases in humans.
This is an exciting prospect, but methods to try and do
this weren't practical or widely-applicable. However, a few years
ago, a gift from biology came from the basic research of
the bacteria immune system, which gave scientists the ability
to easily customizable and precisely edit genomes.
Bacteria evolved ingenious ways of protecting themselves
against pathogens such as viruses by using a system
called CRISPR/Cas. CRISPRs are stretches of DNA
sequence found in the bacterial genome in close proximity
to CRISPR, are the Cas genes, which encode proteins
necessary for the CRISPR system. Up until a few years
ago, what was known about the CRISPR/Cas system
is that bacteria infected by a virus incorporate elements
of the virus's DNA into the CRISPR sequence, this protected
the bacteria from future infection by this virus. Scientists
observed that when a virus invades a bacterium, the CRISPR
DNA produces one or two small RNAs called crRNA and
tracrRNA. These RNAs bound to Cas proteins and formed
complexes that cut the DNA of the invading virus. Thus
protecting the bacteria from infection. But many questions
still remained, how did the small RNAs and Cas work together
to detect and destroy viral DNA? In 2012, a group of scientists
made a major breakthrough and discovered not only how the
CRISPR RNAs and Cas cut DNA, but also how to create a
new technique to specifically change the DNA sequence of
any organism with great ease. This discovery came from
a group of scientists led by Jennifer Doudna at UC Berkeley,
and Emmanuelle Charpentier at Umei University in Sweden.
They published their results in Science, in an article titled
"A programmable dual-RNA guided DNA endonuclease
in adaptive bacterial immunity. So what exactly
" did these scientists find, and why is it so important for
future biomedical research. First, the scientists dissected
how Cas9 and the two RNAs can cut DNA. They found that
the two RNAs, crRNA here in red, and tracrRNA here in green,
pair up, recruit Cas9 protein, and direct it to bind the target
DNA, via the complementary base pairing between the
red CRISPR RNA and the target DNA. Once at the proper DNA
site, Cas9 cleaves both DNA strands. This cleavage occurs
at a very specific and conserved position that is dictated
by the sequence in the red CRISPR RNA molecule.
This process is similar to finding your plane by matching the gate
number to the one on your boarding pass. The scientists then
wondered if they could engineer one RNA molecule that mimicked
the structure of the CRISPR RNA and tracrRNA bound together.
that would guide Cas9 to cut DNA at a specific location.
The scientists designed one RNA molecule that consisted
of the red and green RNA molecules connected together
by a hairpin structure. In this case, they engineered the RNAs
to target specific sequences of the gene encoding the green
fluorescent protein, GFP. They added the engineered RNA
molecules to the GFP DNA sequence, along with the Cas9
protein. And asked whether Cas9 would cut GFP DNA
at specific sequences. And it did!
When the RNA molecules were designed to bind to different
regions of the GFP sequence, the GFP DNA sequence
was cleaved at that specific location. On a gel that separates
DNA according to size, you could see distinctly sized fragments
of the GFP DNA molecule resulting from having been cut
in a specific location. This was huge, as it meant that scientists
could engineer one RNA sequence, introduce Cas9, and
cut DNA at a specific location of their choice.
By having this simple and easily programmable system,
scientists can now induce breaks in the DNA at precise
locations. When the cell tries to repair the broken DNA
strands by ligating them back together, it often causes
a small insertion or deletion that changes the DNA
sequence. Scientists can take advantage of this process
to add or remove specific DNA sequences at the site
of the break. We now have a DNA word processor
that can be used to change genome sequences, including
our own. CRISPR has a bright future ahead, to advance our
understanding of human disease by creating a tool for
basic research that is now widely used in the fields of
molecular biology and genetics to change the genomes of
any organism. But much work needs to be done to make this
system more reliable. Use of the CRISPR system to edit
human embryos is also a controversial issue. And researchers
have already started the conversation about using this technology
ethically and safely to advance human knowledge.
This video has been provided to you by Youreka Science
and iBiology, bringing the world's best biology to you.