This is SARS-CoV-2, the virus behind the current
COVID-19 pandemic. Like all viruses, it is
made up of genetic material wrapped inside
a capsule of protein and fat. SARS-CoV-2 is
uniquely dangerous because it infects the
entire respiratory system and is contagious
even before symptoms show. one important step
in fighting COVID-19 is establishing widespread
testing in our communities. However, this
is no easy task, because most tests are too
expensive, too long, or too difficult to do
on a massive scale. However, recently researchers
have developed a CRISPR-based rapid diagnostic
which can make testing for COVID-19 cheaper,
faster, and more accessible.
CRISPR is a tool scientists have adapted from
bacteria which allows us to accurately and
cheaply edit the genomes of humans, bacteria,
and even viruses. Let’s say we want to edit
the genome of a bacteriophage, a type of virus.
We need to call upon an essential partnership:
A Cas enzyme, usually Cas 9, and a strand
of guide RNA, which can be engineered by researchers.The
guide RNA matches and complements a specific
part of the DNA or RNA within the virus. Then
the partners are sent into the viral genome.
Genomes are like very messy libraries, which
makes it difficult for scientists to accurately
locate a specific gene. Cas9, however, can
easily navigate the genome and locate specific
genes.
First Cas9 tries to find a match to the guide
RNA by looking for a matching PAM sequence.
The PAM sequence acts almost like the first
digits of a book’s Dewey decimal code. By
skipping over genes without the correct PAM,
Cas9 saves a lot of time.
Once a gene with the correct PAM sequence
is located, Cas9 unravels the DNA further
to compare the guide RNA to the gene’s DNA.
If a match is found, Cas9 has found the right
gene! Then Cas9 snips the DNA in two like
a pair of molecular scissors, separating the
strand.
Then the bacteriophage tries to repair the
damage to its DNA. However, usually it doesn’t
do a very good job and the resulting DNA is
unreadable This results in the entire gene
being unusable and “turned off”. But turning
genes off is not all CRISPR can do.
Sometimes instead of cutting the DNA, Cas9
brings along special proteins who will edit
only a specific nucleotide, or even an engineered
replacement to patch the cut DNA. Cas9 can
even bring along a fluorescent molecule that
marks the specific location of a gene like
a glowing bookmark.
In fact, this fluorescence is exactly what
Sherlock uses to detect the presence of COVID-19.
First Cas13 is given a guide RNA that corresponds
to a gene specific to SARS-CoV-2, and then
attached to a suppressed fluorescent molecule.
Then Cas13 is placed in a respiratory sample.
Cas13 searches through the cells, looking
for a match to the guide RNA. If it finds
a match, the Cas13 enzyme yields or wakes
up the fluorescent molecule, which visually
alerts readers to the presence of SARS-CoV-2
in the sample, like in this test. This is
a photo from a SHERLOCK-based test.
Compared to alternatives, CRISPR diagnostics
like this are faster, cheaper and easier to
do. That means that this test and others like
it can lead to more widespread testing.
The development of CRISPR-based testing is
a powerful advancement, and may even help
us against other viruses in the future. Thank
you for watching.
