Crispr Cas9 & Fluorescent Yeast: Genetic Engineering at Home

Crispr Cas9, the relatively new kid on the
genetic engineering block, has taken off in popularity among the non-scientific public. There are even kits you can buy for yourself
and experiment with Crispr and other more conventional genetic modification techniques. Sold by the company, The Odin, The ‘Bacterial
CRISPR and Fluorescent Yeast Combo kit’ is one such kit, with everything you need
to get started. For this Crispr experiment, a non-pathogenic
strain of E. Coli is used, and baker’s yeast, or Saccharomyces cerevisiae is used in the
other. The goal of the yeast experiment is to get
the cells to produce a green fluorescent protein, or GFP for short, so that it ‘glows in the
dark’ under blue or uv light. The goal for the E. coli Crispr experiment
is to make a precise edit to the protein rpsL, or Ribosomal subunit protein. The modified E. coli will be grown on agar
plates that contain streptomycin, an antibiotic, which normally binds to the rpsL preventing
the production of proteins by the cell’s ribosomes. Without proteins the E. coli can’t grow
and replicate. This modification now imparts E. Coli with
antibiotic resistance by preventing streptomycin from binding to the rpsL. The Yeast and E. coli are going to need a
medium to grown on and food to eat. So the first step is making LB Agar plates
for the E. coli, and YPD Agar plates for the Yeast. We will make LB Strep/Kan Agar plates for
the modified E. coli, which contains the antibiotics streptomycin and kanamycin and YPD Agar with G418, or Geneticin another antibiotic which will ensure that only the Yeast with the GFP survive. But first – wear gloves. Agar is like jello, so is the process of making
agar plates. Dump each tube of powdered Agar into the glass
bottle provided. Some sort of funnel is helpful. Pour in 150 mL of water, a half turn of the
cap, and then microwaved at 30 second intervals for a couple of minutes so the agar doesn’t
boil over. Once all the agar has dissolved and it’s
cooled down a bit, but still warm, it’s time to make plates. But don’t let it cool down too much, or
you’ll get clumps. Go slowly, and pour only enough agar to fill
the plate. Each tube is enough for about 7 plates. You only need one each, but experiments fail,
so redundancy is good. Flip the plates over when complete so the
condensation doesn’t drip into the agar. I waited a bit before doing this to make sure
the agar was solid enough. Before continuing, let the plates cool down
for an hour, or at 4 degrees Celsius you can store them in the fridge for later use. We will use the LB Agar and YPD Agar plates
to grow fresh E. coli and yeast, which increases the odds for a successful experiment. I don’t advise doing both experiments at
the same time as it can get really confusing, but for brevity’s sake, I’ll demonstrate
them together in this video. Using an inoculation loop, collect the E.
coli from the bottle along this faint white line called an agar stab. Wildly spearing the Agar is inadvisable and
ineffective. With the loop flat against the agar, gently
streak out the bacteria on the LB Agar plate. Try not to pierce the agar. Add water to the top of one of the tubes of
dried yeast. I was being cautious and used the pipette
for this. Shake the tube until the yeast has dissolved. Set the Pipette to 100 microliters and grab
a tip. Pipette out the yeast solution onto a YPD
Agar plate and carefully streak out the yeast. Allow the E. coli to grow overnight for about
12-18 hours, and the yeast for 12-24 hours. Before any genetic engineering can take place,
we need to get all the components that will do the work inside the cell. The cell walls of yeast and E. coli, by themselves,
don’t usually allow just anything in. Only by successfully making the cell’s competent
are we able to reliably get the modified DNA and the other components pass the cell walls. This process, called transformation, requires
a few components. The Yeast experiment uses Polyethylene Glycol,
or PEG, Lithium Acetate, and Single Stranded Carrier DNA derived from Salmon. The E. coli experiment uses a different concentration
of PEG, Dimethyl Sulfoxide and Calcium Chloride. These components perform several key functions
such as shielding the negative charge of both the DNA and cell wall, DNA would otherwise
be repelled, and making the cell wall more porous. The single stranded DNA in the Yeast experiment,
isn’t performing any modifications, instead it’s used to overwhelm the cell’s defenses
so the nucleases are more likely to digest them rather than the Plasmid DNA that has
the coding for the Green Fluorescent Protein. For both the E. coli and yeast transformation
mixes, pipette out 100 microliters of each and add it into a new microcentrifuge tube. One for each. Discard the tip! With a seperate inoculation loop for both,
gently scrape up some of the yeast and E. coli. Mix it into their respective microcentrifuge
tubes with the transformation mix. About two loop fulls of yeast and E. coli
should be enough. With a new tip for each, you can pipette the
mixture up and down to help the process along. Discard the tip! Mix until the liquid is opaque with no big
clumps. I’ll call these the E. coli or yeast competent
cell mixtures. Set the pipette to 10 microliters and add
a new tip. Grab the tube with the Cas9 Plasmid. Pipette out 10 microliters and add it into
your E. coli competent cell mixture. Then 10 microliters from the guide RNA tube. And a final 10 from the Template DNA tube. Use a new pipette tip for each! Put the E. coli mix in the fridge for 30 minutes. Now back to the yeast. Grab the Yeast GFP Expression Plasmid tube
and pipette out 10 microliters into your yeast competent cell mix. Discard the tip! Both of the competent cell mixes will now
go through heat shock, which is another part of the transformation process to make the
cell walls more permeable. The yeast will undergo its heat shock for
one hour in 42 degree celsius water. I improvised my own method with one of the
measuring tubes provided and a meat thermometer to prevent it from cooling down too quickly
so I wouldn’t have to leave the water running. I occasional put this tube into some hotter
water when needed when the temperature dipped too much. The E. coli only needs 30 seconds in 42 degree
water so I just used a bowl. After the heat shock it’s time to add the
food. Grab a microcentrifuge tube of LB media for
the E. coli competent cell mix, and a YPD one for the yeast. With room temperature water fill each tube
to the top. I used a pipette for the LB media, but feeling
more confident, I used the tap for the YPD media. Shake the tubes until all the media is dissolved. Pipette out 900 microliters of YPD media into
your yeast competent cell mix. Discard the tip! And 200 microliters of the LB media into the
E. coli mix. This pipette can only do 100 microliters,
so you’ll need multiple trips. And then incubate. Lacking a proper incubator, I made my own. I suppose I could have used this for the heat
shock, but the instructions said to use water, and making stuff is fun. Incubate the E. coli mix for 2 hours at 37
degrees celsius or 4 at room temperature if you don’t have an incubator. And 30 degrees for the yeast mix for 4 to
6 hours or overnight at room temperature. I made multiple competent cell mixtures for
each in case of failure. This allows the cells to recover from the
transformation process and let the engineer processes do its thing. The Yeast and E. coli experiment use different
engineering methods. E.Coli bacteria are prokaryotes, and Yeast
cells are eukaryotes. ONE of the important differences, is that
eukaryotes carry their main genetic information inside a membrane bound nucleus, while prokaryotes
have a nucleoid which is not enclosed in a separate membrane from the rest of the cell. Bacterial, as well as some other types of
cells, also have Plasmid DNA. A plasmid is a, “short circular DNA sequence”
that “can replicate independently of the bacterial chromosome.” Using our own plasmids is key to both of these
experiments. For the Yeast experiment we are going to use
a Plasmid that contains the code for a Green Fluorescent Protein, and if successful, it
will be inherited by the daughter cells and glow green under the blue light when viewed
with the yellow filter glasses in the kit. Put simply, a plasmid has a segment called
the Origin of Replication, where the host cell starts the replication process and another
that codes for Antibiotic resistance. After this round of incubation, the modified
yeast will be spread onto a YPD agar plate with G418, or geneticin, an antibiotic. If the yeast doesn’t replicate with the
GFP Plasmid, it will therefore not have the antibiotic resistance and won’t be able
to grow on the plate. Lastly, the insert, sandwiched between restriction
enzyme sites. It’s where you can insert the gene you want
replicated – the Green Fluorescent Protein for Yeast, the Cas9 protein and guide RNA
for the E.Coli experiment. CRISPR Cas9 are acronyms for, Clustered Regularly
Interspaced Short Palindromic Repeats and, CRISPR associated protein 9. The method is derived from the immune defense
system of bacteria. This strain of E.Coli has over 4 million base
pairs, and the Crispr Cas9 process will result in a change to just one of them. Here’s how CRISPR works in this experiment. The E. coli, because of the plasmids we introduced,
start producing the Cas9 protein and the guide RNA or sgRNA. The guide RNA is itself two components, trans-activating
CRISPR RNA, or tracrRNA and CRISPR RNA or crRNA. Next, the guide RNA binds to the Cas9 protein. Together, it locates the specific area of
interest on the E.coli’s genome to which the crispr RNA binds to, having the corresponding
matching base pairs. The Cas9 then cuts both strands of the DNA. The cell would then start to repair its cut
DNA. But the custom DNA that we introduced works
as a repair template, so the cell knows how to repair the cut. Our template DNA is nowhere near as large
as the genome of the E.Coli, but it’s long enough to trick the E.coli to use it as a
repair template, as it has identical base pairs on either side of the cut. The only change is one base pair, a Guanine
– Cytosine substitution for a Thymine – Adenine base pair. That one change will prevent streptomycin
from binding to the rpsL protein. This is a very simplified animation. There are much better explanations and videos
out there. Check out the links in the card above or in
the description for the video by Bozeman Science and the McGovern Institute video. Now it’s time to see if the yeast and E. coli
have been successfully engineered. On a LB Strep/Kahn plate, pipette out 100
microliters of the E. coli competent cell mixture Discard the tip! And on a YPD G418 plate, pipette out 400 microliters
of the yeast mix. Carefully streak out the E. coli and yeast
with an inoculation loop or a plate spreader. Let the plates dry for 10 minutes then put
the lids back on and flip it over. Incubate the E. coli one last time at 37 degree
celsius for 16-24 hours, or 48 hours at room temperature. And 30 degrees celsius for the yeast for 1-3
days or 2-4 at room temperature. Success…for the most part. The E. coli experiment went well, out of the
four plates I incubated only one had no growth. The yeast experiment had some odd results. I had yeast growing on all the plates, but
I only seemed to have one fluorescent yeast colony on each, especially compared to the
example plate provided by The Odin. I noticed this while the yeast was incubating
for the final time so I ran the experiment again, using up all the materials, but with
the same results. Those plates were from both. I figured that the only yeast that should
have survived are ones that replicated the plasmid with the antibiotic resistance. I decided to keep incubating the plates, and
it took a couple of weeks to get some better results. There was still just the one dominant colony,
but it appeared the others were at least somewhat fluorescent under the light. O spoke with a biochemistry professor about this and she explained that microwaving the agar may
have inactivated the antibiotic as it’s usually added into the agar after it’s heated. You can find these kits, and others on The
Odin website, links in the description.


Add a Comment

Your email address will not be published. Required fields are marked *