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The Crime

On the evening of November 1, at approximately 8:15 p.m., Jimmy Sweet entered his bedroom, walked over to his desk, and sat down at his computer. While reaching for the computer's switch he noticed, out of the corner of his eye, that one of the items on a typically well-organized shelf was out of place.

Jimmy shot across the room for a closer examination. Sure enough, the object in question had indeed been disturbed.

The object had been sealed in an air-tight package. The package was now ripped open. The object was still inside, but it was no longer in its original condition. In Jimmy's eyes, it was now worthless.

Jimmy pulled out what had been his most-valued possession— his holographic NOVA lollipop. The confectionery treat was now a sticky mess. Someone had obviously indulged him- or herself in its sugary molecules. The lollipop's holographic image had been licked away.

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The Suspects

The prime suspects in this case are Jimmy's seven sisters: Candy, Cookie, Sugar, Lolly, Honey, Brandee, and Carmela. Each one of these sisters is a notorious candy lover and is easily capable of committing this crime of confection.

The suspects have been detained. DNA fingerprints of each are available.

What You Need to Do

Fortunately for this case, a lollipop cannot be licked without leaving behind a bit of saliva.

Your task, as NOVA Lab's chief technician, is to create a DNA fingerprint from the saliva left on the lollipop. You must then use the fingerprint to single out the culprit of this crime.

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PROBES
AGAROSE GEL
RESTRICTION ENZYMES
X-RAY FILM
NYLON MEMBRANE
DNA
DEVELOPER
STEP 1:
Add restriction enzymes to DNA sample
Add restriction enzymes to DNA sample
You just added the restriction enzymes to the DNA sample. The restriction enzymes work like scissors, cutting the long DNA molecules at different locations. Where those "scissors" cut depends on the code within the DNA molecules and the code within the enzymes. For example, one type of enzyme severs DNA wherever it encounters a sequence of GAATTC. The lengths of these fragments will vary from person to person because the code for every person's DNA is different. Some fragments will be long, others short.

NEXT, STEP 2:
Pour agarose gel into tray on lab counter
Pour agarose gel into tray on lab counter
You just poured agarose gel into the tray on the lab counter. Agarose gel is a thick, porous, Jell-O-like substance. It will act as a molecular strainer, allowing smaller pieces of DNA to move through more easily than larger pieces.




NEXT, STEP 3:
Pour DNA into tray
Pour DNA into tray

You just poured the fragmented DNA into a hole, or depression, made in the agarose gel. The DNA fragments now lie within this hole in the agarose gel.




NEXT, STEP 4:
Push the "POWER" button on the tray to begin electrophoresis
Push "POWER" button on tray to begin electrophoresis
You just turned on the power, which begins electrophoresis, the process of moving molecules with an electric current. The DNA fragments have a slight negative charge, so they move toward the tray's positive end (as with magnets, opposite poles attract). The gel acts like a strainer: smaller DNA fragments travel through the gel more easily (and thus farther toward the tray's opposite end) than do longer ones. When electrophoresis is complete, the fragments are distributed in the gel according to their lengths.


NEXT, STEP 5:
Place nylon membrane on top of the gel
Place nylon membrane on top of the gel
You just placed the nylon membrane on top of the gel. Because the agarose gel is difficult to work with (have you ever tried to pick up a thin layer of Jell-O?), the DNA is transferred to a nylon membrane. The membrane looks like a sheet of paper and the DNA is absorbed into the membrane as the liquid containing the DNA fragments makes contact with it.


NEXT, STEP 6:
Add probes to the nylon membrane in the tray
Add probes to the nylon membrane in the tray
You just added probes to the nylon membrane. The probes are pieces of DNA that have been radioactively labeled. The probes attached themselves to the DNA fragments on the nylon membrane. They attached only where their code encountered a certain sequence of code among the various fragments. Excess probes (all of the material that had not attached to a DNA fragment) are washed away.




NEXT, STEP 7:
Place X-ray film on top of nylon membrane in tray
Place X-ray film on top of nylon membrane in tray
You just placed a sheet of X-ray film on top of the nylon membrane. The radioactivity from the probes, whice are now present at only a few locations on the nylon membrane, exposes corresponding areas on the X-ray film.






NEXT, STEP 8:
Develop film by dragging it to the developer
Develop film by dragging it to the developer
The X-ray film has now been developed. The film displays the locations on the nylon membrane where the probes attached themselves to the DNA fragments. This is your DNA fingerprint.

Notes on the process





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Choose the culprit
Candy
Sugar
Lolly
Honey
Brandee
Carmela
SUSPECTS
SHE'S THE ONE!
Drag the DNA fingerprint below over the suspects' DNA fingerprints to find a match.
Saliva
Congratulations!

You have correctly determined that Honey is the culprit of this crime.

Thanks to your work in the NOVA Lab, the case is solved.

CREDITS


Producer: Rick Groleau
Designers: Anya Vinokour and Tyler Howe
Developers: Brenden Kootsey and Deniel Hart



Image Credits: © WGBH Educational Foundation






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