Create DNA Finger Print

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.

NEXT >

PROBES

AGAROSE GEL

RESTRICTION ENZYMES

X-RAY FILM

NYLON MEMBRANE

DNA

DEVELOPER

STEP 1:

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

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

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

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

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

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

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

Choose the culprit