DNA

DNA evidence is biological material that contains deoxyribonucleic acid (DNA), which is the genetic material that is found in all living cells. DNA evidence can be used to identify individuals, as each person has a unique DNA profile. DNA can be found in blood, saliva, skin cells, and more. This exercise teaches you how to collect DNA evidence at a crime scene.

Objectives

Recognize DNA left at a crime scene

Collect DNA

Preserve DNA

Label DNA using evidentiary materials

Materials

Cotton swabs

Water

Medicine syringe (dropper)

Envelopes (standard)

Clear packing tape

Personal smartphone with camera

Permanent marker

Fake blood*

PPE (Suggested)

Procedures

1. Prepare the Crime Scene: Set up mock DNA evidence using the fake blood. Prepare three types of mock DNA evidence for collection: (1) a wet “bloodstain”; (2) a dried “bloodstain”; and (3) very small stains (less than 1 mml) created by splattering the “blood.” It is recommended you choose surfaces that can be easily cleaned. To create the stains, use a medicine dropper or syringe to make droplets.

2. (Suggested) Put on PPE: To avoid contaminating the mock evidence and protect clothing, skin, and eyes, put on PPE including gloves, coveralls, headcover, and eye protection.

3. Identify DNA Evidence: Carefully examine the mock DNA evidence.

4. Photograph the DNA Evidence: Take photographs of the DNA evidence. This will document what it looked like before it is potentially disturbed, as well as its location.

5. Collect DNA Evidence: Collect the three types of DNA evidence as follows:

   • Wet “bloodstain”: Holding two cotton swabs together, carefully gather as much of the wet bloodstain on the cotton tips of the swabs by twirling them in the sample. Avoid over-swabbing as it can dilute the sample. Allow the swabs to fully dry; elevate the sampled parts of the swab so you do not lose any potential DNA material.

   • Dried “bloodstain”: Holding two cotton swabs together, apply one to two drops of water to the cotton tips. Gently dab the wettened ends carefully over the dried bloodstain. Collect as much of the sample as possible. Avoid over-swabbing as it can dilute the sample. Allow the swabs to fully dry; elevate the sampled parts of the swab so you do not lose any potential DNA material.

   • Very small stains: Take one cotton swab and apply a single drop of water to the very end of the cotton tip. Using the wettened end, rub the cotton swab back and forth over a small dried stain. Repeat this process to collect additional stains. Not all stains need to be collected. Select a few from different spots. Allow the swabs to fully dry; elevate the sampled parts of the swab so you do not lose any potential DNA material.

6. Secure the DNA Evidence: Secure the DNA evidence by sealing each swab or pair of swabs in an individual envelope. Seal the envelope by closing it and taping the envelope flap closed.

7. Label the DNA Evidence: Label each envelope with your initials, date and time of the collection, from where the DNA was obtained, and a brief description of the envelopes’ contents.

Notes

For this exercise, you may use special effects fake blood, red washable paint, or red food coloring to simulate real blood.