1. DNA Analysis
Target 3-2

2. Target 3-2
I can analyze DNA evidence by sequencing steps and interpreting results.

3. What might contain DNA evidence?
Blood
Sweat
Skin cells
Hair (root only)
Saliva
Some clothing may have skin cells or sweat (rough textures – cotton, wool, etc.)

4. Collecting DNA Evidence
DNA is at high risk of contamination!
Wear gloves and masks
All DNA evidence is stored in dry paper (never plastic!) bags – damp evidence can cause mold to form
Keep all DNA evidence separated and labeled

5. Steps for Analyzing DNA Evidence
Purification
Extraction
Amplification
Electrophoresis
Comparison

6. 1. Purification
We need to purify a sample so that it contains only the DNA-containing material
Ex) Pulling skin cells or sweat out of a t-shirt
Ex) Filtering blood out of dirt
We use solvents and filters to do this

7. 2. Extraction Extract the DNA out of the cells in the sample
Separate cells in solution
Break open cell and nucleus (lysis)
Break protein bonds
Separate DNA from solution

8. 3. PCR Amplification
PCR analysis breaks down and copies DNA in the lab for analysis
DNA is heated to separate into 2 half- strands
DNA is treated with enzymes that trim the half-strands and rebuild them with complementary base pairs
Cycles repeat to make millions of copies of the trimmed DNA

9. When heated, the strand splits
3. PCR Amplification
DNA must be heated to separate before it can make a copy
1 original DNA strand
When heated, the strand splits

10. 3. PCR Amplification
Enzymes look for specific sections of DNA to trim each strand based on repeating sections of filler (STRs).

11. X X X X X X 3. PCR Amplification
The DNA fragments will have different lengths based on the number of STR repeats. X X X X X X

12. 3. PCR Amplification
PCR Animation

13. 4. Electrophoresis Separation
After PCR, the DNA is separated into bands based on the length of STR repeats
Remember – different people will have different lengths of the STR repeats!
We use an electric current in a special gel chamber to separate the bands

14. 4. Electrophoresis Separation
DNA travels from (-) to (+).
DNA is negatively charged
It is repelled by the (-) and attracted by the (+)
The shortest bands travel the farthest because they can weave through the gel faster.

15. 4. Electrophoresis Separation

16. 5. Comparison Analysis
Electrophoresis bands from DNA evidence are compared to control samples from victims, suspects, etc.
We need 13 bands to confirm a DNA match.

17. Which samples are a match? Remember: 13 bands must match!
5. Comparison
Which samples are a match? Remember: 13 bands must match!

18. Practice! Which of the following items could contain DNA evidence?
Bloody fingerprint
Hairs found at the crime scene
Skin cells found under a victim’s fingernail
Leather jacket
Soil left in tires
Latent fingerprint left on a table

19. Practice!
Match the steps of DNA analysis to their descriptions: ___ 1. Extraction ___ 2. Purification ___ 3. PCR Amplification ___ 4. Electrophoresis Separation ___ 5. Comparison a. copy and cut DNA b. find evidence c. compare samples d. separate bands e. isolate DNA from the rest of the evidence

20. Practice!
Summarize how PCR and electrophoresis analysis work together to create DNA samples that we can easily compare.

21. Practice!