DNA Analysis Target 3-2

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

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.)

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

Steps for Analyzing DNA Evidence Purification Extraction Amplification Electrophoresis Comparison

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

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

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

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

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

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

3. PCR Amplification PCR Animation

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

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.

4. Electrophoresis Separation

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.

Which samples are a match? Remember: 13 bands must match! 5. Comparison Which samples are a match? Remember: 13 bands must match! 1 2 3 4 5 6 7 8

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

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

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

Practice!