DNA Evidence

You need to know  What is DNA.  What is the purpose of DNA.  Where is DNA located in the cell.  What are some sources of DNA at a crime scene.

What is DNA? What does DNA stand for? What does DNA do? DNA contains genetic information. DNA codes for the proteins our bodies make that are necessary for survival. Deoxyribonucleic Acid

What is DNA? DNA is a code for making proteins AGC TAG CTT ATA CTC TAT CTC TTT Amino Acid Amino Acid Amino Acid Amino Acid Amino Acid Amino Acid The order of amino acids determines what type of protein is made.

What is DNA? Some common proteins are: Hemoglobin - carries oxygen from lungs to cells Insulin - regulates metabolism Many types of enzymes - catalyze reactions in the body, such as the breakdown of sugar for energy DNA also determines how much of these proteins each cell makes.

What is DNA? Among humans, most of the 3 billion bases in the DNA sequence are exactly the same. Our Human DNA is 99.8% similar to each other, but the 0.2% difference is more than enough to distinguish us from one another. NO TWO PEOPLE HAVE IDENTICAL DNA* *except identical twins

Where is DNA?

DNA is found in the cells in our body. Nucleus (Brain of the cell) Mitochondria (more later)

Where is DNA? All types of cells in our body contain a copy of the same DNA. Some cells important to forensic science are: White Blood Cell Sperm CellCheek Cell

Where is DNA? What are sources of DNA at a crime scene? Blood Semen Saliva Tissue Bone Teeth Hair Maggot Crops DNA can be recovered from any substance that contains cells.

Maggot Crop

Forensic DNA Analysis

Collection of Evidence Types of Unknown Samples: Blood, Semen, Stains, Saliva Hair, Tissue, Bones, Teeth Types of Known Samples: Blood or buccal swabs from suspect or victim or other known person

Forensic DNA Analysis Packaging Evidence Package each item individually. Put evidence into paper bags, not plastic. Moisture degrades DNA; air dry samples. Keep samples at room temperature and out of sun.

Forensic DNA Analysis Beware of Contamination Contamination occurs when DNA from another source gets mixed in with the sample being collected. An investigator touches, sneezes, bleeds on a sample. Wear gloves and use disposable instruments Package items separately. Especially, do not mix known samples (from victim or suspect) with unknown samples.

Forensic DNA Analysis Short Tandem Repeats (STRs) Individual identification possible Samples: Blood stains, semen Mitochondrial DNA Used in cases of severely degraded DNA Individual identification not possible Samples: Bones, hair shafts Two main types of analyses (90s - Present) :

Forensic DNA Analysis Basic Steps in Analysis Extraction: Separates DNA from sample Separation: Separates amplified fragments according to size. Amplification or PCR: Amplifies small portions of DNA (STR regions)

The Thermal Cycler Amplifies DNA

FMBio Separates Amplified DNA

Forensic DNA Analysis Color image of gel

Forensic DNA (mitochondria) Mitochondria - The powerhouse of the cell. Mitochondria Mitochondria have their own DNA

Forensic DNA (mitochondria) Nuclear DNA vs. Mitochondrial DNA Double Helix One copy per cell Multiple copies in each mitochondria Multiple mitochondria in each cell One Ring 46 Chromosomes MtDNA used for old or degraded samples

DNA and Statistics The final result is presented as a statistic. Do Say: “The chance that another person has this DNA in the bloodstain is 1 in 300 billion.” Do not say: “The DNA in the bloodstain is John Doe’s DNA.”

  DNA usage