What is DNA? DNA is… Deoxyribonucleic Acid The inherited genetic material that makes us what we are

Characteristics of DNA DNA is inherited from parents (half from mother, half from father). No two individuals have the same DNA profile; except for identical twins. Genes have multiple forms. This variation is the basis of forensic DNA typing.

Human Genome ~3 billion base pairs of DNA 30,000-35,000 genes Population-each gene has multiple forms Allelic variation-basis of forensic DNA typing Dozens of polymorphic loci validated for forensic use

One set of 22 autosomes (plus X) One set of 22 autosomes (plus X or Y) Two alleles* for each autosomal genetic marker Paternity Testing

DNA in the Cell chromosome cell nucleus Double stranded DNA molecule Individual nucleotides

Base Pairing of DNA Strands A = T G  C T = A A = T C  G T C C A G G T A G C T = A C  G A = T G  C 5’ 3’ 5’ 3’ 5’ 3’ 5’ denatured strands hybridized strands Hydrogen bonds C  G G  C Phosphate-sugar backbone Butler, J.M. (2001) Forensic DNA Typing, Figure 2.2, ©Academic Press

Forensic DNA Analysis Evidentiary DNA profile(s) are generated from samples submitted to Forensic Lab. Known profile(s) of suspect/victim (blood or buccal) are compared to DNA profiles from instant case. Evidentiary profiles entered into CODIS database. Suspect’s profile is not entered into CODIS database.

Locard’s Principle of Exchange Anytime there is contact between two surfaces, there will be a mutual exchange of matter across the contact boundary

Sources of Biological Evidence Blood Semen Saliva Urine Hair Teeth Bone Tissue All cells – except RBC

Other Possible items for DNA Testing: 1. cigarette butts 2. gloves, bandanas, ski masks, baseball caps general clothing 3. condoms (inside vs. outside) 4. stains on furniture, pillows, sheets 5. hair clips, lipsticks 6. letters, envelopes, and stamps 7. plant and animal sources of evidence

Transfer of DNA Evidence Locard’s Theory Biological fluids –Saliva –Semen –Blood Epithelial (Skin) Cells –Touch DNA –Fingerprints Primary vs. Secondary Transfer –Dependent upon the source of DNA

How much do you need? What we need today “Touch DNA”

What is Touch DNA? 1.DNA left on evidence by the “handler”. 2.Usually no visible stain. 3.Usually very small amounts of DNA are deposited. 4.Significant increase in number of touch DNA cases in recent years.

From Hands: Gloves Knife handles Weapon handles Firearm grips Plastic bag handles Steering Wheels Rope Shoe laces Electrical cords Common Sources of Touch DNA

Common Sources of Touch DNAcontinued… Common Sources of Touch DNA continued… Wearer: Baseball caps Sweatbands Shirt/jacket collars Socks Waistbands of pants Eyeglasses * Mixtures v. Single Source

TOUCH DNA EVIDENCE : WHAT DOES IT MEAN…….? Shows linkage or association but…. DNA recovered from an object may not be from the last person to touch it. Factors include:DNA recovered from an object may not be from the last person to touch it. Factors include: -Length of contact -Good cell shedder or not -Vigorous contact vs. incidental DNA profiles recovered from touch evidence are often mixtures – multiple individualsDNA profiles recovered from touch evidence are often mixtures – multiple individuals -Elimination known(s) -Lawful owner -Crime scene personnel, officers

VNTRs and STRs Variable Number Tandem Repeats: portions of DNA that are repeated– there are many of these regions. The number of repeats is inherited, and different # of repeats  different alleles STRs are specific VNTRS of a small length that are used in DNA profiling.

Short Tandem Repeats (STRs) Repeat number varies between alleles. PCR primers bind to flanking regions that are constant. 7 repeats 8 repeats AATG Homozygote = Two copies of same allele. Heterozygote = Two different alleles.

STRs (Shown in their positions on chromosomes) These are the loci that are used in forensic DNA analysis.

Activity: Who Robbed The Bank? Given the base pair sequences for a hair from a glove used in a crime scene, along with the sequences for 3 suspects, determine which suspect(s) could have left the hair.

PCR Example 15 STR Markers Can Be Amplified in 1 reaction. Sensitivity = less than 250 pg of DNA. Ability to Handle Mixtures and Degraded Samples. Different Fluorescent Dyes Used to Distinguish STR Alleles with Overlapping Size Ranges.

- Fluorescent dye tags on the primers Fluorescent STR Analysis -Visualize emitted light with a digital camera. -Collect and analyze data with computer.

Capillary Electrophoresis A. Capillary Array B. Polymer Delivery Pump C. Pump block D. Buffer reservoirs E. Detection Block (camera) A. B. C. D. E.

Forensic DNA Analysis