Structure and Function within Forensic Science DNA Structure and Function within Forensic Science

DNA Structure Deoxyribonucleic acid DNA provides the instructions for all proteins DNA is the blue-print for your phenotype. The DNA cannot be used directly to make proteins it must be re-formatted as such: DNA  mRNA  Proteins

DNA Structure Nucleotides are the building blocks of DNA Sugar- Deoxyribose Phosphate group Nitrogenous Base- Adenine Guanine Cytosine Thymine

DNA Structure Sugar and Phosphate bond to form the backbone of DNA

DNA Structure Pyrimidines Purines Pairing Thymine Cytosine Adenine Guanine Pairing A—T G—C

DNA Structure DNA is shaped in a double helix The double helix is formed by the two strands being antiparallel Antiparallel: Each DNA strand runs side-by-side, but in opposite directions 5’ to 3’ 3’ to 5’ The two strands are held together by hydrogen bonds between the nitrogenous bases

DNA Use in Forensic Cases Most are rape cases (>2 out of 3) Looking for match between evidence and suspect Must compare victim’s DNA profile Mixtures must be resolved DNA is often degraded Inhibitors to PCR are often present

Human Identity Testing Forensic cases -- matching suspect with evidence Paternity testing -- identifying father Historical investigations Missing persons investigations Mass disasters -- putting pieces back together Military DNA “dog tag” Convicted felon DNA databases

Sources of Biological Evidence Blood • Semen • Saliva • Urine • Hair • Teeth • Bone • Tissue

DNA Collection & Comparison Analysis of DNA?? Uses various DNA Technologies RFLP PCR STR Mitochondrial DNA Analysis

RFLP: Restriction Fragment Length Polymorphism Analyzes variable lengths of DNA fragments by cutting them out using a restriction enzyme Forensic scientist take out a sequence of 15 to 35 bases and analyze repeats throughout DNA helix The repeats are found using an engineered complimentary sequence (man-made replication) One of the original applications of DNA analysis. Not used as much anymore because it requires a large quantity of DNA sample and samples degraded by the environment do not work well with RFLP

PCR: Polymerase Chain Reaction Replaced RFLPs test Used to make millions of exact copies of DNA from a biological sample DNA polymerase is used to synthesize a region of DNA Steps: Heat DNA Cool DNA and add primers Reheat and add DNA polymerase which rebuilds the double stranded DNA Repeat these steps 28-32 times to engineer a much larger and complete DNA strand Must be very careful about contamination in this process

STR: Short Tandem Repeat Most successful and widely used today Evaluates specific regions (loci) within nuclear DNA Looks at the repeating letters found in our DNA (GAT GAT GAT) which are useful as markers throughout the DNA Very useful when dealing with DNA that has undergone degradation FBI uses 13 standard specific STR regions for CODIS

Mitochondrial DNA Analysis Used for samples that cannot be analyzed using RFLP or STR Uses DNA extracted from mitochondrion rather than nuclear DNA Especially useful in old cases and old samples