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Deoxyribonucleic Acid (DNA) Unit 8A Forensic Science

Unit 8A: DNA – Contents Structure of DNA DNA Typing Collecting and Packaging Biological Evidence Isolating DNA DNA Sequencing: PCR DNA Sequencing: Gel Electrophoresis CODIS

1. Structure of DNA Like fingerprints, DNA is unique to each individual  individual evidence! DNA can definitively link a suspect to a victim or crime scene. The primary unit of DNA is called a gene Each gene contains instructions that control our genetic traits

1. Structure of DNA double helix DNA (deoxyribonucleic acid) is a molecule comprised of repeating units called nucleotides A nucleotide consists of Sugar-phosphate backbone Deoxyribose sugar Phosphate Nitrogenous base adenine, guanine, cytosine, thymine Adenine bonds only to thymine, and guanine bonds only to cytosine The order of bases is our genetic code.

1. Structure of DNA Four types of bases are associated with the DNA structure: adenine (A), guanine (G), cytosine (C), thymine (T).

1. Structure of DNA The long strands of DNA are coiled upon themselves into shapes called chromosomes When paired, chromosomes resemble the letter X Humans have 22 pairs of autosomal (standard) chromosomes, and 1 pair of 2 sex chromosomes Females are XX Males are XY

Unit 8A: DNA – Contents Structure of DNA DNA Typing Collecting and Packaging Biological Evidence Isolating DNA DNA Sequencing: PCR DNA Sequencing: Gel Electrophoresis CODIS

2. DNA Typing DNA typing (a.k.a. DNA Fingerprinting) was developed by British geneticist Sir Alec Jeffreys in 1984. This technique converts DNA into readable bands on a gel With these bands, we can compare suspect and crime scene DNA, or child and possible father, etc.

2. DNA Typing: Short Tandem Repeats (STRs) A common method of DNA typing There are locations (loci) on a chromosome that contain short segments of 3 – 7 bases that repeat themselves STR’s are less susceptible to degradation (breaking down) and can be recovered from bodies or stains that have been subject to extreme decomposition With the technology of PCR one can extract and amplify a combination of different STR’s.

2. DNA Typing: Short Tandem Repeats (STRs) Serve as useful markers for identification because they are found in great abundance throughout the human genome. What is important to understand is that all humans have the same type of repeats, but there is tremendous variation in the number of repeats each of us have.

Unit 8A: DNA – Contents Structure of DNA DNA Typing Collecting and Packaging Biological Evidence Isolating DNA DNA Sequencing: PCR DNA Sequencing: Gel Electrophoresis CODIS

3. Collecting and Packaging Biological Evidence At the crime scene, the following are to be collected and sent to the forensic laboratory: Victim’s clothing Fingernail scrapings Head and pubic hairs Blood Other sources of DNA

3. Collecting and Packaging Biological Evidence Other possible sources of DNA Skin Semen Sweat Urine Blood Hair (root) Mucus Ear Wax Saliva Vaginal or rectal cells Tissue

3. Collecting and Packaging Biological Evidence Photograph evidence first Wear gloves at all times Package each stained article separately in paper or a well-ventilated box (to avoid bacterial or fungal growth) Remove dried blood using a sterile swab moistened with distilled water Store biological evidence in the refrigerator or a cool location until it is delivered to the lab

Unit 8A: DNA – Contents Structure of DNA DNA Typing Collecting and Packaging Biological Evidence Isolating DNA DNA Sequencing: PCR DNA Sequencing: Gel Electrophoresis CODIS

4. Isolating DNA Most DNA extraction procedures consist of two parts: A technique to lyse (cut open) the cells and expose the DNA An enzyme or chemical wash to remove contaminates such as: proteins, RNA, or macromolecules

Unit 8A: DNA – Contents Structure of DNA DNA Typing Collecting and Packaging Biological Evidence Isolating DNA DNA Sequencing: PCR DNA Sequencing: Gel Electrophoresis CODIS

5. DNA Sequencing: PCR

5. DNA Sequencing: Polymerase Chain Reaction (PCR) A technique for making many copies of a specific piece of DNA to be analyzed forensically Can amplify very minute quantities of DNA millions of times! This method works by cycling through different temperatures for each part of the process A device called a thermocycler controls the temperatures, allowing for fast and accurate copying of DNA

5. Polymerase Chain Reaction (PCR) d. The steps of PCR Step 1: Denaturation. Extracted and purified DNA is heated to “unzip” (separate) the double helix This is done at high temperature, about 94°C

5. Polymerase Chain Reaction (PCR) d. The steps of PCR Step 2: Annealing. Short template pieces called “primers” bind with the separated strands for new DNA to build upon. This occurs at ~65°C

5. Polymerase Chain Reaction (PCR) d. The steps of PCR Step 3: Extension/Elongation. Taq Polymerase, a DNA building enzyme, adds free nucleotides from the surrounding solution onto the template primers In this way, new strands are built out of the original 2 separated stands This happens at 72°C new DNA strands

5. Polymerase Chain Reaction (PCR) Each step only requires a few minutes The thermocycler machine cycles through these temperatures for several hours Each cycle doubles the number of copied DNA strands PCR is specific to your “region of interest.” Different primers will selectively amplify different genes

Unit 8A: DNA – Contents Structure of DNA DNA Typing Collecting and Packaging Biological Evidence Isolating DNA DNA Sequencing: PCR DNA Sequencing: Gel Electrophoresis CODIS

6. Gel Electrophoresis

6. Gel Electrophoresis DNA can be visualized through the process of electrophoresis In the lab, DNA molecules are cut by restriction enzymes into fragments of various sizes. Restriction enzymes cut at specific sequences throughout the DNA. The resulting fragments are forced to move along a gel-coated plate under the influence of an electrical field.

6. Gel Electrophoresis The DNA molecules have a slight negative charge, so they are attracted to the positive end of an induced electric field. DNA fragments are separated by size Larger fragments move more slowly Smaller fragments move farther along the gel

6. Gel Electrophoresis After the fragments have “migrated” across the gel, the gel can be stained to show the “bands” or fragments easily Then comparisons can be made Example: crime scene sample to suspect

Unit 8A: DNA – Contents Structure of DNA DNA Typing Collecting and Packaging Biological Evidence Isolating DNA DNA Sequencing: PCR DNA Sequencing: Gel Electrophoresis CODIS

7. Combined DNA Information System (CODIS) CODIS maintains a database of DNA profiles from convicted offenders unsolved crime scene evidence profiles of missing persons

Resources Saferstein, Richard. Forensic Science: An Introduction. New Jersey: Pearson Prentice Hall, 2008 Saferstein, Richard. Forensic Science: An Introduction. 2nd ed. New Jersey: Pearson Prentice Hall, 2011 Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 8th ed. Upper Saddle River, NJ; Pearson Prentice Hall, 2004 http://law2.umkc.edu/faculty/projects/ftrials/clinton/lewinskydress.html http://www.trutv.com/library/crime/notorious_murders/famous/simpson/index_1.html