DNA Evidence How is DNA used to solve crimes?
T. Trimpe 1/31/12 What is DNA? What makes up DNA? Double Helix
DNA stands for deoxyribonucleic acid and contains genetic information. It is found on chromosomes located in the nucleus of our cells. What makes up DNA? The sides or backbone of the DNA molecule are made up of sugar (deoxyribose) and phosphate molecules. The rungs that form the middle of the molecule are made up of pairs of nucleotides or nitrogen bases. Adenine (A) pairs with thymine (T), while guanine (G) always pairs with cytosine (C). The order of the bases determines the genetic code. Double Helix 1/31/12 DNA Image: How is DNA used as evidence?
Only one-tenth of a single percent of DNA (about 3 million bases) differs from one person to the next. Scientists can use these variable regions to generate a DNA profile of an individual, using samples from blood, bone, hair, and other body tissues and products. In criminal cases, this generally involves obtaining samples from crime-scene evidence and a suspect, extracting the DNA, and analyzing it for the presence of a set of specific DNA regions (markers). If the sample profiles don't match, the person did not contribute the DNA at the crime scene. If the patterns match, the suspect may have contributed the evidence sample. 1/31/12 How is DNA used as evidence?
Each persons DNA is different from other people (except identical twins). DNA collected from a crime scene can either link a suspect to the evidence or eliminate a suspect, similar to the use of fingerprints. DNA can identify a victim through DNA from relatives, even when no body can be found. DNA can link crime scenes together by linking the same perpetrator to different scenes locally, statewide, and across the nation. DNA can place an individual at a crime scene, in a home, or in a room where the suspect claimed not to have been. DNA can refute a claim of self-defense and put a weapon in the suspect's hand. It can change a story from an alibi to one of consent. 1/31/12 DNA Strand Image & information: What factors affect DNA evidence?
Several factors can affect the DNA left at a crime scene, such as environmental factors (e.g., heat, sunlight, moisture, bacteria, and mold). Therefore, not all DNA evidence will result in a usable DNA profile. Further, DNA testing cannot identify when the suspect was at the crime scene or for how long. What is CODIS? CODIS stands for COmbined DNA Index System, which is an electronic database of DNA profiles that can identify suspects. DNA profiles from individuals convicted of certain crimes, such as rape, murder, and child abuse, are entered into CODIS and help officers identify possible suspects when no prior suspect existed. Did you know? Each human cell contains three billion DNA base pairs. Our unique DNA amounts to 0.1% or 3 million base pairs. 1/31/12 DNA information: Which three statements below are true?
True or False? Which three statements below are true? 1. The DNA in a man's blood is the same as the DNA in his skin cells and saliva. 2. Each person's DNA is different from every other individual's. 3. DNA can be found in all the cells in our bodies except the blood cells. 4. DNA can have forensic value even if it is decades old. 5. DNA evidence was first used to get a conviction in a trial in 1987. Watch the video segment from NOVA: "The Killer's Trail" and be ready to answer the questions on the next slide. 1/31/12 History of Forensic DNA Analysis
DNA typing, introduced in the mid-1980s, has revolutionized forensic science and the ability of law enforcement to match perpetrators with crime scenes. Thousands of cases have been closed and innocent suspects freed with guilty ones punished because of the power of a silent biological witness at the crime scene. 'DNA fingerprinting' or DNA typing (profiling) was first described in 1985 by an English geneticist named Alec Jeffreys, who found that certain regions of DNA contained DNA sequences that were repeated over and over again next to each other. He also discovered that the number of repeated sections present in a sample could differ from individual to individual. By developing a technique to examine the length variation of these DNA repeat sequences, Dr. Jeffreys created the ability to perform human identity tests. 1/31/12 Restriction Fragment Length Polymorphism
These DNA repeat regions became known as VNTRs, which stands for Variable Number of Tandem Repeats. The technique used by Dr. Jeffreys to examine the VNTRs was called Restriction Fragment Length Polymorphism (RFLP) because it involved the use of a restriction enzyme to cut the regions of DNA surrounding the VNTRs. The past 15 years have seen tremendous growth in the use of DNA evidence in crime scene investigations as well as paternity testing. Today over 150 public forensic laboratories and several dozen private paternity testing laboratories conduct hundreds of thousands of DNA tests annually in the United States. The number of laboratories around the world conducting DNA testing will continue to grow as the technique gains in popularity within the law enforcement community. 1/31/12 Polymerase Chain Reaction
The evolution of DNA testing advanced significantly when Dr. Kary Mullis discovered that DNA could be copied in the laboratory much as it is in the natural world. The copying process, known as Polymerase Chain Reaction (PCR), uses an enzyme (polymerase) to replicate DNA regions in a test tube. By repeating the copying process, a small number of DNA molecules can be reliably increased up to billions within several hours. RFLP analysis requires a biological sample about the size of a quarter, but PCR can be used to reproduce millions of copies of the DNA contained in a few skin cells. Since PCR analysis requires only a minute quantity of DNA, it can enable the laboratory to analyze highly degraded evidence for DNA. On the other hand, because the sensitive PCR technique replicates any and all of the DNA contained in an evidence sample, greater attention to contamination issues is necessary when identifying, collecting, and preserving DNA evidence. 1/31/12 Short Tandem Repeats Short Tandem Repeat (STR) technology is a forensic analysis that evaluates specific regions (loci) that are found on nuclear DNA. The variable nature of the STR regions that are analyzed for forensic testing intensifies the discrimination between one DNA profile and another. The likelihood that any two individuals (except identical twins) will have the same 13-loci DNA profile can be as high as 1 in 1 billion or greater. The Federal Bureau of Investigation (FBI) has chosen 13 specific STR loci to serve as the standard for CODIS to ensure that all forensic laboratories are uniform. If the CODIS index is to be used in the investigative stages of unsolved cases, DNA profiles must be generated by using STR technology and the specific 13 core STR loci selected by the FBI. 1/31/12 Sources of DNA Evidence
Since only a few cells can be sufficient to obtain useful DNA information to help your case, this list identifies some common items of evidence that you may need to collect, the possible location of the DNA on the evidence, and the biological source containing the cells. Remember that just because you cannot see a stain does not mean there are not enough cells for DNA typing. 1/31/12 Mitochondrial DNA Mitochondrial DNA (mtDNA) analysis allows forensic laboratories to develop DNA profiles from evidence that may not be suitable for RFLP or STR analysis. While RFLP and PCR techniques analyze DNA extracted from the nucleus of a cell, mtDNA technology analyzes DNA found in a different part of the cell, the mitochondrion. Old remains and evidence lacking nucleated cells --such as hair shafts, bones, and teeth--that are unamenable to STR and RFLP testing may yield results if mtDNA analysis is performed. It is important to note that all maternal relatives (for example, a person's mother or maternal grandmother) have identical mtDNA. This enables unidentified remains to be analyzed and compared to the mtDNA profile of any maternal relative for the purpose of aiding missing persons or unidentified remains investigations. 1/31/12 Steps of DNA Analysis Following collection of biological material from a crime scene or paternity investigation, the DNA is first extracted from its biological source material. After isolating the DNA from its cells, specific regions are copied with a technique known as the polymerase chain reaction, or PCR. PCR produces millions of copies for each DNA segment of interest and thus permits very minute amounts of DNA to be examined. Multiple STR regions can be examined simultaneously to increase the informativeness of the DNA test. The resulting PCR products are then separated and detected in order to characterize the STR region being examined. The separation methods used today include slab gel and capillary electrophoresis (CE). 1/31/12 What is Electrophoresis?
Electrophoresis has proved to be an invaluable tool in the analysis of crime scene evidence, especially in the area of DNA fingerprinting. Electrophoresis involves the separation of chemicals along a solid medium in the presence of an applied potential difference. In electrophoresis, chemicals such as blood proteins, DNA or inorganic ions can be separated according to differences in their mass and/or charge. The solid medium used in electrophoresis is usually an agarose or polyacrylamide gel. 1/31/12