Read Aloud the Text Content

This audio was created by Woord's Text to Speech service by content creators from all around the world.

Text Content or SSML code:

scientific approaches to answer questions about the facts of criminal or civil cases. Prior to 1986, forensic scientists had a limited array of tools with which to link evidence to specific individuals or suspects. These included some reliable methods such as blood typing and fingerprint analysis, but also many unreliable methods such as bite mark comparisons and hair microscopy. Since the first forensic use of DNA profiling in 1986 (Box 1), DNA forensics (also called forensic DNA fingerprinting or DNA typing) has become an important method for police to identify sources of biological materials. DNA profiles can now be obtained from saliva left on cigarette butts or postage stamps, pet hairs found at crime scenes, or bloodspots the size of pinheads. Even biological samples that are degraded by fire or time are yielding DNA profiles that help the legal system determine identity, innocence, or guilt. Investigators now scan large databases of stored DNA profiles in order to match profiles generated from crime scene evidence. DNA profiling has proven the innocence of people who were convicted of serious crimes and even sentenced to death. Forensic scientists have used DNA profiling to identify victims of mass disasters such as the Asian Tsunami of 2004 and the September 11, 2001, terrorist attacks in New York. They have also used forensic DNA analysis to identify endangered species and animals trafficked in the illegal wildlife trade. The applications of DNA profiling extend beyond forensic investigations. These include paternity and family relationship testing, identification of plant materials, verification of Samples degraded by fire or time are yielding DNA profiles that help determine identity, problems associated with DNA profiling, and INNOCENCE, or guilt’. Forensic science (or forensics) uses technological and military casualties, and evolutionary studies. In this Special Topics chapter, we will explore how DNA profiling works and how the results of profiles are interpreted. We will learn about DNA databases, the potential the future of this powerful technology. The era of DNA-based human identification began in 1985, with Dr. Alec Jeffreys’s publication on DNA loci known as “Even biological mini-satellites, or variable number tandem repeats (VNTRs). As described earlier in the text (see Chapter 11), VNTRs are located in noncoding regions of the genome and are made up of DNA sequences of between 15 and 100 base pairs (bp) long, with each unit repeated a number of times. The number of repeats found at each VNTR locus varies from person to person, and hence VNTRs can be from 1 to 20 kilobases (kb) in length, depending on the person. For example, the VNTR is composed of three tandem repeats of a 16-nucleotide sequence (highlighted in bold). DNA Forensics VNTRs are useful for DNA profiling because there are as many as 30 different possible alleles (repeat lengths) at any VNTR in a population. This creates a large number of possible genotypes. For example, if one examined four different VNTR loci within a population, and each locus had 20 possible alleles, there would be approximately 2 billion possible genotypes in this four-locus profile. To create a VNTR profile (also known as a DNA fingerprint), scientists extract DNA from a tissue sample and digest it with a restriction enzyme that cleaves on either side of the VNTR repeat region (Figure ST 5.1). The digested DNA is separated by gel electrophoresis and subjected to Southern blot analysis (which is described in detail in Chapter 17). Briefly, separated DNA is transferred from the gel to a membrane and hybridized with a radioactive probe that recognizes DNA sequences within the VNTR region. After exposing the membrane to X-ray film, the pattern of bands is measured, with larger VNTR repeat alleles remaining near the top of the gel and smaller VNTRs, which migrate more rapidly through the gel, being closer to the bottom. The pattern of bands is the same for a given individual, no matter what tissue is used as the source of the DNA. If enough VNTRs are analyzed, each person’s DNA profile will be unique (except, of course, for identical twins) because of the huge number of possible VNTRs and alleles. In practice, scientists analyze about five or six loci to create a DNA profile. A significant limitation of VNTR profiling is that it requires a relatively large sample of DNA (10,000 cells or about 50 ug of DNA)—more than is usually found typical The Pitchfork Case: The First Criminal Conviction Using DNA crime scene. In addition, the DNA must be relatively intact (nondegraded). As a result, VNTR profiling has been used most frequently when large tissue samples are available such as in paternity testing. Although VNTR profiling is still used in some cases, it has mostly been replaced by more sensitive methods, as described next. Profiling In the mid-1980s, the bodies of two schoolgirls, Lynda Mann and Dawn Ashworth, were found in Leicestershire, England. Both girls had been raped, strangled, and their bodies left in the bushes. In the absence of useful clues, the police questioned a local intellectually disabled porter named Richard Buckland. During interrogation, Buckland confessed to the murder of Dawn Ashworth; however, police did not know whether he was also responsible for Lynda Mann’s death. In 1986, in order to identify the second killer, the police asked Dr. Alec Jeffreys of the University of Leicester to analyze the crime scene evidence using a new method of DNA analysis called VNTR profiling. Dr. Jeffreys’s VNTR analysis revealed a match between the DNA profiles from semen samples obtained from both crime scenes, suggesting that the same person was responsible for both rapes. However, neither of the DNA profiles matched those from a blood sample taken from Richard Buckland. Having eliminated their only suspect, the police embarked on the first mass DNA dragnet in history by requesting blood samples from every adult male in the region. Although 4000 men offered samples, one did not. Colin Pitchfork, a bakery worker, paid a friend to give a blood sample in his place, using forged identity documents. Their plan was detected when their conversation was overheard at a local pub. The conversation was reported to police, who then arrested Pitchfork, obtained his blood sample, and sent it for analysis. His DNA profile matched the profiles from the semen samples left at both crime scenes. Pitchfork confessed to the murders, pleaded guilty, and was sentenced to life in prison. The Pitchfork Case was not only the first criminal case resolved by forensic DNA profiling, but also the first case in which DNA profiling ted to the exoneration of an innocent person. The development of the polymerase chain reaction (PCR) revolutionized DNA profiling. As described in Chapter 17, PCR is an in vitro method that uses specific primers and a heat-tolerant DNA polymerase to amplify specific regions of DNA. Within a few hours, this method can generate a millionfold increase in the quantity of DNA within a specific sequence region. Using PCR-amplified DNA samples, scientists are able to generate DNA profiles from trace samples (e.g., the bulb of single hairs or a few cells from a bloodstain) and from samples that are old or degraded (such as a bone found in a field or an ancient Egyptian mummy). The majority of human forensic DNA profiling is now done by amplifying and analyzing regions of the genome known as microsatellites, or short tandem repeats (STRs). STRs are similar to VNTRs, but the repeated motif is shorter—between two and nine base pairs, repeated from 7 to 40 times. For example, one locus known as D8S1179 is made up of the four base-pair sequence TCTA, repeated 7 to 20 times, depending on the allele. There are 19 possible alleles of the locus that are found within a population. Although hundreds of STR loci are present in the human genome, only a subset is used for DNA profiling. At the present time, the FBI and other U.S. law enforcement agencies use 20 STR loci as a core set for forensic analysis. Most European countries now use 12 STR loci as a core set. Several commercially available kits are used for forensic DNA analysis of STR loci. The methods vary slightly, but generally involve the following steps. As shown in Figure ST 5.2, each primer set is tagged by one of four fluorescent dyes—represented here as blue, green, yellow, or red. Each primer set is designed to amplify DNA fragments, the sizes of which vary depending on the number of repeats within the region amplified. For example, the primer sets that amplify the THO, vWA, D21811, D78820, D5$818, TPOX, and DYS391 STR loci are all labeled with a fluorescent tag indicated as yellow. The sizes of the amplified DNA fragments allow scientists to differentiate between the yellow-labeled products. For example, the amplified products from the D21S11 locus range from about 200 to 260 bp in length, whereas those from the TPOX locus range from about 375 to 425 bp, and so on. After amplification, the DNA sample will contain a small amount of the original template DNA sample and a large amount of fluorescently labeled amplification products (Figure ST 5.3). The sizes of the amplified fragments are measured by capillary electrophoresis. This method uses thin glass tubes that are filled with a polyacrylamide gel material similar to that used in slab gel electrophoresis. The amplified DNA sample is loaded onto the top of the capillary tube, and an electric current is passed through the tube. The negatively charged DNA fragments migrate through the gel toward the positive electrode, according to their sizes. Short fragments move more quickly through the gel, and larger ones more slowly. At the bottom of the tube, a laser detects each fluorescent fragment as it migrates through the tube. The data are analyzed by software that calculates both the