Forensic Escape Room: Design Your Own Biotech Adventure
and free oxygen molecules. The hemoglobin loses electrons during this process, which are then donated back to hemoglobin by the phenolphthalein molecule. This causes the indicator to change from clear to pink, indicating blood. The Kastle-Meyer test is valued for its speed, specificity for hemoglobin, and reliability, even with dilute samples. However, it may yield false positive results due to chemicals like iron and copper
oxides that can react with the hy- drogen peroxide. While it remains an efficient presumptive test, its susceptibility to false positives necessitates caution. The next step in blood testing is to confirm the identity of the sample using a test that definitively detects blood. One such confirmatory test is ABO blood group testing, which categorizes blood into types A, B,
Antigen on Red Blood Cells
Antibody in Blood
Percentage of Population
Blood Type
A B AB O
A B A & B O
anti-B anti-A none anti-A & anti-B
42% 10% 4% 44%
Figure 1: Types of Blood in the Population
AB, and O based on the presence or absence of A and B antigens on the surface of red blood cells. Testing for blood groups relies on the precipitation of an antigen-antibody complex, called agglutination, using specific antibodies that bind to the surface proteins. Blood samples are mixed with antibodies to either the A or B antigens, and the samples are allowed to incubate. If either the antibody or the blood cells are in excess, there is no visible reaction. However, when both components are present at a similar concentration, the interactions between the antigens and the antibodies form large complexes that precipitate out of solution in a state known as equivalence. The mixture in the wells will look granular instead of smooth, which is easy to detect by eye. Only blood will produce this agglutination, which is why it is classified as a confirmatory blood test. Although confirmatory tests like blood typing are more time-consuming and costly than presump - tive tests, they offer superior accuracy. While it cannot pinpoint a specific perpetrator, blood typing can help narrow down suspects by identifying groups of individuals with matching blood types or by eliminating those with incompatible blood types, aiding in criminal investigations. The next step would be DNA fingerprinting, to identify the origin of the blood sample more conclusively. DNA FINGERPRINTING Once a sample has been confirmed to be human blood or tissue, the DNA is extracted and analyzed using DNA Fingerprinting. In humans, DNA is packaged into 23 pairs of chromosomes that are inherited from an individual’s biological parents. Although most of this genetic material is identical in every person, small differences, or “polymorphisms”, in the DNA sequence occur throughout the genome. For example, the simplest difference is a Single Nucleotide Polymorphism (or SNP). Changes in the number and location of restriction enzyme sites result in Restriction Frag- ment Length Polymorphisms (or RFLPs). Short repetitive stretches of DNA at specific locations in the genome can vary in number to produce STRs (Short Tandem Repeats) and VNTRs (Variable Number of Tandem Repeats). Although most polymorphisms occur in non-coding regions of DNA, those that disrupt a gene can result in disease. Analyzing several different polymorphisms within a person’s genome generates a unique DNA “fingerprint” that can allow us to distinguish one individual from another, or even to determine familial relationships. The best-known application of DNA fingerprinting is in forensic science. DNA fingerprinting tech - niques are utilized to interpret blood, tissue, or fluid evidence collected at accidents and crime scenes. After DNA is extracted from these samples, forensic scientists can develop a DNA finger - print. Following collection, the DNA is extracted from the cells, amplified using the Polymerase Chain Reaction (or PCR, box 2), and fragmented into smaller pieces using specific restriction enzymes. These fragments are analyzed using agarose gel electrophoresis, a technique which uses a porous
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