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DNA Profiling in Forensic Science (Forensic Molecular Biology)

Updated: Aug 14, 2023


DNA (Deoxyribonucleic acid) is a double helical polymer made up of two polynucleotide chains. It is a molecule that carries genetic information from one generation to another generation. The main function of a DNA molecule is to store information for a long-time duration. In a cell, DNA is organized into structures called chromosomes in the nucleus and mitochondria. Every cell nucleus contains two sets of homologous (diploid) chromosomes from each parent. Genes are part of a DNA molecule that carries genetic information. A DNA molecule chemically consists of long polymers of two simple units called nucleotides, the backbone of which contains sugar and phosphate groups linked with ester bonds. These polymer chains are antiparallel to each other. Chromosomes replicate before the cell divides, through a process called DNA replication. DNA molecules are found stored in the nucleus of a cell in eukaryotic organisms. The basic structure of DNA consists of: - • Sugar molecules • Phosphate group • Nitrogen Group
i. Adenine ii. Thymine iii. Guanine iv. Cytosine
DNA Profiling in forensic science has a lot of applications. DNA analysis, to identify culprits or parentage. DNA profiling in forensic science is basically genetic fingerprinting. Every individual has a unique DNA profile and this technique is very useful for the identification of a person involved in crime. Identical twins have the same DNA profile and this is the only exception. DNA fingerprinting or DNA analysis or DNA typing is the study of differences or similarities in certain segments or chromosomes (called markers, alleles, or loci) in the molecular structure of a person's DNA molecule. DNA evidence is most useful and authentic when combined with other trace pieces of evidence present on a crime scene, and eyewitnesses. History British anthropologist Sir Francis Galton published his first study of digital fingerprints to identify specific person, in the year 1880. Another important step in establishing the identification was the discovery of ABO blood grouping by an Austrian doctor Karl Landsteiner in the 20th century. After 1950, the laboratory tested various types of blood and tissue antigens, ultimately resulting in the study of major histocompatibility complex (HLA). DNA fingerprinting was coined by Sir Alec Jaffrey and he was the first who said that certain regions of DNA contain repetitive sequences that differ from one person to another. Sir Jeffrey was the one who demonstrated the importance of genetic fingerprinting in personal identification in criminal cases such as paternity disputes, sexual assault, etc. Kary Mullis discovered PCR which is Polymerase Chain Reaction in 1983. This was a new step towards DNA analysis. This is an enzymatic process in which a region of DNA is amplified (duplicated) 28-34 times, creating approximately one billion copies. The process involves repeating base units and converting them into numerical units called genetic mapping. He aimed at making copies of DNA fragments of sufficient quality with the result of amplification to obtain a usable genetic map. This method is suitable for biological samples containing <50 picograms of DNA matrix., Biological organisms refer to chemical structures in advanced stages of decay that contain one or more bases. DNA analysis results in the last two decades are admissible as evidence in many countries. Sources of DNA from Biological Evidence The human body comprises many different types of cells, most of which contains the nucleus, apart from red blood cells. Every nucleated cell has two copies of the individual’s genome, which are used for the creation of a DNA profile. Generally, the sample shows some degradation, but when the degree of degradation is higher, the cell material must have a DNA profile. Chemical samples containing nucleated cells are important for genetic analysis, for example: • Liquid and dry blood. • Liquid and dry semen. • Biological secretions such as saliva, semen, or a mixture of secretions originate after sexual activities. • Rooted hair. Collection of Biological Samples. Biological materials such as blood are considered one of the most common sources of DNA. Store in anticoagulant first and store at 4°C for a week. Thereafter, DNA samples are stored at lower temperatures for longer periods. After collection, store them in a dry place at room temperature. The basic steps of DNA analysis consist of four steps – 1. DNA Extraction- There are various DNA extraction methods, some commonly used methods are the Chelex-100 method, silica-based DNA extraction, and phenol-chloroform method. 2. DNA Quantification- After DNA extraction, the quantity and quality of the DNA extract must be accurately measured. The best amount of DNA is done in a short time period when added to PCR. 3. DNA Amplification- Polymerase Chain Reaction (PCR) is a method used for DNA amplification. PCR helps in the amplification of certain regions of the DNA sample; A smaller molecule of DNA can be amplified up to a billion times with a certain number of amplification cycles. The steps of PCR include Denaturation, Annealing, and Extension. 4. Detection of DNA amplification products- For human forensic identification the following are used: i. Autosomal Short Tandem Repeat (STR) Analysis
ii. Analysis of Y Chromosome iii. Analysis of Mitochondrial DNA iv. Autosomal single-nucleotide polymorphism (SNP) typing.

By Ankita Sinha

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