DEOXYRIBONUCLEIC ACID (DNA)
DNA
It is vital to understand the structure and function of DNA and how this relates to DNA analysis in forensic science. DNA, deoxyribonucleic acid, is a molecule arranged into a double-helix. It is composed of nucleotide trisphosphate molecules, referred to as the ‘building blocks’ of DNA. These molecules consist of a trisphosphate group, a deoxyribose sugar and one of four nitrogenous bases. The four bases involved in a DNA molecule are adenine and guanine (purines) and thymine and cytosine (pyrimidines). These bases bond to the deoxyribose sugar and one of the other bases to form base pairs, with adenine and thymine bonding through two hydrogen bonds, and guanine and cytosine bonding with three hydrogen bonds. DNA is essentially the molecule that holds all genetic information and ‘instructions’ for an organism. The human genome is composed of over 3 billion base pairs of information organised into 23 chromosomes. The majority of cells making up the human body are diploid cells carrying identical DNA, with the exception of haploid gametes (egg and sperm) and red blood cells (which have no nucleus). Several types of biological evidence are commonly used in forensic science for the purpose of DNA analysis, including blood, saliva, semen, skin, urine and hair, though some are more useful than others. The use of biological evidence in DNA and genetic analysis varies, with areas of study including blood typing, gender determination based on chromosome analysis (karyotyping), DNA profiling and, more recently, forensic DNA phenotyping. |
THE HISTORY OF DNA
Since the advent of DNA profiling in the 1980s, it has been successfully utilised in criminal cases, disaster victim identification and paternity testing to name a few. However despite their merits, DNA fingerprints are not ideally used as the sole piece of evidence in a case, and in certain countries, such as the United Kingdom, DNA fingerprints must be presented in conjunction with other evidence. It is through the extensive study of the genome that DNA fingerprinting has been produced as a useful and reliable technique in forensic science. Sources of DNA Evidence & DNA Extraction In terms of forensic DNA analysis, there is a variety of possible sources of DNA evidence. The more useful sources include blood, semen, vaginal fluid, nasal secretions and hair with roots. It is theoretically possible to obtain DNA from evidence such as urine, faeces and dead skin cells, though this is often classed as a poor source due to the lack of intact cells and high levels of contaminants preventing successful analysis. DNA Profile Interpretation The primary purpose of forensic DNA profiling is to obtain a DNA ‘fingerprint’ from a biological sample and compare this to profiles obtained from DNA from a crime scene, an individual or profiles stored on a database. The process of modern DNA profiling includes statistically determining the chance that two people will share the same profile by establishing how common a genotype or collection of genotypes is within a population. Allow Attorney Cummings to use her crime scene investigations experience with her fingerprint knowledge to help resolve your case. |
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