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When a crime has been committed, samples are obtained from the crime scene and sent to the forensic laboratory. In a typical case, a known sample is obtained from a person, mostly in form of a buccal swab, and the results of the DNA analysis compared to the Questioned sample from the crime scene (Thompson 540). Later, a report is made based on the relationship between the donor of known sample and Questioned sample. The paper seek to examine different DNA assessment tools and their uses.
During the DNA investigation in the laboratory, an analysis is performed in nuclear DNA. This 3.3 billion base pairs long chromosomal DNS resides in the nucleus and is available in two copies per cell (Butler 32). It recombines during meiosis before it is passed down from parents to their children. In the early stages of human identification, Bernatzky discovered long repeating sequence in the nuclear DNA and investigated these by performing Restriction Fragment Length Polymorphism studies (4). This technique employs enzymes that cut the DNA at designated sites, which shows a particular pattern of size separation by gel electrophoresis (Bernatzky 5).
Currently, the forensic DNA investigation depends on the assessment of short tandem repeats (STRs) from nuclear DNA (Wambaugh 55). These short DNA sequence repeats occur in abundance throughout the non-coding regions of the nuclear genome. In the laboratory, the STRs are amplified with primers that are appended with fluorescent dyes, and the resulting amplicons are separated by size with capillary electrophoresis to determine the variation in repeats between individuals (Butler 32). Since nuclear DNA recombines in a loci-independent manner, it is very suitable for human identification because this lays a basis for combining the results for all STR loci. By doing so, it is possible to establish identity from a DNA sample.
In some case, STRs may be unfit for investigation because of the low quantity of present DNA to obtain an STR profile (Butler 32). This is mostly the case with hairs and aged teeth and bone samples. In such situations, mitochondrial DNA is the best alternative strategy for DNA investigation (Budowle, Joseph, and Mark 6). Unlike nuclear DNA, mitochondrial DNA is maternally inherited, which limits its resolution to a maternal lineage. However, this characteristics makes is specifically useful in kinship testing and the identification of human remains. The mitochondrial genome resides in the mitochondria of a cell. It is circular, approximately 16.5 kb long and is present in thousands of copies depending on the cell type, compared to the two copies of nuclear DNA in a single cell. The mitochondrial genome is categorized into a coding and non-coding region. The coding region codes for certain genes (Budowle, Joseph, and Mark 8). The non-coding control region of the mitochondrial genome mutates at a higher rate than the coding region. This allows forensic DNA experts to focus their analysis on two or three hypervariable regions found in the non-coding region which contain the most intraindividual variation. since it is smaller in size, the total amount of mitochondrial DNA in a cell is lower than that of the nuclear DNA, but its higher copy number, its shape and the extra level of protection from the environment provided by the double membrane of the mitochondrion increases the level of sensitivity and lays a foundation for greater possibility of recovering substantial mitochondrial DNA for typing of degraded samples (Butler 32).
The evidentiary values of mitochondrial DNA differs from that of nuclear DNA because of its material inheritance; it cannot be used to establish identity. Since there is no recombination of the mitochondrial DNA, all maternal relatives have the same profile, apart from germ-line mutations that may have developed.
References
Bernatzky, Robert. “Restriction Fragment Length Polymorphism.” Plant Molecular Biology Manual. C.2. (1988): 1-8
Budowle, Bruce, Joseph A. DiZinno, and Mark R. Wilson. “Interpretation Guidelines of mtDNA Control Region Sequence Electropherograms in Forensic Genetics.” Tenth International Symposium. (1999) Web.
Butler, John M. Forensic DNA Typing: Biology and Technology behind STR Markers. San Diego, CA: Academic Press, 2001. Print.
Foran, David R. “Relative Degradation of Nuclear and Mitochondrial DNA: An Experimental Approach.” Journal of Forensic Sciences 5.4 (2006): 766-70. Print.
Thompson, William C. “DNA Testing.” Encyclopedia of Crime and Punishment. By David Levinson. Vol. 2. Thousand Oaks, Calif: Sage Publications, 2002. 537-44. Print.
Wambaugh, Joseph. The Blooding. New York: Morrow, 1989. Print.
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