The victim was found lying on the floor, facing the ceiling, in between the couch and the dusty fireplace. There were no signs of serious injury; she had some bruising on her face and forearms, reddish colouration around her eyes (suspected to be petechiae) and some skin under her fingernails. Her top was cut neatly across the middle, which shows no sign of a struggle; the victim was possibly unconscious during the assault, and from her waist to her feet, void of clothing.
There were two pools of blood on the floor around her; one close to her hand and another, a few feet above her head. A bloodstained knife was on the floor, a few feet away from her and a bloodstained rag also, to the left of the fireplace. On the table adjacent to the door, there was a dark bottle labelled “Chloroform” and a knife with traces of fabric.
On the couch, opposite the victim, there were some traces of biological fluid that fluoresced under UV light; bloody fingerprints were seen on the doorknob, doorframe and the armrest of the couch. Tiny droplets of blood were also seen on the floor. This seems to be a case of murder, rape and sexual assault. The scene and evidence locations were documented and photographed.
Vaginal, vulvar, cervical and anal swabs were obtained and smeared onto labelled slides for microscopic analysis. The swabs and smears were air-dried, labelled and packaged in manila envelopes accordingly.
The bloody rag was packaged in a paper bag, the top sealed and labelled accordingly. The bloodstained knife and chloroform bottle will be processed for latent fingerprints. The latent fingerprints will be visualized by using a fluorescent magnetic powder, photographed, and then lifted with lifting tape and packaged individually. The blood print on the couch will be fixed with 5-sulphosalicylic acid, visualized using Amido black and cut out using a sharp scalpel, then packaged individually. A control sample will also be collected, by cutting out another portion of the couch and packaged separately. The victim’s clothing, flowered cut top and brassiere, were packaged in separate paper bags. The smears will be viewed for the presence of spermatozoa and the swabs processed for seminal fluid/sperm cells and subjected to DNA analysis via STR Typing using polymerase chain reaction as the amplifier and capillary electrophoresis as the detector.
STRs (short tandem repeats) are locations on the chromosome containing short sequences of nucleotide bases that repeat themselves within the DNA molecule. STRs are short sequences of about 2-7 base pairs in length. They are very important markers for identification because of their abundance in the human genome.
Firstly, the DNA in the vaginal swabs is extracted from the swab via a series of procedures, that involves breaking open the cells, denaturation of proteins with a protease, DNA precipitation with sodium acetate and cleansing of DNA with cold ethanol, followed by suspension in Tris buffer and quantification using agarose gel electrophoresis.
Furthermore, the extracted DNA is then subjected to amplification via the direct polymerase chain reaction (PCR). The amplification occurs in four stages. Firstly, the extracted DNA is heated to about 90°, causing the bonds to break and the DNA strand to unwind and become two single strands. Secondly, the fluorescent-labelled primers (specific for certain STR loci) are annealed to the single DNA strands when the temperature is cooled to about 50°. Thirdly, the primer is extended by the addition of deoxyribonucleotide triphosphates (ddNTPs) complementary to the template strand; the addition stops when it encounters another primer on the DNA strand. Lastly, the process of heating and cooling is repeated until the required quantity of DNA is obtained. This process is carried out in a thermal cycler; each cycle of PCR doubles the quantity of DNA inside the cycler.
Finally, the STR profile of the crime scene DNA is detected via capillary electrophoresis. The amplified evidentiary DNA is prepared in a tube, alongside the positive, negative and reagent controls, then all tubes are closed and placed in a tube rack, which is placed on the capillary electrophoresis machine. The sample is drawn up from the tube into the capillary tube. After a voltage is applied, the sample moves along the tube travelling from the negative terminal (anode) to the positive terminal (cathode). As they move along the tube, the molecule separates into smaller molecules, which then move individually. This movement is based on their mass-to-charge ratios; therefore, the smaller DNA molecules migrate at a faster speed, compared to the larger DNA molecules, to the detector. As the samples reach the detector, they are excited and emit wavelengths which are recorded and displayed as an electropherogram. The resultant electropherogram of the evidentiary DNA is observed and the presence of two peaks at the amelogenin gene confirmed the presence of sperm cells in the victim’s vaginal swab, thus confirming the occurrence of rape before the victim’s murder.
STR typing is widely accepted for use in DNA profiling. Due to its small required sample size, it is also suitable when the sample quantity is low. The existence of different STRs in a population and the implementation of multiplexing helps to narrow down the owner of the sample by using the product rule. By characterizing different STRs and multiplying the probabilities for the occurrence of each, individualization of an electropherogram is achieved
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