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Trace DNA or touch DNA or Low Copy Number (LCN) has attracted the interest in the latest years because of it may be chosen as an alternative when other evidences (biological) are not there. In many cases the genomic DNA left behind by anyone in the crime scene is enough to produce the DNA profile. There are a few factors that positively effect for the betterment in the DNA profiling. Those factors include status of shedding, temperature, type of surface or pressure applied during the contact.
Touch DNA is referred to the transfer of epithelium cells or body fluids when the surface is touched. The count of the cells left behind are very small but are capable for performing DNA profiling. Over the last 2 decades the process has gained its attention. The procedure of DNA recovery and collection are the most critical for DNA analysis. DNA typing success depends upon the available DNA template.
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Different type of analysis of touch is done bases on the surface it has been found from or analyzed. Swab sampling is generally favored for smooth surfaces, heavily shedding cloth and non-adsorbent surfaces while tape sampling is more suitable for ridged or absorbent surfaces, usage of solvent pairing and swab, with a non-polar surface may be chosen for the task and these processes do not intervene with the DNA amplification. DNA extraction, short tendem repeat (STR) amplification and quantification are usually involved in the process of forensic DNA analysis. The portion of original sample is usually consumed for the process of the pre-amplification quantification and extraction.
Extensively PCR are directly used in molecular biology as an ability to introduce samples without an intermediate process into amplification reactions with zero prior DNA purification and extraction. Because these touch DNA sample evidences already have very little biomass samples by nature, it is mostly avoided to directly introduce them to into STR amplification a there is chance of loss of the sample in the extraction processes. A chance of reduction of the sample contamination is an advantage to this outlook by reducing the middle handling that is usually done. All these benefits combine together to increase the chances to maximum of being able to obtain the usable DNA profiles from touch DNA.
Touch DNA, trace DNA or Epithelial cells as evidence can be defined as evidence that would likely contain DNA but with no or least visible staining which is often form the results of the transfer of epithelial cells to an object from the skin. Do you leave behind the skin cells just by simply touching any object? According to a statement published by the forensic scientist, cells as few as five to six may be used for DNA Profiling. However, there is no guarantee that a simple touch on the surface and few left behind cells may help obtain a successful DNA profiling. Detection and being able to obtain an interpretable DNA profile Are two different aspects or concepts.
Jones and Van Oorschot were the prime two scientists to report that there are chances to recover DNA sample from epithelial cells from an object that was handled in the year 1997. The possibilities of the usage of DNA evidence increased in different cases, especially including on sexual assaults, rapes, and murder, where before the DNA recovery had not been considered or given a try.
The same STR typing method is performed by most public crime labs for testing the touch DNA testing performed for the last 10–15 years by DNA analysts. Not being a totally new tecnology. Different methods for the detection of DNA from a lower count of cells ranging from 15–30 diploid cells are present. This is called the Low Copy Number (LCN) testing and utilizes advanced techniques for obtaining DNA profiles. There is a disagreement between the experts and courts on its validation, use, interpretation and acceptance. There is mostly a requirement of 70-150 cells for the process of STR to make a DNA profile result.
Tools used in the recovery of Touch DNA
- 4N6 flocked nylon swabs and EZ-1 extraction (control method)
- Wetted microFLOQ® swabs
- MicroFLOQ swab wetted with SpeedBeads™
- Cellulose discs
- By elution in liquids
- Interdental brushes
- Silicone toothpicks
- PCR amplification of touch DNA samples
- Capillary electrophoresis and analysis
Processing the Touch DNA Crime Scene
A crime scene technician must always be wearing the appropriate personal protective equipment (PPE). Apart from wearing latex gloves, other open body parts like no hat, no mask, a short sleeved shirt and dusting an item for prints may be leave behind Epithelial cells from the head, arms, mouth, and nose on crime scene by the investigator himself. The equipments such as brush and powder should be disposed for preventions from mishaps. Contamination can be defined as the unintentional pollution of the samples in the lad or the crime scene by bringing in the outside DNA. Contaminated DNA might be seen as background DNA, a DNA profile from a single source, the major or minor profile within a mixture, or all of the above. There is no absolute time for these things to take place. Thay can happen before\after the crime has been committed, during discovery or investigation or even the lab that it has been sent to for the analysis. In these times with techniques like touch DNA, a scene of crime must be approached with minimum contamination sinceno one can see or test for touch DNA.
Items that should be swabbed for Touch DNA
Contamination detection, control and monitoring
Some ways that may be used or employed are:
- Entry to the forensic lab should be limited and controlled to not allow the access of un authorized personnel.
- Different areas of work for evidence examination, extraction of DNA, and amplification of DNA and typing.
- Samples are either examined at different times or at different locations.
- The control samples that are taken up are from the evidence test and directly though to extract and amplify.
- Introduction contamination logs monitor of exogenous DNA and for identifying the sources.
- The files of the analyst or support staff that contain the elimination profiles of both the past and present are maintained.
Problems usually faced
- When a partial profile is tested it gives mostly a very low statistics.
- There is absolutely no scope for the presumptive test to take place with the low count of cells.
- A profile made sometimes may not be enough to enter system (CODIS).
- Sometimes there are chances that DNA samples may be mixed by contamination during and after the crime has been committed or even from the background DNA that is already present there, this increases the difficulty for the identification of the relevant profile.
- The touch DNA examination may be requested late due to because it may be a shared evidence.
- Re-examination of old cases in which there may not have been collection, storage, or examination of trace DNA.
- The Touch DNA does not give any information to “when” or “how” DNA was deposited on the surface.
- There are chances that the DNA found as the background DNA may be of the manufacturer or someone who has handled the cloth.
- A stain of blood that contains epithelial cells from a different source.
According to an experiment that was conducted in the Australian soil, showed us that it is feasible to achieve interpretable DNA profiles from an outside surface, such as a table glass recovered 7-14 days after deposition (at average temperatures and humidity),and a DNA profile from a glass slide stored in a cold and dark place for up to six weeks since deposition. There was difference observed in the lifetime of both cases that had its direct impact on the persistance of the DNA. This difference was majorly in the environmental conditions like moisture surrounding the sample’s surface, weather conditions (humidity, high temperature and exposure to UV- light). A DNA sample taken from a moist environment is more vulnerable to hydrolysis as well as oxidation base damage. The prime target of hydrolysis is the base sugar bond, resulting in loss of base through depurination and spoiling the entire DNA. The rise in the rate of hydrolysis of the samples is directly proportional to the increase in heat. Consequently, leading to direct cleavage of DNA strands due to drying.
The quality of the already collected DNA is directly affected by the collection technique like the use of minitapes, different swab tip types that is used in collection of evidences from the scene of itself. While, the different type of surface from where the samples are collected can directly effect on the efficiency of some collection method. There is a very high recommendation that the teams that search and collect for samples re-think about their ways of collecting samples that may have touch DNA for a better production of the results. Also, more works need to be done for the evaluation of the factors affecting touch DNA, and better internal validation should be used for making the choice of the most suitable collection methods on different types of surfaces.
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