Unlike clean laboratory samples, biological materials collected at the scene would carry-over all kinds of the substrates in the environment. These contamination may interfere the PCR in 3 areas:
1. interference with the cell lysis
2. interference by degrading nucleic acid
3. inhibition of polymerase activity during the PCR amplification
And the third one is usually the major problem leading to failure of target DNA amplification.
Let's take a look what compounds are in this category:
various body fluid component (haemoglobin, melanin, erea, etc.)
chemical reagents(heparin, formalin, Ca ion, etc)
The compounds used in cell lysis and DNA purification (proteolytic enzymes or denaturants, phenolic compounds)
polysaccharides, urea, humic acids, haemoglobin, melanin in hair samples, indigo dyes from denim and a very spread-used supplementary in skin care products, EDTA.
Gladly, forensic scientists have worked out several methods to remove these contaminants
If it is regular samples, three primary techniques will be used: the phemol-chloroform extraction method (the gold sandard to judge new methods), Chelox 100 extraction (Bio-Rad),and magnetic affinity solid-phase extraction.
These methods work well on regular samples. However, some forensic samples could be pose special challenges, such as sperm and skeletal remains.
On a semen-contaminated vagina swab, female cells are predominant. How can we identify the cells specific to the male suspect?A special differential lysis treatment for forensic samples from sexual assault cases can separate epithelial cells from sperm cells, called differential extraction. Sperm nuclei are resistant to the usual SDS method, but can be lysed with DTT so as to breakdown sperm nuclear membranes
To extract DNA from an individual bone sample, two steps are need: pulverization and decalcification (EDTA).
I visited the 911 memorial. The pools and building debris remind people the mass disaster.
To identify the victims in the World Trade Center attack, a high-throughput bone-fragment-based DNA identiication is necessary. After pulverization, Guanidinum thiocyanate and detergent Triton X-100 is used for cell lysis instead of decalcification.
Large forensic labs are using manufactured robot machine to do these tedious work. That's why engineering major are easily to find a job. Think from another way, it is important for science students to stay at the edge and push forward the technology.
Reference:
Molecular Foensics. Ralph Rapley, David Whitehouse
