Forensic DNA analysis is the majority of cells assemblies up the human body are diploid cells carrying identical DNA, with the omission of haploid gametes i.e., egg and sperm and red blood cells which have no nucleus.
In terms of forensic DNA analysis, there is a range of possible sources of DNA evidence. The most functional sources of biological evidence that are used for the purpose of forensic DNA analysis includes blood, saliva, semen, skin, hair with roots, urine, vaginal fluid, and nasal secretions. It is theoretically possible to obtain DNA from evidence such as urine, faeces and dead skin cells, though this is classified as a poor source due to the lack of intact cells and high levels of contaminants that prevents successful analysis. Such type of samples will be collected depending upon on the type of sample.
The use of biological evidence in DNA and genetic analysis varies with the areas of study including blood typing, gender determination based on their chromosome analysis (karyotyping), DNA profiling and recently forensic DNA phenotyping. Since the introduction of DNA profiling in the 1980s, it has been effectively utilized in criminal cases, disaster victim identification and paternity testing.
Prior to analysis, it will be necessary to extract DNA from the sample. The steps involved are
(i) The sample cells should be lysed (broken down) in a buffer solution.
(ii) Denatured proteins and fats are pelleted through centrifugation.
(iii) The cleared lysate is passed through a column, which often contains a positively charged medium that binds to the DNA.
(iv) Contaminating proteins, fats and salts are removed through several washes.
(v)The DNA is recovered in a buffer solution or water.
(vi) The amount of DNA is often quantified using spectrophotometric techniques like UV spectrophotometry.
(vii) Various methods of DNA extraction have been devised for different types of sample.
Even though of their merits, DNA fingerprints are not preferably used as the sole piece of evidence in a case. In few countries like the UK, DNA fingerprints must be presented in conjunction with other evidence or proof.