DNA Profiling Technology

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Below I have explained in details DNA profiling and how evidences are collected, different tests runs and how all those are used to tie suspects to a crime scene. You can take out what's relevant to answer your questions.

DNA PROFILING:
DNA profiling relates to the procedure where a small set of DNA variations that are very likely to be different in all unrelated individuals are exploited by forensic investigators to discriminate one individual from another. This technique was originally developed by Alec Jeffreys from the Leicester University in the year 1985. Short Tandem Repeat sequences (STR) are used these days to profile a person's DNA. STR analysis has made this technique extremely sensitive and even trace amount of skin cells, hair or a tiny drop of blood (even dried blood) is required to run the test and get confirming results. The applications of this method are plenty, following are some of the areas this technique is employed:
• In forensic and criminal investigation DNA is used to link a person to the crime scene from samples like blood, hair left by the criminal. DNA profiles of repeat offenders are kept by the police to link them to any crime scene.
• DNA is also used in matching the DNA of parents to their child. A paternity test is the most definitive way of confirming a child's paternity.
•DNA testing can be used to search ancestors. People can track their long lost relatives via DNA testing.

DNA Profiling process:

99.9% of the human DNA sequences are same in every person but there is also a considerable amount of DNA (the remaining 0.1 percent of the genome—3 million nucleotide bases—for telltale variations) that is different between people. DNA profiling technique exploits this variance to discriminate between different individuals.
In order to link a criminal to a crime scene, a single droplet of blood or stray eyelash collected at a crime scene is all that is required by investigators since it carries all the genetic information of an individual. Once the evidence is taken back to the lab the following steps are undertaken:

1. DNA extraction: DNA is first extracted from the sample (blood, hair or semen). As little as 100μg (microgram) of DNA is required by forensic experts to construct a DNA profile from the samples retrieved at the crime scene. Just to give an idea how little 100μg of DNA is, imagine a few cells of saliva left on a straw while drinking a soda or a tiny blood smear left in the crime scene which belongs to the murderer or skin cells collected inside the nails of the victim during struggling or one strand of hair found inside the hand of the victim belonging to the suspect, should be sufficient to extract that amount of DNA to profile the criminal.
2. DNA Amplification by Polymerase Chain Reaction (PCR): Once the DNA is extracted from the sample, this DNA needs to be amplified. DNA has coding sequences and non-coding sequences. Coding sequences code for functional proteins whereas non-coding sequences have no role in protein expression. Short Tandem Repeat (STR) sequences are 4-10 nucleotide sequences that are present in these non-coding region of a person's DNA. Each and every individual have unique STR sequences, which means the nucleotide numbers vary from one individual to another. More genetically close two individuals are the similar the STR sequences become but non-related individuals have very different STR sequences. For DNA profiling investigators use ~13 different types of STR sequences. Using DNA primers, each of these STR sequences are amplified. During PCR, fluorescent dyes are attached to all the STR copies that get made—one type of dye ...