The world is full of obvious things which nobody by any chance ever observes.
– Sherlock Holmes
It was said a century ago when there was no concept of DNA. But today DNA identification is used as the ultimate and irrefutable tool in criminal justice system and paternity disputes, immigration, endangered species movement, among many other applications. Forensic science has moved a long way from the time of Sherlock Holmes and the obvious things are surely observed in specialized laboratories with utmost care.
On surface, it seems that the technology should be ideal for routine use. But there are problems which hinder its wide-scale use in daily life. In the CSI: Crime Scene Investigation or SVU: Law and Order series of the cable TV, investigators are shown to be able to solve any mystery within few hours with the help of DNA analysis. But in reality it is far from the truth. With so many complexities involved, it takes a good amount of man hours to generate a reliable DNA profile.
DNA or Deoxyribo Nucleic Acid is a nucleic acid present in every cell of any organism, including humans. It is called the User Manual or a Master Code of the body. Everything we do from birth to death is controlled by the information encoded in it. It is composed of small units called nucleotides and denoted by 4 alphabets, A, C, G and T. However, a large amount of this DNA is currently considered as junk DNA and it is usually in the form of repeats e.g. AGTA. These repetitions are of interest to forensic scientists. Humans have different number of repeats at different places on their DNA. These are in the form of successive repeats and are thus called Short Tandem Repeats (STR). For example, one person may have 6 repeats of AGTA while another may have 7 and some other 8 and so on. These are called alleles and determining how many repeats a particular location or locus (plural loci) has is called allele count. For example, the 6 repeat is called allele 6 and so on. The more loci we use for allele determination, the lesser chance we will have of a random match. CODIS (Combined DNA Index System, USA) is a database of DNA profiles which uses 13 loci of the DNA to identify different humans. The data from all these 13 loci are called a DNA Profiles. CODIS is a database which stores DNA profiles from convicted offenders as well as evidence recovered from a crime scene and used for identifying a match. By using this system, there is a probability of 1 in a few trillions that a DNA profile from one human will exactly match the profile from another individual except identical twins.
The first step of DNA testing is PCR or Polymerase chain reaction which amplifies the sample DNA into amount millions of copies which can be used for testing. We only need to amplify the regions of interest and not the whole DNA. This is accomplished by providing the starting points of amplification in the form of short DNA pieces or fragments, called primers. It could be compared with the postal service. If we write the proper address, we can deliver our mail to only one specific address in the whole world. These primers are labeled with dyes so that the amplified DNA can be visualized for allele count. The next step is generation of DNA profile with the help of electrophoresis in which depending on the size, DNA fragments move differently. For example a 100 base pair fragment will move faster than a 150 base pair fragment. A standard or ladder DNA is also run along the sample so that size is determined using this is a reference. All alleles of the sample DNA are determined and this information is stored in the database for future use. If another profile having exactly the same DNA profile comes, we can say that it is the same person or a twin of that person.
|Simplified DNA Profiles of a DNA sample found at a crime scene and two suspects. You can find which one is the true perpetrator|
Each person inherits one allele from father and one from mother. So if we have DNA samples from a parent and a child, we can determine the other parent. For example, if a child has alleles 5 and 6 and mother has alleles 6 and 7 then we can be sure that allele 6 in the child is contributed by the mother and hence, allele 5 must be contributed by the father. This is done for all the 13 locations and then a match or mismatch is confirmed. This technique is used in resolving paternity disputes.
The most important point in DNA testing is collection of evidence and its interpretation. Hence, in many cases, convicted people are exonerated after many years, on the basis of DNA testing. As in CSI, Gil Grissom says: “You can be wrong, I can be wrong but the evidence is just the evidence.”
P.S. This is an old post but I posted it again for 2 reasons:
1. In the stories about Sherlock Holmes, there was a bit about DNA Testing and some people were asking about it.
2. I would be writing something on DNA in the coming posts and that is why I wanted to make kind of a foundation.