Just like the government uses an Aadhaar number to uniquely identify an individual for social security purposes, a person’s DNA fingerprint can be used for biological purposes. This is why DNA analysis is such a big deal. Today, it is most often (but not exclusively) used together with technologies like PCR, capillary electrophoresis, and fingerprinting.
What is DNA?
Each of an individual’s cells — in one’s skin, blood, teeth, bone, etc. — contain 46 DNA molecules. One set of 23 is inherited via sperm from the father and the other 23 via the egg from the mother. Sperm and egg cells are exceptional because they have only one copy of the genome each, not two. These facts underpin the generation and use of DNA fingerprints to identify individuals and their relatives.
The DNA is packed inside chromosomes. For example, chromosome no. 3 contains 6.5% of the total DNA in each cell. The chromosome 3 from the father contains DNA that is largely similar to that derived from the mother. The few parts that are different are called polymorphisms.
DNA polymorphisms can be used to differentiate one person from another. They can also tell us whether a paternal chromosome no. 3 came from the father’s mother or father, and likewise for a maternal chromosome no. 3. That is, polymorphisms make the tracing of ancestry possible. DNA profiles are typically generated using polymorphisms in parts of the DNA called short tandem repeats (STRs).
What are STRs?
DNA has two strands. Each strand is a sequence of four chemical bases: adenine (A), cytosine (C), guanine (G), and thymine (T). The strands are anti-parallel and complementary. ‘Anti-parallel’ means the strands run in opposite directions. ‘Complementary’ means the As and Cs of one strand bond with Ts and Gs on the other. When a cell divides and needs to make a copy of its DNA, it pulls the strands apart and makes a new complementary strand for each one: As on one strand get Cs on the other and Ts get Gs. Thus, the daughter DNA has the same sequence of bases. In rare cases, the cell will make a mistake and insert the wrong base. This is called a mutation. In general, mutations occur less than once per billion base-pairs per generation.
An STR is a short sequence of base-pairs on the DNA that is repeated some number of times, such as GATCGATCGATCGATC (on the complementary strand, this STR will have the sequence CTAGCTAGCTAGCTAG). STRs are often polymorphic. Unrelated individuals almost certainly have different numbers of repeat units in at least some STRs.
How do we make copies of DNA?
Scientists need to make many copies of the DNA present in particular STRs to make studying them easier. They use a laboratory procedure called Polymerase Chain Reaction (PCR) to make copies of a particular stretch of DNA (up to 5,000 base-pairs). Even if they have a very small amount of DNA to begin with, PCR can make millions of copies in a short span of time.
First, scientists extract some genetic material from tissue that contains the DNA of interest. They heat it to about 95º C for 25 seconds to split the strands apart. Second, they introduce multiple molecules into the sample. The first is the primer, a short single-stranded piece of DNA that binds to a single strand: As and Ts on the primer bind with Ts and As in the strand and Cs and Gs with the Gs and Cs. The sequence of bases on the primer is configured so that it binds to the portion of interest on the DNA. The temperature in this phase is lowered to around 60º C.
Next, another molecule called DNA polymerase enters the reaction. It holds the bound primer, like for grip, and synthesises the rest of it according to the complementary bases on the strand.
To help, scientists add more bases into the sample. If a polymerase called Taq polymerase is used, the temperature is held at around 72º C. In these three steps, called one cycle, a new copy of the DNA segment is ready. If there is only one starting copy and each cycle operates with 100% efficiency, making a million copies of the segment will take about 50 minutes. The device that performs these steps is called a thermocycler.
What is a DNA fingerprint?
New copies of DNA can be made to move through a capillary under the influence of an electric field. The smaller ones move faster. This technique is called capillary electrophoresis. Multiple STRs can simultaneously be sized in the same test. The sizes of the various paternal/maternal variants of the different STRs are compiled in a table. This table is unique to each individual and is called their DNA fingerprint (only monozygotic twins have the same DNA fingerprints).
This fingerprint can be established using DNA from teeth, bones, blood (a drop is enough), spit, semen, skin cells, etc.
DNA fingerprints can establish parent/child relationships and help identify individuals from their mortal remains at disaster sites. DNA from blood stains, sweat, and spit from cloth or soil, retrieved from crime scenes, can be used to identify suspects. Experts also use them to identify or exclude potential donors during organ donation.
DNA is very stable. Scientists have extracted it almost intact from 65,000-year-old human remains preserved only by desiccation or cold. Many ‘cold cases’ have been solved by examining new profiles made from DNA extracted from crime scene materials in the archives. DNA fingerprints have also exonerated wrongly accused convicts.
D.P.Kasbekar is a retired scientist. He is an INSA Honorary Scientist in the Centre for DNA Fingerprinting and Diagnostics, Hyderabad.
Published – March 25, 2025 08:30 am IST
