What is DNA fingerprinting?
DNA fingerprinting is also referred to as DNA profiling.
It is a technique used by forensic scientists in the laboratory to establish a link between a DNA sample taken from a crime scene and a possible suspect in the investigation.
The forensic scientist will compare the DNA found on a sample from a crime scene and the DNA of a suspect in order to figure out if there is a connection.
How does DNA fingerprinting work?
After collecting a fingerprint or a sample, such as a hair follicle, skin, or blood, a forensic scientist can run a chemical DNA profiling test.
This test allows the forensic scientist to view the genetic makeup of the evidence.
The chemicals used in the test will separate the DNA from the sample into strands. This reveals parts of the genome, which is the complete set of compounds in DNA.
All DNA has different genomes, except for identical twins. The results of the genome are a pattern of stripes or bands.
After viewing the genetic makeup using DNA fingerprinting, forensic scientists can match similar genomes between evidence and suspects.
Steps of DNA fingerprinting
Extracting the DNA
To begin DNA fingerprinting, the forensic scientists will extract the DNA from any cells or tissue, such as hair, saliva, or tissue.
They extract the DNA by adding certain chemical substances. After extracting the DNA, they will store it in a small tube in a fridge.
DNA Replication
There is usually only a very small amount of DNA extracted from the sample.
If the forensic scientist needs more DNA in order to continue DNA fingerprinting, they will put the DNA in a tube and undergo DNA replication in order to make more DNA to be tested.
Restriction enzymes
Then, the forensic scientist will add restriction enzymes to the DNA. The restriction enzymes will cut the DNA at specific sites in the nucleotide sequence.
The restriction enzymes are needed to cut the DNA because the DNA strands need to be shortened to undergo the next step: gel electrophoresis.
Gel Electrophoresis
The DNA is then added to a gel mixture. The gel is placed on the gel electrophoresis machine and a current is run through the gel.
The current separates the strands of DNA throughout the gel based on the size of the DNA strand. Then the gel is placed in an ethidium bromide solution. The solution sticks to the DNA strands and when a UV light hits it, the DNA can be seen as a smear.
Southern Blotting
After gel electrophoresis, the fragments of DNA are taken out of the gel by a nylon membrane. The DNA is then converted into single strands so that the pattern of the genome can be seen better. These DNA fragments are then transferred onto white nylon nitrocellulose paper using heat.
Radioactive Probe
A radioactive liquid, which is the radioactive probe, is placed over the paper. The radioactive probe is some radioactive DNA of a particular genome sequence. The probe sticks to particular parts of the DNA sequence that is being tested on the paper.
The part of the probe that sticks onto matching parts of the DNA sequence will be seen as the band patterns. After parts of the probe bind to the DNA sequence, the excess probe is washed off the paper.
X-Ray Film
Finally, the forensic scientist will enter a dark room to develop the DNA pattern. The paper is placed on X-ray film to develop the parts where the probe stuck onto the DNA sequence.
After the development period, the pattern of bands of DNA, which is the DNA sequence, is visible. The forensic scientists can now compare the patterns of bands from different samples to try and find a connection.