DNA fingerprinting is a technique to identify an individual by looking at unique patterns in their DNA.
- In human’s 99.9% of the base sequences of DNA are same & are referred as Bulk genomic DNA.
- The difference lies in remaining 0.1%. It is these differences which make every individual unique in their phenotypic appearance. This DNA has small stretches of repetitive sequences. They are referred as Repetitive DNA.
- The only exception is monozygotic twins.
Repetitive DNA can be separated from bulk genomic DNA by density gradient centrifugation.
- Bulk DNA forms a major peak while repetitive DNA forms several small peaks and are referred as satellite DNA.
- Satellite DNA contains short nucleotide sequences repeated thousands of time tandemly.
- Satellite DNA is divided into different categories according to their size i.e., micro-satellites, mini-satellites etc.
- Micro-satellite consists of 2 – 6 bp repeats.
- Mini-satellite consists of 10 – 100 bp repeats, they are referred as VNTRs.
- Every person’s DNA has coding and non-coding sequences, the coding sequences are exons and non-coding sequences are introns.
- Introns consist of repetitive DNA which is referred as VNTRs i.e., variable number of tandem repeats.
- The size of repeats and the number of repeats differ from person to person. They come from the genetic information donated by parents.
- They form a large portion of the human genome and do not code for any proteins.
These sequences show a high degree of polymorphism and form the basis for DNA fingerprinting.
Polymorphism is variation at genetic level which occurs due to mutation. These are single nucleotide changes.The probability of such variations is more in introns as they don’t have any immediate effect in an individual. Such polymorphism plays an important role in evolution and speciation.
The technique of DNA fingerprinting:
Alec Jeffery developed the technique of DNA fingerprinting. Also known as DNA profiling.
To find out VNTR of a person, Southern blotting is done
- Isolation of DNA: DNA is isolated from the nucleus of cells such as blood, hair follicle, skin, bone, saliva, sperm etc.
- Gene Amplification by Polymerase Chain Reaction.
- DNA is cut into fragments by using Restriction endonucleases.
- Gel electrophoresis is used to separate fragments according to their size.
- DNA is denatured by treating with alkaline chemicals or by heating.
- DNA fragments are transferred to a nitrocellulose paper or a nylon membrane (this is Southern blotting).
- Hybridization reaction: Denatured DNA is put into a plastic bag having saline solution along with a Radioactive DNA probe (single stranded repetitive DNA whose base sequence is known and one base is made radioactive). The probe binds to the DNA fragments wherever complementary sequences are present. This is hybridization reaction. Excess of probe/fragments is washed off.
8. An X-ray film of the hybridization reaction is taken. The region where DNA probe binds to the denatured DNA fragments appears as dark bands (autoradiography). These bands give a characteristic pattern for an individual DNA.The pattern that results is the DNA fingerprint.
Applications of DNA fingerprinting:
- Paternity/Maternity disputes: helps in identifying the real genetic mother, father, and the offspring.
- Forensics: it is very useful in detecting criminals.
- Help to identify genes connected with heredity diseases like thalassemia, sickle cell anemia etc.
- It helps in determining population and genetic diversities. Helping to identify racial groups.