DNA finger printing, DNA profiling,genetic profiling , DNA typing
- DNA finger printing, is a method to prepare a picture of sequences of DNA of any individual. Each individual has a unique and different sequence in its DNA.
- Genes are segments of DNA and DNA are differ in sequence of nitrogen base (nucleotide). Most Segments of DNA (gene) code for a particular protein but all segments of DNA do not code for protein.
Repitetive sequences in DNA finger printing
- There is small portion of DNA in which Sequences are repeated in many time. This portion of DNA is called as repetitive DNA.
- These repetitive DNA are separated from main bulk of DNA through the during density gradient centrifugation.
- The bulk DNA forms a major portion and the repetitive DNA form small portion . The small portion is referred to satellite DNA.
- Depending on base composition, length of segment, and number of repetitive units, the satellite DNA is classified into micro-satellites, mini-satellites etc. These sequences normally do not code for any proteins, but they form a large portion of human genome.
- Repetitive sequence show high degree of polymorphism and form the basis of DNA fingerprinting. Since DNA from blood, hair-follicle, skin, bone, saliva, sperm etc from an individual show the same degree of polymorphism, they become very useful identification tool in forensic applications. Further, as the polymorphism are inheritable from parents to children, DNA fingerprinting is the basis of paternity testing, in case of disputes.
- As polymorphism in DNA sequence is the basis of genetic mapping of human genome as well as of DNA fingerprinting, it is essential that we understand what DNA polymorphism means in simple terms. Polymorphism (variation at genetic level) arises due to mutations.
- Allelic sequence variation has traditionally been described as a DNA polymorphism if more than one variant allele at a locus occurs in human population with a frequency greater than 0.01. In simple terms, if an inheritable mutation is observed in a population at high frequency, it is referred to as DNA polymorphism.
- The VNTR belongs to a class of satellite DNA referred to as mini-satellite. A small DNA sequence is arranged tandemly in many copy numbers. The copy number varies from chromosome to chromosome in an individual.
- The numbers of repeat show very high degree of polymorphism. As a result the size of VNTR varies in size from 0.1 to 20 kb. Consequently, after hybridisation with VNTR probe, the autoradiogram gives many bands of differing sizes.
- These bands give a characteristic pattern for an individual DNA . It differs from individual to individual in a population except in the case of monozygotic or identical twins.
Steps involved in DNA fingerprinting
- The technique of DNA fingerprinting was developed by Alec Jeffreys.
- For DNA fingerprinting, DNA is isolated from the blood, hair,semen ,skin and other body parts.DNA fingerprinting involves following steps -
Step -1
- DNA is extracted from the cell by using the technique of centrifugation.
Step-2
- Large numbers of copies of DNA are produced by Polymerase chain reaction (PCR).
- The production of multiple number of Copies of DNA by using Polymerase chain reaction is called Amplification.
Step -3
- DNA is cut into fragments by using restriction enzymes.
Step-4
- DNA fragments are separated by gel electrophoresis in a medium of agarose gel.
- After the separation of DNA fragments, these fragments are visualised under ultraviolet radiation after the staining.
Step-5
- The separated DNA sequence is transferred on the membrane of nitrocellulose or nylon membrane. This step is called Southern blotting.
Step-6
- The radioactive DNA probes are added in this step. These probes are capable of making hydrogen bonding according to a complementary sequence of separated DNA fragments. This step is known as hybridisation.
Step-7
- The separated DNA fragments along with the probe are exposed on X ray film.
- On x-ray film, a unique pattern of DNA is visualised on x-ray film in the form of Dark and light bands , which look like barcodes. These bands differ from person to person.
- This whole technique is called Autoradiography.
Advantages of DNA finger printing.
- DNA fingerprinting is used to identify the criminals, to settle the paternity dispute, to identify racial groups in terms of biological evolution, to verify the immigrant where he/she is a close relative.
Comments
Post a Comment