To know the technique one should have prior knowledge of DNA structure, replication and base pairing. Forensic science encounter with problem like identification of culprits from the blood stain, hair, skin or any other minute body parts left at the site of the crime. It may also be necessary to ascertain parenthood of one controversial child. In such cases a forensic expert takes the help of southern blot technique. We know that there is a large amount of “junk DNA”- DNA that does not code for protein-in the human genome. Junk DNA is made up of repeated sequences that are called repeats. 

Although individuals may have identical genes, there may be different numbers of repeats between these genes. For example, one person may have 7 while another has 12. The more repeats, the longer the junk DNA between genes. One method of DNA fingerprinting- which produces a Southern Blot – begins by taking a DNA sample from something such as skin, saliva, blood, or hair. The DNA is cut into pieces using restriction enzymes. The resulting collection of DNA pieces will consist of some pieces of junk DNA and some genes. The sample DNA pieces and placed into a clear gelatin, where an electric current pushes the DNA pieces through the gel. 

Short pieces move farther than long ones, so a piece of DNA that had 7 repeats would move faster than a piece of DNA with 12 repeats. Since DNA has no color more steps must be completed so scientists can “see” particular DNA pieces. The sequences are denatured so only a single strand remains. They are transferred onto a nylon sheet where the strands are permanently fixed. A radioactive probe with a known sequence is then added. 

After a radioactive probe of single stranded DNA has been allowed to bond by base pairing with the denatured DNA on the paper an X-ray reveals only the areas where the radioactive probe sits. These are the only things that will show up on the film. This allows researches to identify, in a particular person’s DNA, the occurrence and frequency of the particular genetic pattern contained in the probe. This is then match with the DNA of suspected persons or their close relative to pinpoint the offenders.