DNA Fingerprinting

DNA fingerprinting is a method used to identify an individual from a sample of DNA by looking at unique patterns in their DNA.

PROCESS OF DNA FINGERPRINTING

How is the first DNA fingerprint produced?

1. The first step of DNA fingerprinting was to extract DNA from a sample of human material, usually blood.

2. Molecular ‘scissors’, called restriction enzymes were used to cut the DNA. This resulted in thousands of pieces of DNA with a variety of different lengths.

3. These pieces of DNA were then separated according to size by a process called gel electrophoresis?

4. The DNA was loaded into wells at one end of a porous gel, which acted a bit like a sieve. 

5. An electric current was applied which pulled the negatively-charged DNA through the gel.

6. The shorter pieces of DNA moved through the gel easiest and therefore fastest. It is more difficult for the longer pieces of DNA to move through the gel so they travelled slower. 

7. As a result, by the time the electric current was switched off, the DNA pieces had been separated in order of size. The smallest DNA molecules were furthest away from where the original sample was loaded on to the gel.

8. Once the DNA had been sorted, the pieces of DNA were transferred or ‘blotted’ out of the fragile gel on to a robust piece of nylon membrane and then ‘unzipped’ to produce single strands of DNA. 

9. Next the nylon membrane was incubated with radioactive probes. 

10. Probes are small fragments of minisatellite DNA tagged with radioactive phosphorous.

11. The probes only attach to the pieces of DNA that they are complementary? To – in this case they attach to the minisatellites in the genome.

12. The minisatellites that the probes have attached to were then visualized by exposing the nylon membrane to X-ray film. 

13. When exposed to radioactivity a pattern of more than 30 dark bands appeared on the film where the labelled DNA was. This pattern was the DNA fingerprint. 

14. To compare two or more different DNA fingerprints the different DNA samples were run side-by-side on the same electrophoresis gel.