DNA fingerprinting: stages from extraction to analysis

DNA is extracted from a sample

if only a small amound of DNA is available it can be ampllified using the polymerase chain reaction(PCR)

fragments are separated on the basis of size using a process called gel electrophoresis

DNA fragments are injected into wells and an electric current is applied along the gel

DNA is negatively charged so it is attracted to the positive end of the gel

the gel is immersed in alkalli in order to separate the double strands into single strands

the pattern of fragments are transferred to a nylon membrane by a process called southern blotting

radioactive probes are used to attach to the cores sequences (hybridisation)

the probes have base sequences which are complementary to the core sequences

any probes not bound are washed off

the membrane is dried

the nylon sheet is placed under X-ray film

the radioactive probes on the DNA fragments expose the film

this produces visible pattern of light and dark bands which is unique to each individual

the pattern of fragment distribution is then analysed

the pattern of the bands is unique to every individual (except identical twins)

stage 5

stage 4

stage 3

stage 2

stage 1

southern blotting

a thin nylon membrane is laid over the gel

the membrane is covered with several sheets of absorbent paper, which draws up the liquid containing the DNA by capillary action

this transfers the DNA fragments to the nylon membrane in precisely the same relative positions they occupied on the gel

the DNA fragments are then fixed to the membrane using UV light