DNA fingerprinting
DNA fingerprinting compares repetitive DNA such as VNTRs between people. The band pattern on an autoradiogram is unique to each person, except identical twins.
Why every person's DNA pattern is different
About 99.9% of the bases are the same in all people. The remaining 0.1%, about 3 × 10⁶ bp, makes each person unique. DNA fingerprinting looks at differences in repetitive DNA, where a short stretch is repeated many times. In density gradient centrifugation, bulk DNA forms a major peak, and repetitive DNA forms small separate peaks called satellite DNA. Satellite DNA is grouped into micro-satellites, mini-satellites and others by base composition, length of segment and number of repeats. It does not code for proteins, but it shows a high degree of polymorphism (variation). These variations arise by mutation and are inherited from both parents. Every tissue of a person, such as blood, hair root, skin, bone, saliva or sperm, shows the same pattern.
Steps of DNA fingerprinting
- 1Isolate DNA from the sample (blood, hair root, saliva).
- 2Digest the DNA with restriction endonucleases to cut it into fragments.
- 3Separate the fragments by size with gel electrophoresis.
- 4Transfer (blot) the separated fragments onto a synthetic membrane of nitrocellulose or nylon (Southern blotting).
- 5Hybridise the membrane with a labelled VNTR probe, which sticks to matching fragments.
- 6★Detect the hybridised fragments by autoradiography: X-ray film shows dark bands.
- 7Compare the band patterns. The test is more sensitive with PCR: DNA from a single cell can be amplified enough.
★ key step
Uses of DNA fingerprinting
| Use | How it helps |
|---|---|
| Forensic science | DNA from blood, hair or other traces at a crime scene is matched with suspects |
| Paternity testing | Each band in a child must come from the mother or the biological father |
| Population and genetic diversity studies | Band patterns compare the variation within and between populations |
Reading a paternity test
A child's DNA fingerprint has 8 bands. Of these, 4 match the mother. Man P matches the other 4 bands; man Q matches only 1 of them. Who is the likely biological father?
- Every band in a child comes from either the mother or the biological father.
- Set aside the 4 bands that match the mother.
- The remaining 4 bands must have come from the father.
- Man P matches all 4 of them; man Q matches only 1.
Man P is the likely father, and man Q is excluded, because all the child's non-maternal bands match P.
How to draw a DNA fingerprint comparison
- Draw a rectangle for the gel or film. Mark small slots (wells) along the top edge and label it (−); label the bottom edge (+).
- Draw one vertical lane under each well and label the lanes, e.g. Mother, Child, Man P, Man Q (or Crime scene, Suspect 1, Suspect 2).
- Draw bands as short dark horizontal bars in each lane.
- Put matching bands at the same height across lanes; smaller fragments travel further down.
- Write a one-line conclusion under the drawing, such as 'All bands of the child are shared with the mother or Man P'.
A national DNA fingerprinting centre in Hyderabad helps police solve crimes and families settle paternity disputes. DNA tests are also used to check whether basmati rice has been mixed with cheaper rice.
3 marks: “List the steps involved in DNA fingerprinting.” Isolation of DNA; digestion by restriction endonucleases; separation by gel electrophoresis; blotting onto nitrocellulose or nylon; hybridisation with labelled VNTR probe; detection by autoradiography.
2 marks: “What are VNTRs? Why are they used as probes in DNA fingerprinting?” Variable number tandem repeats: mini-satellite sequences repeated in tandem, whose copy number varies between people. Their sizes (0.1 to 20 kb) are highly polymorphic, so they give a band pattern unique to each person.
3 marks: “What is satellite DNA? Why is it useful in forensic science?” Repetitive DNA that separates as small peaks from bulk DNA in density gradient centrifugation. It does not code for proteins but is highly polymorphic and is the same in every tissue of a person, so any trace can be matched.
Steps in order: 'I Don't See Big Hairy Apes' = Isolate, Digest, Separate, Blot, Hybridise, Autoradiograph.