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Genetic Fingerprinting

Biology: AQA A Level, topic 8.4.7

VNTRs

VNTRs are sections of DNA that do not code for any proteins called variable number tandem repeats. VNTRs are used in genetic fingerprinting.

Structure

Structure

  • VNTRs are sections of DNA found throughout the genome.
  • The sequences of VNTRs consist of a series of bases that is repeated in the same sequence many times (e.g. CAGCAGCAG).
  • The number of times the sequence is repeated is varied.
  • This is why VNTRs are called variable number tandem repeats.
Function

Function

  • The number of repeats in a VNTR can vary among individuals.
  • It is very unlikely that two individuals will have the exact same number of VNTRs in the same place in the genome.
  • This allows comparisons to be drawn between individuals.
  • VNTRs are used in this way in genetic fingerprinting.

Gel Electrophoresis

Gel electrophoresis is the technique used to compare the genetic fingerprints of individuals. Every individual has a different genetic fingerprint. The steps involved in this technique are:

1) Amplification

1) Amplification

  • The DNA sample is extracted from the individual. This is done by taking a swab inside someone's mouth or taking a blood sample.
  • The DNA sample is amplified many times using PCR (polymerase chain reaction).
  • PCR generates many copies of the same sample.
2) Labelling

2) Labelling

  • The DNA fragments produced from PCR are labelled using a fluorescent label.
  • The label allows the DNA fragments to be identified when exposed to UV light.
3) Inserting the DNA

3) Inserting the DNA

  • The many DNA fragments are inserted into a well in a gel.
  • The gel is covered in a buffer solution that conducts electricity with a positively charged electrode at one end of the gel and a negatively charged electrode at the other end.
  • DNA is inserted at the negative end of the gel.
4) Movement of DNA

4) Movement of DNA

  • DNA is negatively charged so when an electric current is passed through the gel, the DNA will move away from the negative electrode towards the positive electrode.
  • Smaller DNA fragments will move through the gel more quickly and travel further than larger fragments.
5) Ladder of DNA

5) Ladder of DNA

  • The electric current is removed after approximately 10 minutes.
  • The DNA fragments that are different lengths in a sample will have moved differing distances up the gel.
  • The presence of DNA fragments in the gel form bands of DNA.
  • The different bands of DNA in an individual sample produce a 'ladder' of DNA.
6) Identifying DNA fragments

6) Identifying DNA fragments

  • The different DNA fragments in a sample can be identified by exposing the gel to UV light.
  • The DNA fragments are fluorescently labelled so UV light shows the bands of DNA present.
7) Genetic fingerprinting

7) Genetic fingerprinting

  • The lengths of DNA fragments are determined by the number of repeats in a VNTR.
  • The number of repeats varies between individuals.
  • This means the DNA fragments in an individual will move different distances and the ladder of DNA for every individual will be unique.
  • The ladder of DNA in an individual is considered their genetic fingerprint.

Using Genetic Fingerprinting

Genetic fingerprinting has a number of applications. These include:

Genetic relationships

Genetic relationships

  • Genetic fingerprinting can be used to identify the genetic relationship between individuals.
  • VNTRs are inherited from an individual's parents. This means offspring will have a combination of their mother's genetic fingerprint and their father's genetic fingerprint.
  • Genetic fingerprinting can be used in this way for paternity tests.
Genetic variability

Genetic variability

  • Genetic variability is how much genetic diversity exists in a population.
  • Genetic fingerprinting can be used to identify the genetic variability in a population.
  • The more varied the genetic fingerprints of a population are (e.g. the fewer overlapping bands in gel electrophoresis), the greater the genetic variability.
Forensic science

Forensic science

  • Genetic fingerprinting can be used to compare DNA at a crime scene to potential suspects.
  • DNA is sampled from a crime scene and a fingerprint is produced using gel electrophoresis.
  • The DNA from the scene can be used to find an individual with a closely matched genetic fingerprint.
Medical diagnosis

Medical diagnosis

  • Some genetic disorders are produced from multiple mutations.
  • The mutations involved in a disorder may produce a specific genetic fingerprint.
  • These disorders can be diagnosed by comparing the genetic fingerprint of an individual to the genetic pattern of the condition.
  • E.g. Sarcomas (a type of tumour) can be identified by comparing the genetic fingerprint of a patient's tumour to the genetic fingerprint of a known sarcoma.
Inbreeding

Inbreeding

  • Inbreeding between animals and plants is problematic in agriculture because it reduces genetic diversity.
  • Low genetic diversity increases the chance of genetic disorders and extinction.
  • Inbreeding can be avoided by using genetic fingerprints.

Test yourself on Genetic Fingerprinting

  • Which of the following statements about VNTRs are true?

  • Which of these statements is NOT true for VNTRs?

  • Stages in Gel Electrophoresis

  • In which direction does DNA move in gel electrophoresis?

  • Which of the following statements about gel electrophoresis are true?

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