Genetic Fingerprinting

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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

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

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

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

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

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

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

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

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

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 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 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

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

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 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.