21- Genetic Fingerprinting

What does genetic fingerprinting rely on?

• The DNA of every individual, except identical twins, is unique

• The genome of most eukaryotic organisms contains VNTRs

What is different in the VNTRs between individuals?

Their length

What are VNTRs?

• Variable number tandem repeats

• Non-coding DNA bases

What is the probability of two individuals having identical VNTR sequences?

Very small

What is the relationship between the relatedness of two individuals and the similarity of their VNTRs?

The more closely related two individuals are, the more similar their VNTRs will be

What is gel electrophoresis used for?

Used to separate DNA fragments according to their size

Which DNA fragments will travel the furthest in gel electrophoresis?

The smallest ones

Why do the large fragments of DNA travel slowly in gel electrophoresis?

Due to the resistance of the agar gel

What are the stages in gel electrophoresis?

• DNA fragments are placed on to an agar gel

• Gel is immersed in alkali in order to separate the double strands into single strands

• A voltage is applied across it

• DNA fragments move towards the positively charged anode at different speeds based on their sizes

What are the five stages in genetic fingerprinting?

• Extraction

• Digestion

• Separation

• Hybridisation

• Development

What happens during the extraction stage of genetic fingerprinting?

• Take tissue sample from organism

• Separate DNA from the rest of the cell using homogenisation and centrifugation

• Amplify DNA using the polymerase chain reaction

What happens during the digestion stage of genetic fingerprinting?

• Extracted DNA is fragmented using restriction endonucleases

• Restriction endonucleases are chosen based on their ability to cut close to the target DNA

What happens during the separation stage of genetic fingerprinting?

DNA fragments are separated according to their size using gel electrophoresis

What happens during the hybridisation stage of genetic fingerprinting?

• Radioactive or fluorescent DNA probes are used to bind with VNTRs

• Probes have base sequences complementary to the base sequences of the VNTRs

• They bind to the VNTRs under specific conditions, such as temperature and pH

What happens during the development stage of genetic fingerprinting?

• If a fluorescent probe has been used, positions of DNA are located visually

• If a radioactive probe has been used, transfer DNA fragments onto X-Ray film, which exposes them- this is known as autoradiography

• Series of bars is revealed, unique to every individual except identical twins

What are the four potential uses of genetic fingerprinting?

• Paternity testing

• Forensics

• Medical diagnosis

• Plant and animal breeding

What possible explanations are there for having a close match between a suspect’s DNA and DNA at the crime scene, other than that the suspect carried out the crime?

• DNA may have been left on a separate (and innocent) occasion

• DNA may belong to a very close relative

How could genetic fingerprinting be used for medical diagnosis?

• Producing genetic fingerprint for patient

• Comparing against genetic fingerprints of people with and without the disease and seeing how similar they are

What would genetic fingerprinting be used for in plant and animal breeding?

• To prevent undesirable inbreeding of animals

• To identify plants and animals with desirable alleles