Genetic Fingerprinting Flashcards
What is genetic fingerprinting
Is a diagnostic tool used widely in forensic science, plant and animal breeding, and medical diagnosis. It is based on the fact that the dna of every individual, expect identical twins, is unique
What are DNA bases that are non coding for
Variable number tandem repeats VNTRS
Can VNTRs be the same
They are different in all individuals except identical twins, and the probability of two individuals having identical sequences of these VTNRs is extremely small.
What is gel electrophoresis used to do
Separate DNA fragments according to their size
Describe how electrophoresis works
The DNA fragments are placed on to the afar gel and a voltage is applied across it.
The resistance of the gel means that the larger the fragments, the more slowly they move.
Therefore over a fixed period, the smaller fragments move further than the larger ones.
In this way DNA fragments of different lengths are separated
If DNA fragments are labelled for example with radioactive DNA probes, their final positions in the gel can be determined by placing a sheet of x ray Film over the ages gel for several hours.
The radioactivity from each dna fragment exposes the film and shows where the fragment is situated on the gel.
Only DNA fragments up to around 500 bases can be sequenced jn this way
Larger genes and whole genomes must therefore be cut into smaller fragments by restriction endonucleases
Describe the genetic fingerprinting technique
Extraction - dna is extracted from the sample (as amount of DNA is usually small, it’s quantity can be increased by using the polymerase chain reaction
Digestion - restriction endonucleases cut the DNA into fragments
Seperation - fragments are separated according to size by gel electrophoresis under the influence of an electrical voltage. DNA fragments are transferred from the gel to nylon membrane
Hybridisation- radioactive probes are now used to bind with VNTRs. The probes have base sequences which are complementary to the base sequences of the VNTRs, and bind to them under specific conditions such as temp and pH. The process is carried out with different probes, which bind to different target DNA sequences
Development - membrane with radioactively labelled dna fragments is placed onto an x ray Film. Development of the x ray Film reversals dark bands where the radioactive DNA probes have attached
How do we interpret the results from DNA fingerprints for example from blood at the scene of a crime and from a suspect
The pattern of bars of each fingerprint is passed through an automated scanning machine, which calculates the length of the DNA fragments from the bands.
It does this using data obtained by measuring the distances travelled during electrophoresis by known lengths of dna
Finally the odds are calculated of someone else having an identical fingerprint.
The closer the match between the two patterns, the greater the probability that the two sets of DNA have come from the same person
How can DNA fingerprinting be hard in resolving questions of paternity
As you inherit half of genetic material from Mum and half from dad.
Therefore each band on a DNA fingerprint of an individual should have a corresponding band in one of the parents
This can establish whether someone is the genetic father of a child
Even if there is a close match between a suspects DNA and the DNA found at a crime. What are the other possible explanations that need to be investigated
The DNA may have been left on some other, innocent occasion
The DNA may belong to a very close relative
The DNA sample may have been contaminated after the crime, either by the suspects DNA or by chemicals that affected the action of the restriction endonucleases used in preparing the fingerprint
Suggest how the genetic fingerprint of someone with the allele for Huntington’s disease might differ from that of someone who does not have the allele
In a person with the allele for Huntington’s disease, some of the dna fragments will be larger than those in a person without the allele because of the edges repeating units on the gene. These will travel a shorter distance in the electrophoresis gel and so there will be more thick bands nearest the start of the fingerprinting ( where the initial sample was located)
Explain how genetic fingerprinting can be used to ensure that interbreeding is avoided
Genetic fingerprinting can determine how closely any two individuals are related. The closer the match between their fingerprints, the close they are related. Therefore, to avoid the problems caused by interbreeding, it is advisable to mate animals whose fingerprints differ the most
Suggest how chemicals that affect the action of restriction endonucleases can alter the genetic fingerprint of a DNA sample
The chemicals may inhibit some of the restriction endonucleases, which would then fail to cut some sections of DNA. There would therefore be a greater number of longer DNA fragments than normal and the fingerprint would be different
Explain why it is often necessary to use the polymerase chain reaction when producing a genetic fingerprint
PCR is used to increase the quantity of DNA because the quantity available is often very small
