Genetic fingerprinting Flashcards
1
Q
What is genetic fingerprinting?
A
- 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, except identical twins, is unique
- this technique relies on the fact that the genome of most eukaryotic organisms contains many repetitive, non-coding bases of DNA
- indeed, 95% of human DNA is currently not known to code for any characteristic but may yet be found to be functional
- DNA bases which are non-coding are known as variable number tandem repeats (VNTRs)
- four every individual the number and length of VNTRs has a unique pattern
- they are different in all individuals except identical twins, and the probability of two individuals having identical sequences of these VNTRs is extremely small
- however, the more closely related two individuals are, the more similar the VNTRs will be
2
Q
What is Gel electrophoresis?
A
- Gel electrophoresis is used to separate DNA fragments according to their size
- the DNA fragments are placed on to an agar 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 move further than the larger ones
- in this way DNA fragments. Of different lengths are separated
- if the DNA fragments are labelled, for example with radioactive DNA probes, their final positions in the gen can be determined by placing a sheet of x-ray film over the agar 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 long can be sequenced in this way
- larger ends and while genomes must therefore be cut into smaller fragments by restriction endonucleases
3
Q
Wheat are the stages of making a genetic fingerprint?
A
- extraction
- digestion
- separation
- hybridisation
- development
4
Q
What is extraction?
A
- even the tiniest sample of animal tissue, such as a drop of blood or a hair root, is enough to give a genetic fingerprint
- whatever the sample, the first stage is to extract the DNA by separating it from the rest of the cell
- as the amount of DNA is usually small, its quantity can be increased by using the polymerase chain reaction
5
Q
What is digestion?
A
- the DNA is then cut into fragments, using the same restriction endonucleases
- the endonucleases are chosen for their ability to cut close to, but not within, the target DNA
6
Q
What is separation?
A
- the fragments of DNA are next separated according to size by gel electrophoresis under the influence of an electrical voltage
- the gel is then immersed in alkali in order to separate the double strands into single strands
7
Q
What is hybridisation?
A
- radioactive (or fluorescent) DNA probes are now used to bind with VNTRs
- the probes have base sequences which are complementary to the base sequences which are complementary to the base sequences of the VNTRs, and bind to them under specific conditions, such as temperature and pH, which bind to different target DNA sequences
8
Q
What is development?
A
- finally, an x-ray film is put over the nylon membrane
- the film is exposed by the radiation from the radioactive probes
- 9if using fluorescent probes, the positions are located visually)
- because these points correspond to the position of the DNA fragments as separated during electrophoresis, a series of bars is revealed
- the pattern of the bands is unique to every individual except identical twins
9
Q
How do you interpret the results of DNA fingerprinting?
A
- DNA fingerprints from two samples, for example, from blood found at the scene of a crime and from a suspect, are visually checked
- of there appears to be a match, 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 gravelled 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
10
Q
What arte the uses of DNA fingerprinting?
A
- genetic relationships and variability
- forensic science
- medical diagnosis
- plant and animal breeding
11
Q
How is DNA fingerprinting used in genetic relationships and variability?
A
- DNA fingerprinting can be used to help resolve questions of paternity
- individuals inherit half their genetic material from their mother and half form their father
- therefore each band on a DNA fingerprint of an individual hold have a corresponding band in one of the parents parents’ DNA fingerprint
- this can be used to establish whether someone is the genetic father of a child
- genetic fingerprinting is also useful in determining genetic variability within a population
- the more closely two individuals are related the closer the resemblance of their genetic fingerprints
- a population whose members have very similar genetic fingerprints has little genetic diversity
- a population has greater genetic diversity
12
Q
What is forensic science?
A
- DNA is often left at the scene of a crime, for example, blood at the scene of a violent crime, semen at the scene of a rape and hair at the scene of a robbery
- genetic fingerprinting can establish whether a person is likely to have been present at the crime scene, although this does not prove they actually carries out the crime scene, although this does not prove they actually carried out the crime
- even if there is a close match between a suspect’s DNA and the DNA found at the crime scene, it does not follow that the suspect carried out the crime
- finally, the probability that someone else’s DNA might match that of the suspect has to be calculated
- this calculation is based on the assumption that the DNA which produces the banding patterns is randomly distributed in the community
- this may not always be the case, for example, it may not apply where religious or ethnic groups tend to have partners from within their own small community
13
Q
What are other possible explanations for a possible match between someone’s DNA and the DNA found at a crime scene?
A
- 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 suspect’s DNA or by chemicals that affected the action of the restriction endonucleases used in preparing the fingerprinting
14
Q
What is medical diagnosis?
A
- genetic fingerprints can help in diagnosing diseases such as Huntington’s disease
- this is a genetic disorder of the nervous system
- it results from a three-base sequence (AGC) at one end of the gene on chromosome 4 being repeated over ad over again - a sort of genetic stutter
- people with fewer than 30 repeats are unlikely to get the disease, while those with more that 38 repeats are unlikely to get the disease, which those with more than 38 repeats are almost certain to do so
- if they have over 50 repeats the onset of the disease will occur earlier than average
- a sample of DNA from a person with the allele of Huntington’s disease can be cut with restriction endonucleases and a DNA fingerprint prepared
- this can then be matched with fingerprints of people with various forms of the disease and those without the disease
- in this way, the probability of developing the symptoms and when can be determines
- genetic fingerprints are also used to identify the nature of a microbial infection by comparing the fingerprint of the microbe found in patients with that of known pathogens
15
Q
What is plant and animal breeding?
A
- genetic fingerprinting van be used to prevent undesirable interbreeding during feeding programs on farms or in zoos
- it can also identify plants or animals that have a particular allege of a desirable gene
- individuals with this allele can be selected for breeding in order to increase the probability of their offspring having the characteristic that it produces
- another application is the determination of paternity in animals and thus establishing the pedigree (family tree) of an individual
