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Summary Notes on Genetic Fingerprinting - AQA A Level Biology
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3.8.4.3 Genetic Fingerprinting – AQA A Level Biology Summary NotesGenetic fingerprinting
- Diagnostic tool used widely in forensic science, plant and animal breeding and medical
diagnosis
- Relies upon being able to seperate fragments of DNA by size, a technique called gel
electrophoresis
Electrophoresis
- Migration of charged molecules under the influence of an electric field
- Because DNA is negatively charged, DNA molecules will move towards a positive electrode
(anode) when exposed to a potential difference
Steps to carry out electrophoresis:
1. A gel made from agarose (a carbohydrate with a mesh-like structure) is created. Wells are
present at one end
2. Gel is placed in tank and covered in a buffer solution that conducts electricity
3. Mixture of DNA fragments of different lengths to be separated is put in wel at one end of gel
4. Negative electrode is at well-end of the gel and positive electrode is at other end. A
potential difference is applied
5. DNA fragments move towards the end of gel where the positive electrode is. Smaller
fragments will move more quickly than larger fragments, so DNA fragments are seperated by
size
As mentioned in 3.8.4.2, the gel may then be used in Southern Blot and DNA probes applied
Often one of the wells is loaded with a ‘ladder’ of fragments of DNA of known size (‘markers’)
This allows scientists to compare the position of unknown fragments of DNA that they are separating
with the ladder to find out how large each DNA fragment is
, An organism’s genome contains many variable number tandem repeats (VNTRs). The probability of
two individuals having the same VNTRs is very low
VNTRs are sections of DNA that do not code for a protein, and consist of a number of bases that
repeat and are found next to each other
Different VNTRs are found on different loci on different chromosomes, but all individuals have VNTRs
at same loci (just like different genes are found on different chromosomes, but in any two individuals
the same genes are found in the same positions on the same chromosomes)
VNTRs are said to be variable number because the number of times the base sequence
repeats is different in different individuals (think of this like different alleles of a gene)
Here, VNTR sites are frequently flanked by
restriction sites recognised by restriction
enzymes
This means that VNTRs can be cut out from
individual’s DNA and separated by size using
electrophoresis
DNA probes can be added to visualise the
position of VNTRs. This leaves a unique
genetic fingerprint
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