21.5 genetic fingerprinting Flashcards
what does genetic fingerprinting rely on
the fact the genome of most eukaryotic organisms contains many repetitive non-coding bases of DNA which are unique for every individual (excluding twins)
DNA bases which are non-coding are known as
Variable number tandem repeats (VNTRs)
what is gel electrophoresis used for
to separate DNA fragments according to their size
how does gel electrophoresis work
DNA fragments placed on agar gel and a voltage is applied across it
resistance of the gel means the larger the fragments the more slowly they move
over fixed period small fragments move further than the large ones.
if DNA fragments labelled e.g. with radioactive DNA probes
final position in gel can be determined by placing a sheet of X-ray film over the agar gel for several hours
radioactivity from each DNA fragment exposes the film and shows where the DNA is situated on gel
drawbacks of gel electrophoresis
solution
only DNA fragments up to 500 bases long can be sequenced this way
larger genes must be cut into smaller fragments by restriction endonucleases
5 main stages of making a genetic fingerprint
- Extraction
DNA is extracted from the sample - Digestion
restriction endonucleases cuts DNA into fragments - Seperation
fragments are separated using gel electrophoresis
DNA fragments are transferred from gel to nylon membrane - Hybridisation
DNA probes added to label the fragments. these radioactive probes attach to specific fragments - Development
membrane with radio actively labeled DNA fragments is placed onto x-ray film.
development of x-ray film leaves dark bands where the radioactive DNA probes have attached
interpreting results e.g. 2 samples blood found at scene of crime and blood from a suspect
first visually checked
if a match the pattern of bars of each fingerprint is passed through an automated scanning machine which calculates length of DNA fragments.
closer the match the greater the probability that 2 sets of DNA have come from same person
