Genetic fingerprinting Flashcards
What is genetic fingerprinting?
A persons DNA profile, 99.9% of the human genome is the same in every person, remaining 0.1% makes an individuals fingerprint unique.
It only shows the non-coding portions of DNA, relies on 2 techniques:
-Gel electrophoresis.
-Polymerase chain reaction to make large number of copies.
The DNA of a genetic profile?
Exons are sections of coding DNA within a gene.
Introns are blocks of repeated nucleotides, each type is called an STR (short tandem repeat), exist in HVRs (hyper-variable regions), identical STRs are called a satellite.
DNA is extracted from a sample, cut into small fragments using restriction enzymes, these fragments will be different lengths in different people.
DNA is extracted and repeat sequences can be ‘cut out’ by enzymes.
What is gel electophoresis?
DNA fragments are separated by this.
DNA is placed into wells at one end of an agarose gel and a voltage is applied across the gel.
DNA is negatively charged due to the PO4-, it migrates towards the + anode.
Smaller the fragment, the faster it moved through gel.
Fragments separate into bands on the gel, each individual has different sized fragments of DNA the pattern formed by the distance moved by their DNA is unique to each.
What is southern blotting? (used to make the DNA visible)
- Gel is covered with a nylon membrane, alkaline solution and special paper.
- DNA fragments transfer upwards into this membrane.
- Nylon membrane (contains DNA fragments) is removed.
- Sodium hydroxide is added making DNA single.
- DNA probes are added to membrane, these attach to specific strands of fragment.
- Membrane washed to remove any non-bonded probes.
- Membrane placed under x-ray film, radioactive probes cause dark bands to appear.
- Every person has a different number of repeated patterns (autoradiograph).
What is a DNA probe?
Short single-stranded DNA that are complementary to part of the DNA fragment and also labelled with radioactivity or fluorescense.
What happens during a paternity suit and criminal conviction?
Bands are inherited from both parents so the technique can be used to identify parents in suits.
WBC are taken from mother and possible father, bands of mother are subtracted from child’s pattern, if man is the father he must possess all remaining bands in child’s fingerprint.
What is polymerase chain reaction (PCR)?
If a small sample of DNA found at crime scene, DNA is amplified (create identical copies) to provide sufficient DNA for fingerprinting.
PCR produces many molecules of DNA from an initial single molecule. Uses DNA polymerase enzymes with a high op temp.
PCR and the use of primers?
The DNA to be copied is mixed with buffer, DNA polymerase, DNA nucleotides and short pieces of DNA called primers.
Primers are about 10 bases long and bind to single stranded DNA to make double strands. DNA polymerase which replicates DNA, cannot bind directly, first made single and then replicates using polymerase and primer follows.
The method of PCR??
- Melting- original DNA is heated to 95, seperates 2 strands by breaking hydrogen bonds, DNA is single now.
- Annealing- solution cooled to 55 triggering primers to join to base sequences.
- Extension- solution heated to 70 (op for polymerase), primers used as start point, enzyme catalyses the formation of double strand.
- Repeat 1-3 many times, doubling quantity each time.
What are the limitations of PCR??
- Contamination; any DNA entering by accident may be amplified, could be air borne, from experimenter or using same apparatus.
- Error rate; DNA polymerases sometimes insert the wrong nucleotide, most of the time correct itself, taq cannot.
- Fragment size; most efficient at 1000-3000 base pairs as taq cannot proof read.
- Molecules in sample may act as inhibitors and PCR is very sensitive to them.
DNA profiling in society?
- Paternity; DNA from WBC, bands in childs compared with mothers, remaining must be dads so compared.
- Twins; monozygotic (identical) have identical banding patterns in profile.
- Siblings; people adopted may want to check blood relatives where profiles will be very similar.
Benefits of DNA profiling?
- Non invasive methods to obtain biochemical samples, e.g. mouth swabs, urine, hair.
- The technique can be used on small DNA samples.
- Can be used to reverse wrongful convictions when used with other forensic tools.
- Efforts are being made to store genetic material from people around the world.
Concerns about DNA profiling??
- Any request for DNA samples can be considered by some as a violation of privacy.
- Concerns that storage of genetic info in computer may be vulnerable or hacked.
- Access to and use of genetic data must be carefully regulated, insurance companies could deny coverage.
- Employers could avoid employing if certain traits.
- Data profiling may produce wrongful convictions (if errors in procedure).
