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DNA Profiling (DNA fingerprinting)
What is DNA Profiling? A technique used by scientists to distinguish between individuals of the same species using only samples of their DNA
Who Invented it? • The process of DNA fingerprinting was invented by Alec Jeffreys at the University of Leicester in 1985. • He was knighted in 1994.
What is a DNA fingerprint? • Every cell of an individual carries a copy of the DNA • a cell collected from a person’s skin or hair folicle contains the same DNA as from that persons heart tissue or white blood cells • Order of base pairs in the DNA of every individual is different except identical twins
How do we distinguish one person’s DNA from another? • We do not need to sequence the entire 3 billion base pairs of a person’s DNA to distinguish it from another person’s DNA • Junk DNA contain sequences that are 20-100 bp in length that are repeated at different locations (loci) along the chromosome. • CGGCTACGGCTACGGCTA(repeated 3 times at this location; at another location, it may be repeated 9 times) • These sequences are called Short Tandem Repeats (STRs) or VNTRs
STRs • Each person has some STRs that were inherited from mother and some from father • No person has STRs that are identical to those of either parent • The number of repeats at each loci on chromosome is highly variable in the population, ranging from 4 to 40. • The length of the DNA after cutting the chromosome with a restriction enzyme, and its position after electrophoresis will depend on the exact number of repeats at the locus
The uniqueness of an individual’s STRs provides the scientific marker of identity known as a DNA fingerprint. • In the United States the FBI has standardized a set of 13 STR assays (13 different locations on the chromosomes) for DNA typing, and has organized the CODIS database for forensic identification in criminal cases. • The United States maintains the largest DNA database in the world: The Combined DNA Index System, with over 60 million records as of 2007.
Preparation of a DNA fingerprint • Specimen collection • blood, semen, etc • easy to contaminate a DNA sample with DNA from other sources (bacteria, DNA of person collecting sample) • DNA is not stable for very long-it degrades • sunlight • heat • moisture
Stages of DNA Profiling • Stage 1: Cells are broken down to release DNA If only a small amount of DNA is available it can be amplified using the polymerase chain reaction (PCR)
PCR allows you to make millions of copies of the STR region from a single copy of DNA you recovered from crime scene.
Stages of DNA Profiling • Step 2: The DNA is cut into fragments using restriction enzymes. Each restriction enzyme cuts DNA at a specific base sequence.
Stages of DNA Profiling • The sections of DNA that are cut out are called restriction fragments. • This yields thousands of restriction fragments of all different sizes because the base sequences being cut may be far apart (long fragment) or close together (short fragment).
Stages of DNA Profiling Stage 3: • Fragments are separated on the basis of size using a process called gel electrophoresis. • DNA fragments are injected into wells and an electric current is applied along the gel.
Stages of DNA Profiling DNA is negatively charged so it is attracted to the positive end of the gel. The shorter DNA fragments move faster than the longer fragments. DNA is separated on basis of size.
Stages of DNA Profiling • A radioactive material is added which combines with the DNA fragments to produce a fluorescent image. • A photographic copy of the DNA bands is obtained.
Stages of DNA Profiling Stage 4: • The pattern of fragment distribution is then analysed.
Uses of DNA Profiling • DNA profiling is used to solve crimes and medical problems
Crime • Forensic science is the use of scientific knowledge in legal situations. • The DNA profile of each individual is highly specific. • The chances of two people having exactly the same DNA profile is 30,000 million to 1 (except for identical twins).
Biological materials used for DNA profiling • Blood • Hair • Saliva • Semen • Body tissue cells • DNA samples have been obtained from vaginal cells transferred to the outside of a condom during sexual intercourse.
DNA Profiling can solve crimes • The pattern of the DNA profile is then compared with those of the victim and the suspect. • If the profile matches the suspect it provides strong evidence that the suspect was present at the crime scene (NB:it does not prove they committed the crime). • If the profile doesn’t match the suspect then that suspect may be eliminated from the enquiry.
Example • A violent murder occurred. • The forensics team retrieved a blood sample from the crime scene. • They prepared DNA profiles of the blood sample, the victim and a suspect as follows:
Was the suspect at the crime scene? Suspects Profile Blood sample from crime scene Victims profile
Solving Medical Problems DNA profiles can be used to determine whether a particular person is the parent of a child. A childs paternity (father) and maternity(mother) can be determined. This information can be used in • Paternity suits • Inheritance cases • Immigration cases
Example: A Paternity Test • By comparing the DNA profile of a mother and her child it is possible to identify DNA fragments in the child which are absent from the mother and must therefore have been inherited from the biological father.
Is this man the father of the child? Mother Child Man
Famous cases • In 2002 Elizabeth Hurley used DNA profiling to prove that Steve Bing was the father of her child Damien
Famous Cases • Colin Pitchfork was the first criminal caught based on DNA fingerprinting evidence. • He was arrested in 1986 for the rape and murder of two girls and was sentenced in 1988.
Simpson/Goldman Murder • Pretrial hearings announced that blood collected at crime scene matched that of O.J.s • Defense argued that contamination could have occurred during sample collection and between collection of different samples • Technician admitted mislabeling samples • Possibility that evidence might be tainted was obvious to both the court and the jury • DNA evidence was not allowed as evidence • When rules of evidence are not followed, DNA samples lose their value in court.