Genetic fingerprinting Each DNA profile is unique!

Learning Objectives: What is genetic fingerprinting? How is genetic fingerprinting carried out? How are the results interpreted? For what purposes is it used?

What is Genetic fingerprinting? A technique used by scientists which is based on the fact that DNA of every individual, except identical twins, is unique. It is also known as DNA fingerprinting or DNA profiling.

Genetic fingerprinting Regions of chromosomes that code for proteins are called exons. Other regions that are non-coding are called introns. 95% of Human DNA does not code for any characteristics

Genetic fingerprinting Introns contain blocks of repeated nucleotides called core sequences. It is the number of times that these core sequences are repeated that produces the variations in individuals. The more closely related two individuals are the more similar the core sequences will be.

Genetic fingerprinting

How it is done? Stage 1: DNA is extracted from a sample (blood, hair, semen, skin). If only a small amount of DNA is available it can be amplified using the polymerase chain reaction (PCR). OR Cells are broken down to release DNA. The DNA is cut into millions of small fragments using restriction endonucleases. The restriction endonucleases are chosen for their ability to cut close to but not within the core sequences.

How it is done? 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).

How is it done? Stage 2: 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. DNA is negatively charged so it is attracted to the positive end of the gel.

How it is done? The smaller the fragment – the faster it moves. DNA is separated into bands according to size of the fragments.

How it is done? Stage 3: The gel is immersed in alkali in order to separate the double strands into single strands. The pattern of fragments are transferred to a nylon membrane by a process called Southern blotting.

Southern Blotting A thin nylon membrane is laid over the gel. The membrane is covered with several sheets of absorbent paper, which draw up the liquid containing the DNA by capillary action. This transfers the DNA fragments to the nylon membrane in precisely the same relative positions they occupied on the gel. The DNA fragments are then fixed to the membrane using UV light.

How it is done? Stage 4: Radioactive probes are used to attach to the core sequences (hybridisation). The probes have base sequences which are complementary to the core sequences. Any probes not bound are washed off. The membrane is dried.

How it is done? Stage 5: The nylon sheet is placed under X-ray film. The radioactive probes on the DNA fragments expose the film. This produces visible pattern of light and dark bands which is unique to each individual. The pattern of fragment distribution is then analyzed. The pattern of the bands is unique to every individual (except identical twins).

Uses of genetic fingerprinting Stage 6: The results can then be analyzed and interpreted. Genetic fingerprinting is used to solve crimes and medical problems. 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).

Genetic fingerprinting can solve crimes The pattern of the DNA profile is 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.

Solving Medical Problems DNA profiles can be used to determine whether a particular person is the parent of a child. A child’s 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. Mother Child Man

Genetic variability within a population A population whose members have very similar genetic fingerprints has little genetic diversity. A population whose members have a greater variety of genetic fingerprints has greater genetic diversity.

Tasks Complete the cut and stick activity. Answer the application questions.