Gel electrophoresis

Gel electrophoresis is used to separate DNA fragments. Electrophoresis uses an electric current to separate different-sized molecules in a sponge-like matrix. Smaller molecules move more easily through the gel pores than larger molecules.

Agarose gel can be used to separate DNA. First the gel is submersed in a tank filled with a salt solution which conducts electricity.

Using a pipette, DNA samples are loaded into slots made in the gel. The DNA is colorless but a blue dye is added, this makes it easier to track the DNA and its migration through the gel.

The phosphate groups in the DNA backbone carry negatively charges oxygen’s, giving DNA an overall negative charge. In an electric current, the negatively charged DNA moves towards the positive pole of the electrophoresis chamber.

DNA molecules then move through the gel by ‘reptation’-a reptile like snaking action through the pores of the gel. Smaller DNA fragments migrate faster and further over a given period of time than do larger fragments. This is how DNA fragments can be separated by size in the gel.

The introduction of a florescent dye, ethidium bromide, was used to stain the DNA. Ethidium bromide binds to the DNA double helix and glows in ultra-violet light. This lets researchers see where the separated DNA fragments end up.

DNA Profiling

Genes make up the blueprint for our bodies and almost every cell in the human body contains a copy of the blueprint, stored inside the nucleus. These genes are on the DNA on strands known as chromosomes. Certain portions of DNA are unique to each individual and DNA profiling is a way of establishing identity and is used in a variety of ways, such as finding out whether twins are fraternal or identical. DNA samples are usually obtained from blood.

DNA profiling is a technique used by forensic scientists to identify an individual by their respective DNA profiles. DNA profiles are encrypted sets of numbers that reflect a person's DNA makeup, which can also be used as the person's identifier.

DNA profiling involves the (partial) sequencing of genomes. Profiles tend to focus on areas of satellite of junk DNA which vary significantly between individuals. Junk DNA is used because by sequencing a number of sections a unique ‘genetic fingerprint’ can be created for an individual.

The uses for profiling DNA:

•             Paternity - to find out if the alleged father is actually the biological father of the child.

•             Twins - identical twins share the same genetic material, while fraternal (non identical) twins develop from two eggs fertilised by two sperm and are no more alike than individual siblings born at different times. It can be difficult to tell at birth whether twins are identical or fraternal.

•             Siblings - for example, adopted people may want to have DNA tests to make sure that alleged biological siblings are actually their blood brothers or sisters.

•             Immigration - some visa applications may depend on proof of relatedness.

•             Criminal justice - DNA testing can help solve crimes by comparing the DNA profiles of suspects to offender samples. Victorian law allows the collection of blood and saliva samples from convicted criminals and suspects. DNA profiles are then kept on a database.

Advantages of DNA profiling:

•             DNA tests can be applied to any human sample that contains cells with nuclei, such as saliva, semen, urine and hair.

•             DNA tests are extremely sensitive, and can be conducted using samples that would be too small for other serological tests.

•             DNA resists degeneration even after contamination with chemicals or bacteria.

•             The ability of DNA profiling to exclude a suspect means the police are able to confidently drop that line of enquiry and continue their investigation down other avenues.

Limitations of DNA profiling

•             DNA profiling can give incorrect results, due cross-contamination of samples.

•             Old DNA profiling technologies are more prone to errors, which could give false-negative or false-positive results.

•             DNA profiles can only offer statistical probability (for example, one in a million), rather than absolute certainty.

•             The more people tested, the lower the statistical probability. For example, the probability of one in a million may nosedive to one in 10,000 if enough people are profiled for a single test.

•             DNA databases stored on computer are vulnerable to exploitation via hackers.

•             Some critics point out that holding a person’s DNA profile on record is, in a sense, a violation of that person’s DNA ‘ownership’.

•             DNA evidence is easily planted at a crime scene.