Since Francis Crick and James Watson discovered its structure in 1953, deoxyribonucleic acid, or DNA, has dominated scientific and medical research.

Criminal cases have been built on the discovery of DNA evidence; conversely, it has freed innocent men from jail. It's used as a tool in archeology, anthropology and zoology to track humans and species long dead or very much alive.

But how is this material obtained? And what are its limits? In order to understand the implications of DNA evidence, here is a look at the basics of DNA retrieval.

What is DNA?

Deoxyribonucleic acid is a long, double-stranded molecule shaped like a twisted rope ladder.

The backbone of DNA carries on it four bases — adenine, thymine, cytosine and guanine — which pair up and form the steps of the ladder. The sequence of these pairs forms the genetic code and allows proteins to copy the information and transfer it other cells.

DNA could be called the blueprint for life.

All living things contain DNA, except for some viruses, and all plants and animals store their DNA in the nucleus of the cell.

For humans, DNA found in the nucleus comes from genetic material passed on from both parents and is present in nearly every tissue in the body. With the exception of identical twins, each person's DNA is unique to them.

This unique feature has made DNA evidence increasingly popular for use in forensic science, both for identifying blood, hair follicles, skin, semen, sweat and other tissue samples at crime scenes and for use in missing persons investigations.

But scientists can only identify an individual through DNA if they can compare it to some other source of genetic material.

How do scientists isolate and analyze it?

When scientists take small biological samples from something like blood cells, they have to clear all the protein and lipids surrounding the DNA inside the cell.

Though there are dozens of different methods, most use some combination of enzymes and detergent to break down the cell and bond with the lipids but not the DNA, and an acid to remove impurities. The genetic material is spun out using a centrifuge or other separation machine.

But DNA can break apart over time. The older the sample, the more likely it is that the sequences obtained after separation will be incomplete or small in size. To create larger samples, scientists use a process called polymerase chain reaction (PCR) amplification.

Essentially, the DNA is warmed until its two strands split apart and form a template after which a primer material can pattern itself.

The primer make copies of the pattern at lower temperatures and once the temperature is raised, the primer joins with the template to effectively double the amount of the DNA sample.

This process can be repeated 30 or more times to turn a small sample of just a few dozen cells into a sample large enough to work with and analyze.

Can DNA be obtained from any other source?

Blood, skin and the other sources of DNA found in the nucleus are not the only substances that leave traces of DNA. Bones also hold mitochondrial DNA inside them as well, providing crime investigators with a means of identifying bodies from the coldest of cases and archeologists and archeologists with a useful tool for looking into centuries past.

Mitochondrial DNA is found outside of the nucleus in the mitochondria, where a cell generates energy.

Since cells can contain thousands of mitochondria, mitochondrial DNA is more plentiful than nuclear DNA. Mitochondrial DNA can come from hair shafts or bone cells lacking a nucleus, or from smaller, older fragments of evidence.

However, unlike nuclear DNA, the vast majority of mitochondrial DNA is inherited from the mother and so it doesn't offer a unique fingerprint of every person. Siblings, mothers and grandmothers may share mitochondrial DNA, so tests cannot discriminate between these family members.

How far back can scientists trace DNA?

The oldest widely accepted DNA sample comes from 400,000-year-old plants found in ice in Siberia. The oldest DNA recovered from a hominid is 100,000 years old — from a Neanderthal tooth found in Scladina cave in Meuse Basin in Belgium.

Most archeological sites don't last nearly that long. After an organism dies bacteria, oxidation, hydrolysis and even radiation can slowly break down genetic material.

"We don't have a good understanding yet of the rules of DNA preservation," said Dr. Dongya Yang, an archeology professor at Simon Fraser University. "Each burial environment is unique, and each set of conditions can affect the samples differently."

In general, high temperatures and moisture are bad for DNA survival, as is soil that is either too acidic or basic. The best-preserved samples come from sources frozen in Arctic regions, an environment where bacteria are not as active.

How accurate is the evidence?

Like all evidence, a sample of DNA is more reliable if it hasn't been contaminated. Contamination can occur from environmental conditions like those listed above or simply if the sample comes in contact with other DNA.

A bone sample, for example, might provide a wealth of information about the genetic makeup of a human from centuries ago. But if countless people have held the bone in their hands before it finally makes its way to a lab for an examination, it's highly likely some skin DNA from those individuals will have rubbed off on the sample and show up in a test, possibly spoiling the results.

A number of methods to remove foreign DNA are used in any DNA separation process, but the cleaner the starting sample, the more likely it is that the final sample will be clean.

While seen as an accurate process, the constant changes in the technology can lead to second-guessing of past tests. In the United Kingdom, the Association of Chief Police Officers said in February 2007 that some 200 cases involving DNA testing would be revisited because of concern that crucial evidence may have been missed due to differences in the way forensic labs tested for DNA samples between 2000 and 2005.

DNA evidence — whether it is used to put a person at the scene of a crime or determine whether uncovered bones were from one hominid or another — is often only as good as the surrounding evidence, said Yang.

"DNA itself cannot itself provide an answer," said Yang. "You have to put it in context and make sure you're interpreting the other information around you."

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