Many of us like to think we know a thing or two about DNA and its use in solving crimes.

We've watched TV shows like CSI or Silent Witness and we know it is imperative that crime scenes are treated reverentially, with minimal disturbance, and that anyone entering them must be dressed in full protective gear.

But while we're all armchair experts these days, that wasn't always the case.

As has been made abundantly clear over the past fortnight in the trial of Bradley Edwards for the 1996 and 1997 Claremont serial killings, our understanding of DNA has progressed enormously in the intervening decades.

Today it is widely known that a person's DNA can be left at a scene by the most casual of contact, even if they don't touch anything. Without adequate precautions, contamination can easily occur.

Forensic police examine the bush crime scene where Ciara Glennon's body was found. ( ABC News )

But 20 years ago, few had such a sophisticated understanding and crime scenes were treated very differently.

When protective clothing was optional

Crime scene videos played in the WA Supreme Court this week of the burial sites of Jane Rimmer and Ciara Glennon captured vision of a senior forensic officer holding an evidence bag without gloves and even using his bare hands to push clumps of dirt and other material collected at the scene into the bag.

The body of Ms Rimmer, a 23-year-old childcare worker, was found naked and semi-covered in vegetation in bushland at Wellard, south of Perth, on August 3, 1996, nearly two months after she went missing.

Jane Rimmer's body was found partly covered in vegetation off a road in Wellard. ( Supplied: Supreme Court of WA )

The remains of 27-year-old lawyer Ms Glennon were found at Eglinton, north of Perth, also semi-covered in vegetation in an area of low-lying coastal scrub, on April 3, 1997. She had been reported missing three weeks earlier, on March 15.

Ciara Glennon's body was found in bushland in Perth's northern suburbs. ( ABC News )

Witness after witness — most of them police officers and many forensic experts — testified that the use of protective clothing and gloves was largely optional at crime scenes in the mid-1990s.

Keeping officers' clothes and hands clean and preventing infection was the main reason they were used, if at all, and little thought was given to contamination — either of the scene in general or of individual exhibits.

"DNA was not a factor," former forensics senior sergeant Barry Mott told the court this week, agreeing with defence counsel Paul Yovich SC that he "certainly did not go to a crime scene thinking that [he] could potentially contaminate an exhibit without even touching it".

Mr Mott, who is no longer a police officer, even admitted he may have accidentally trodden on Ms Glennon's body as he helped collect evidence at the scene.

"I just honestly can't recall whether I — whether I stepped on it or I've inadvertently stepped on it," he said.

"Trying to move a deceased's body is quite difficult, especially one in that condition."

OJ Simpson case put spotlight on DNA

Ms Rimmer's body was discovered a year after one of the highest-profile crime cases on the planet had been run in a Los Angeles court — the trial of OJ Simpson for the murder of his ex-wife Nicole Brown and her friend Ron Goldman.

It was the first time DNA evidence had come into the spotlight in such a major case, as US prosecutors presented 108 separate DNA exhibits that experts testified linked the former American football star to the bloody crime scene.

Some of the arguments used by OJ Simpson's lawyers are similar to those run by Edwards's defence team. ( Reuters: Fred Prouser )

But his defence team successfully argued that the evidence had been "compromised, contaminated and ultimately corrupted", and Simpson was acquitted.

Whether the OJ Simpson case played on the minds of officers who attended the Rimmer and Glennon crime scenes is impossible to know, but a quarter of a century later, some of the arguments used by Simpson's lawyers are similar to those run by Edwards's defence team — the evidence could have been contaminated and therefore cannot be accepted as proof of guilt beyond reasonable doubt.

Just as in the OJ Simpson case, DNA is crucial to the prosecution's case in the Claremont serial killings trial.

DNA evidence, which first came to prominence in the OJ Simpson case (right) will be crucial in the trial of Bradley Edwards (left). ( ABC News, Reuters )

The defence is making every effort to show that the way in which evidence was collected at the crime scene was cavalier at best and therefore utterly unreliable.

The Simpson case didn't have the same limitations as the Claremont trial, however, where the two bodies were not discovered until some time after the young women's deaths, left out in the open and subjected to the elements for several weeks.

And while Californian prosecutors had 108 items of DNA to use to boost their case, DNA in the Claremont case is largely confined to just two crucial exhibits labelled as AJM 40 and AJM 42 — scrapings taken from Ms Glennon's left thumbnail and right middle fingernail.

Ciara Glennon fingernail scrapings crucial

Laboratory testing obtained a mixed DNA profile from the fingernail scrapings that came from two people — Ms Glennon herself and another person.

Until 2009, police had no idea who this second person was.

But on January 16, 2009, PathWest scientist Laurie Webb made a crucial link. He discovered that the male component of the DNA on the Glennon samples matched the male component of DNA extracted from intimate swabs taken from the teenage victim of a violent rape at Karrakatta Cemetery in 1995.

Both lots of samples had been stored in the same DNA extract freezer at PathWest — the state pathology lab.

In her opening address, state prosecutor Carmel Barbagallo SC was quick to quash any suggestion the samples could somehow have come into contact while at PathWest, describing potential contamination in this way as an "exercise in errant fantasy".

"DNA within a particular sample doesn't just fly around a laboratory," she said, arguing the evidence would prove the sample from the rape victim "never came anywhere near AJM 40 and 42 exhibits, both in terms of time and place within the laboratory".

"It cannot pass through a barrier such as plastic," Ms Barbagallo said, and crucially, "it cannot selectively contaminate an exhibit with only part of the sample from which it is contained".

Prosecutor Carmel Barbagallo says suggestions of DNA contamination of some exhibits are an "exercise in errant fantasy". ( ABC News: Carmel Barbagallo )

In other words, because the rape victim's samples contained both her DNA and that of Edwards, had any contamination occurred between the two samples, her DNA would also have been found on the Glennon evidence.

Yet it was not.

Edwards confessed to the 1995 rape of the 17-year-old on the eve of his murder trial.

Scientists' DNA found on evidence

Mr Yovich said in his opening address that his client did not contest the fact his DNA had been found on the sample taken from Ms Glennon's fingernails and that he "does not have any explanation for the DNA".

"We accept as well that again the scientific literature suggests that the chance of contamination in a lab is usually remote, although secondary transfer is known and documented in the literature," he said.

Defence lawyer Paul Yovich argues DNA contamination in the laboratory cannot be safely ruled out. ( ABC News: Hugh Sando )

But he also pointed out four pieces of evidence in the case had already been contaminated.

The DNA of two different PathWest scientists was found on swabs taken from the two bodies — fingernail samples from Ms Rimmer were found to also contain Ms Glennon's DNA and a swab from a branch placed on top of Ms Rimmer's body was found to contain DNA matching the profile of the victim of a separate crime whose samples had been processed at PathWest around the same time.

Jane Rimmer's body was found nearly two months after she disappeared from Claremont. ( Supplied )

In these circumstances, Mr Yovich said, contamination in the laboratory could not be safely ruled out.

His opening address also identified another key line of attack the defence planned to use with regard to the DNA evidence — that it was "trace" DNA and scientists could not say where it was from nor how it got there.

"So the DNA evidence on which the state relies, and without which they cannot prove the case against the accused beyond reasonable doubt, consists of DNA that was extracted from cellular material that cannot be precisely identified," Mr Yovich said.

DNA evidence is a central plank of the prosecution's case against Bradley Edwards. ( ABC News: James Carmody )

The evidence, he said, had apparently come from "on or around Ciara Glennon's thumb and left middle finger but no other fingers, or perhaps from the sides of the yellow-top containers in which that debris had been stored, or perhaps from a combination of some or all of these things".

These arguments have yet to be fully teased out by the defence and we can expect some hard questioning from Mr Yovich once DNA experts are called to the stand in the coming weeks.

Forensic scientist set to testify

Mr Yovich is likely to focus on the role played by forensic biologist Laurie Webb, who was the state's leading DNA analyst at the time.

Mr Webb was involved with the handling of exhibits taken from Ms Glennon's body from the outset, working with a colleague to create DNA extracts from her fingernail scrapings.

Ciara Glennon was the third young woman to disappear from Claremont in 15 months. ( Fairfax Media )

He also took four plastic jars containing the fingernail samples to the UK — including the all-important samples AJM 40 and 42 — where they were examined by forensic experts who made the breakthrough discovery that the samples contained a mixed DNA profile.

Mr Webb then went on to link the male part of the sample to material extracted from swabs taken from the Karrakatta rape victim.

He is expected to testify at the trial in the coming months and Mr Yovich's cross-examination is likely to be protracted and thorough.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 3 minutes 59 seconds 3 m 59 s A timeline of the Claremont serial killings

Sloppy record keeping and the lack of continuity in exhibit handling have also been seized on by the defence as significant flaws in the state's case.

Evidence from police integrally involved in the case pointed to a number of irregularities in the collection and storage of exhibits, which were sometimes not numbered sequentially or were recorded against the wrong number.

In other cases, police appeared to lose track of exhibits for months at a time because systems meant to keep track of such items were not updated when they should have been.

Defence seeks impression of unreliability

This was true of key exhibits, including the black shorts worn by the teenager raped at Karrakatta Cemetery in 1995, the year before the Claremont killings began.

Edwards has pleaded not guilty to murdering the three women in 1996 and 1997. ( Supplied: WA Supreme Court )

The shorts were incorrectly labelled as a skirt, a mistake not picked up for some time, and were recorded on police paperwork as being kept together in a bag with the victim's underpants, although police testified that they had been kept separately.

The shorts were unaccounted for over an eight-month period, police records showed, an error retired detective Jonathan Adams attributed to a "human data entry lag".

Another officer involved in the recording of evidence at the site where Ms Glennon's body was found, Sergeant Gary Hyde, admitted to numerous mistakes in recording information, including incorrect quantities of evidence, mixed-up dates and inconsistent descriptions of evidence.

An electronic database known as the Property Tracking System (PTS) was in use by WA Police at the time and was meant to record what items had been taken from crime scenes as evidence, give them an identifying number and log where they were stored at any given time and where they had been.

But numerous instances of its shortcomings and inaccuracies have been detailed by witnesses after prompting from Mr Yovich, including its lack of detailed descriptors for evidence.

Mr Yovich's detailed probing of police witnesses has sometimes appeared to be nit-picking, but has been part of a strategy to slowly create an overall impression of unreliability — that there were so many errors made, the evidence cannot be safely relied upon.

The senior defence counsel left no doubt during his opening address about what his evidentiary focus would be.

"The process of collection, storage, handling and examination of exhibits will be live in the trial, especially when we say there are large gaps in the chain of continuity of important exhibits, and also where exhibit handling, recording and forensic processing protocols were not always satisfactory," he said.

The trial is due to resume on Monday.