Dr. Robin Cotton, director of the nation’s largest private DNA laboratory, began her testimony by delivering a basic primer on how DNA works, peppering her lecture with a series of metaphors intended to illuminate the scientifically dense topic. At various points, Cotton compared DNA to an alphabet, a thread, a zipper, a pair of interwoven ribbons, a chapter in the genetic book that is the chromosome and a blueprint for human development similar to the plans for erecting a building.

If we make the assumption that a blueprint contains all the information for how to build your house, she said, the analogy is that DNA contains all the information on how to build you.

Though she did not describe any results of DNA tests performed by her laboratory in this case, prosecutors say those results will reveal a trail of blood linking Simpson to the murder scene showing that blood with some of his genetic characteristics was at the scene and that blood apparently from both victims, Ronald Lyle Goldman and Nicole Brown Simpson, was found inside his car and at his Brentwood estate.

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Simpson has pleaded not guilty to the June 12, 1994, killings, and his legal team is prepared to mount an aggressive challenge to the DNA evidence.

Looking directly at jurors and speaking in a soft, clear voice, Cotton illustrated her testimony Monday with neatly drawn charts on sheets of butcher-block paper. She told jurors that degradation of DNA samples never would cause a sample to falsely point to a suspect, a notion that defense attorneys have hinted at for weeks.

Testifying on a day cut short by the funeral of a revered Los Angeles police detective, one that Superior Court Judge Lance A. Ito and other trial participants wanted to attend, Cotton was guided through her testimony by Deputy District Attorney George Clarke, one of the most experienced DNA prosecutors in California.

Although many trial watchers have braced for at least a month of dull, scientific testimony on DNA, Cotton’s initial appearance was delivered brightly and illustrated simply.

Under questioning from Clarke, Cotton wove her observations about the basic science of DNA with her qualifications and with the history of her company, Cellmark Diagnostics in Germantown, Md.

Many of the jurors took copious notes and most seemed attentive during Monday’s session.

Cotton, an experienced witness who said she has testified in roughly 90 cases, held that attention with an approach like that of a high school biology teacher. She meticulously addressed each of her points, starting with basic definitions of terms such as molecular biology, biochemistry and DNA and building toward the specific techniques employed by Cellmark, one of several laboratories that have tested samples in the Simpson case.

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Cotton told jurors that DNA is present in nearly all human cells with the major exception of red blood cellsand reminded the panelists that all people get their DNA from their parents. Although more than 90 percent of a person’s DNA is common to all peoplea common stock that reflects the similarities among all humansthe balance is highly individual, she said.

That’s intuitively obvious because you can look around even in a very large room and you’ll see that with the exception of identical twins, people are different, Cotton said. Even closely related people, brothers, two brothers, are different. You can always tell them apart. And the exception is identical twins. And identical twins are identical because they have identical DNA, and that is the only exception.

Scientific concepts such as that are important building blocks in the prosecution case because within a day or two, prosecutors expect to begin introducing DNA test results showing that blood found at the crime scene contained genetic markers matching those found in a sample of O.J. Simpson’s blood. The odds of those two samples having come from different people are extremely small, a notion that is based on the highly individual nature of DNA.

Near the end of her testimony, Cotton gave jurors a preview of what is to come, as she testified that her lab had indeed gotten results from DNA analysis known as RFLP testsone of two major types of DNA testing. Clarke also posed a series of questions to her intended to discredit the defense suggestion that degraded blood samples might have produced false results in some tests.

If DNA suffers this degradation process, will that result in changing of the types from the original type of sample to a new and different type? Clarke asked.

No, she responded. It will not.

So this process of degradation, can it change my DNA into looking like your DNA? the lawyer continued, emphasizing his point.

Or your DNA into looking like the court’s DNA? Clarke then asked, nodding in the direction of Ito.

No, she said again.

Cotton, who will return to the witness stand Tuesday, was called after a brief appearance by tow-truck driver Bernie Douroux, who testified about towing Simpson’s Ford Bronco from his Rockingham Avenue estate on the day after the killings.

Douroux spent less than 30 minutes on the stand, a record for speedy testimony in a case where some relatively minor witnesses have been subjected to extraordinarily probing questioning.

Although neither side made much of Douroux’s account, prosecutors called him to establish the chain of custody for the carwhich witnesses already have said was found stained with bloodand Simpson attorney Johnnie L. Cochran Jr. used his brief questioning to raise still more doubts about the police handling of evidence in the case.

Douroux testified that he never had unlocked Simpson’s car or touched the door handle and that he had left it inside a locked LAPD print shack after driving it across town from Simpson’s house on the day after the murders. Under cross-examination, Douroux acknowledged that he had briefly left Simpson’s Ford Bronco unattended outside police headquarters that afternoon and also said he had not noticed blood inside or outside the car when he picked it up or dropped it off.

LA TIMES-WASHINGTON POST05-08-95 2056EDT