LOS ANGELES (AP) _ The most critical part of the state's case against O.J. Simpson began Monday with a biochemist explaining the ABC's of DNA to jurors but waiting to reveal whether Simpson's genetic fingerprints link him to two murders.

Coincidentally, on the day that the scientist explained that everybody is made up of genes inherited from their parents, Simpson's elderly mother, Eunice, returned to court for the first time in weeks. She sat in a wheelchair near the front of the courtroom.

Robin Cotton, director of Cellmark Diagnostics in Germantown, Md., was the first prosecution witness on the crucial subject of DNA. With the even, no-nonsense tone of a biology teacher, she used charts and her own drawings to explain how genes passed to a child from mother and father form the ``genetic blueprint'' of the body.

``If a blueprint contains all information on how to build a house,'' she told jurors, ``the DNA contains information on how to build you.''

Under questioning by Deputy District Attorney George ``Woody'' Clarke, a DNA specialist on loan from San Diego County, Cotton tried to make the highly technical aspects of DNA accessible to jurors by using elementary metaphors.

``DNA has four basic components,'' Cotton explained to the jury, which has only two college graduates. ``They are referred to as bases, spelled b-a-s-e-s, so there are four bases that make up all the DNA. And they have the names adenine, guanine, thymine, and cytosine. And they are abbreviated ... just by their first letter, A, T, G, and C.

``These four bases are the DNA alphabet,'' she continued. ``It's just like if you look at the English alphabet, it has 26 letters. And you can put those letters together to make words which, in turn, makes sentences, which, in turn, makes paragraphs.''

Two strands of DNA, she said, are connected as if with a zipper. At one point, she referred to the DNA pairs as ``happy'' when they match up and ``pretty happy'' when they are separate _ phrases that drew some eye-rolling from jurors. Most jurors appeared to pay close attention, keeping their eyes on the charts being displayed. Only a few took notes.

At the defense table, Simpson, himself, yawned at one point.

The jurors' comprehension of the concept of DNA testing is vital to the prosecution, which hopes to show that the genetic blueprint in blood samples at the scene of the June 12 murders of Simpson's ex-wife and her friend match the genetic blueprint of Simpson's blood.

Cotton gave no results of the DNA analysis of blood in the Simpson case. Instead, she focused on familiarizing jurors with the technology, explaining such terms as ``gels'' and ``autorads,'' showing them small black bars on pieces of film.

Legal experts gave Cotton high marks for her thoroughness and clear delivery and said her explanations are crucial to whether jurors accept DNA evidence.

``She doesn't have to bring all these people up to the master's level of DNA, '' said Loyola University law professor Laurie Levenson, ``but she has to get them familiar with the vocabulary, and that's a lot of what today focused on.''

Levenson said the most important thing is that the jurors as a whole get a sense of the DNA.

``They're either going to say, `We believe in this science or we don't,' '' Levenson said.

Simpson is on trial for the slashing deaths of his ex-wife, Nicole Brown Simpson, and her friend Ronald Goldman. Because there were no known eyewitnesses and no murder weapons recovered, prosecutors are relying on analysis of blood, hair and fibers to prove they have arrested the right man. Defense lawyers have suggested a conspiracy theory in which police framed the black former football star by planting traces of his blood, and perhaps his ex-wife's, at the murder scene and at his estate.

The defense also has waged a pitched battle to show jurors that scientific evidence _ no matter how incriminating _ is too flawed to be believed. They have cited contamination in collection procedures and sloppy laboratory handling of samples.

Clarke asked Cotton whether any ``degrading'' of blood samples could make changes in DNA analysis results.

``This process of degradation, can it change my DNA into looking like your DNA?'' he asked.

``No,'' Cotton said.

Cotton took the stand after brief testimony from Bernie Douroux, a tow truck driver who hauled Simpson's Ford Bronco from his home to police headquarters the day after the killings. He said he didn't notice any blood stains but didn't open the locked vehicle. And he said he left the Bronco unattended for perhaps three minutes while he went in search of a detective.

Before testimony began, defense attorney Robert Shapiro complained to Judge Lance Ito that the coroner's office was charging the defense an inflated price of $85 each for slides taken of tissues during autopsies. He said the slides cost $4 to $5 each and normally are provided to the defense for free.

Ito suggested Shapiro file a formal motion to be heard by the supervising judge because the matter involves a coroner's office policy.

The trial was cut short Monday so that Ito could attend the funeral of Detective ``Jigsaw'' John St. John, the legendary LAPD investigator who solved at least 1,000 murders in his 51 years on the force.