SANFORD, Fla. -

Two weeks and about three dozen witnesses later, prosecutors are about to rest their case in George Zimmerman's second-degree murder trial.

Returning from a day off to observe the Fourth of July, the prosecution was expected to wrap up their questioning of witnesses Friday -- although they still may call Trayvon Martin's parents to testify. The parents have said it's their 17-year-old son screaming for help on 911 calls recorded during a fight the unarmed teen had with Zimmerman, a neighborhood watch volunteer, last year before Zimmerman fatally shot him.

Zimmerman's father has said it is his son. The screams are crucial pieces of evidence because they could determine who the aggressor was in the confrontation.

Zimmerman has pleaded not guilty, claim he was acting in self-defense when he fatally shot Trayvon Martin.

Zimmerman's attorneys will start their case after the prosecution rests

On Wednesday, prosecutors called forensics experts and college professors who they hoped would weaken Zimmerman’s claims of self-defense and professed ignorance of Florida’s “stand-your-ground” defense law.

Anthony Gorgone Zimmerman trial Florida Department of Law Enforcement DNA expert Anthony Gorgone on Wednesday testified that Martin’s DNA was not found on the grip of George Zimmerman’s gun, and Zimmerman’s DNA was not found under the unarmed teen’s fingernails. Zimmerman says he shot Martin in the chest to protect himself as Martin reached for his gun during a fight.

While cross-examining Gorgone, defense attorney Don West focused on the packaging of the DNA samples, suggesting they could have led to the samples being degraded. Gorgone told him Martin’s two sweatshirts had been packaged in plastic while wet, instead of a paper bag where they can dry out, and when he opened the samples they smelled of ammonia and mold.

Florida Department of Law Enforcement analyst Amy Siewert also testified that tearing and residue on Martin’s clothing showed the gun was directly against him when it fired.

The forensic experts’ testimony came on the same day prosecutors presented evidence about Zimmerman’s work in a college criminal justice course, which they say shows the former neighborhood watch volunteer knew about Florida’s self-defense law and had aspirations of becoming a police officer. They also showed he had applied for a police officer job in northern Virginia and applied to go on ride-alongs with Sanford Police officers.

George Zimmerman, 6-24-13 2 Prosecutors have sought to portray Zimmerman as a vigilante who profiled Martin as the teen walked through the central Florida gated community on a rainy night.

Zimmerman had maintained in an interview with Fox News last year that he did not know about the law. Prosecutors say he did have knowledge of it, however, because the subject was covered in the college class. They called as a witness Alexis Francisco Carter, the military attorney who taught Zimmerman’s class that covered the “stand-your-ground” law, which says a person has no duty to retreat and can invoke self-defense in killing someone if it is necessary to prevent death or great bodily harm. Carter described Zimmerman as one of his better students and said the neighborhood watch volunteer got an “A’’ in his class.

Under cross-examination, Carter gave two definitions of legal concepts that seemed to bolster the defense’s case. He explained that a person can make a self-defense argument if the person has a “reasonable apprehension” of death or great bodily harm.

“It’s imminent fear. The fact alone that there isn’t an injury doesn’t necessarily mean that the person didn’t have a reasonable apprehension or fear,” Carter said. “The fact that there are injuries might support there was reasonable apprehension and fear.”

Carter also explained the concept of “imperfect self-defense,” when a person is being threatened but then counters with a force disproportionately greater than the force used against them.

“They would have the right to defend themselves?” said defense attorney Don West.

“Right,” Carter said.

Another instructor, Seminole County State College professor Scott Pleasants, testified that Zimmerman had taken his online criminal justice class. Pleasants’ testimony via Skype from Colorado, broadcast live on television, was interrupted when he started getting inundated with Skype calls.

Zimmerman, 29, is charged with second-degree murder in the shooting death of Martin last year. Martin was black; Zimmerman identifies himself as Hispanic. The case sparked nationwide protests after there was a 44-day delay in Zimmerman’s arrest, and it touched off a debate about race and self-defense.

Prosecutors said Zimmerman’s ability to understand criminal investigations and desire to be a police officer doesn’t show wrongdoing, but is relevant to Zimmerman’s state of mind on the night Martin was killed.