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Trayvon Martin's DNA Not on Zimmerman's Gun

Prosecutors question Zimmerman's credibility over claims of self-defense and struggle over gun.
4:09 | 07/04/13

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Transcript for Trayvon Martin's DNA Not on Zimmerman's Gun
Let's get to the high-stakes george zimmerman trial. The prosecution, close to wrapping up its case, with trayvon martin's mother expected to take the stand as early as tomorrow. Abc's matt gutman has the story. Reporter: Before recessing for the fourth of july holiday, the prosecution continuing its assault on george zimmerman's credibility wednesday. Bringing in zimmerman's former college law instructor to the stand. How are you doing, george? Reporter: At issue, what did zimmerman know about florida's controversial stand your ground law, which allows someone to use deadly force in threatened. Did you discuss self-defense and stand your ground laws in connection of violent crime, such as murder? Yes. Reporter: It was the day before that jurors watched zimmerman saying the opposite. The prosecution playing this 2012 fox news interview. I'm just curious, prior to this night, this incident, had you even heard stand your ground? No, sir. Reporter: It turns out zimmerman got an "a" in that legal class, two years before shooting trayvon martin, he claims in self-defense. The defense pointed out he may have learned a different lesson from that class. You don't have to wait until you're almost dead before you defend yourself. No. I would advice you not do that. Reporter: A chucker from zimmerman. And more levity followed with another of his former professors. Calling in via skype. The problem, the user name was broadcast on live tv. Pranksters started calling in. We're being toyed with. Reporter: Eventually, they resorted to a low-tech solution, telephone. But the lighter atmosphere short-lived. The prosecution plans to close its case on friday with an emotional crescendo, putting trayvon martin's mother on the stand. Let's going in anisa de rossi. It seems like the prosecution's strategy is to poke holes in zimmerman's account. Do you think they made some progress yesterday? I do. This is not the be-all, end-all evidence. It's important to show his mind of who is george zimmerman. There's nothing wrong with wanting to be a police officer or taking the criminal law cases, by a long shot. It shows his background what he came into that night. And it's going to go towards the prosecution when they sum it up to the jury. This is a neighborhood watchman. But somewhat disappointed in where his aspirations have led him. He wanted to be the guy who caught the suspect in the terms that he used. And they're starting to paint that picture better towards the end of the trial. There's also a credibility issue. Saying on the one hand, I don't know what stand your ground is. And the professor is coming in saying, I taught you stand your ground. People started watching saying, why do they put in statement after statement after statement of george zimmerman to solidify his account. But like you said, here he is talking on national television in an interview, saying I knew nothing about this until after trayvon martin was shot. And you have a professor saying, not only did he know, we talked about it and he got an "a." Why is he making those things up? Maybe because he realized that he went too far. Trayvon martin's mother, going to be emotional and has substance to this case. It's a great way to end their case. To bring it back to remember. This is not just a technicality of evidence, but a young boy was killed. She is going to say, I heard the calls and that is my son yelling for help. That coupled with the autopsy, to remember what happened to the 17-year-old is probably what the prosecution is going to do to end their case. And we may see a relatively quick defense case after that. I think so. They've laid out how they're going to sum up with the prosecution's case.

This transcript has been automatically generated and may not be 100% accurate.

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