"It is not necessary for George Zimmerman to disprove anything. Nor is George Zimmerman required to prove his innocence," she said.
"It is up to the state to prove George Zimmerman's guilt by evidence."
Nelson also reminded jurors that Florida law allowed residents to use lethal force to defend themselves, under the state's "stand your ground" law.
"The killing of a human being is justifiable and lawful if necessarily done while resisting an attempt to murder or commit a felony upon George Zimmerman, or to commit a felony in any dwelling house in which George Zimmerman was at the time of the attempted killing," Nelson said.
Zimmerman's lawyer closed his case by telling jurors that his client feared he was going to die when he shot Martin, 17.
"He's not guilty of anything but protecting his own life," Mark O'Mara said.
Zimmerman faces possible life in prison if convicted of second degree murder.
Jurors were told they could also consider the lesser charge of manslaughter if they were not convinced he was guilty of murder.
Defence lawyers rested their case without calling Zimmerman to the witness stand, after three weeks of evidence and testimony.
The trial has been nationally broadcast on US television.
The shooting became a cause celebre last year after authorities initially did not bring charges against Zimmerman, prompting civil rights activists to charge that law enforcement would have pursued the case more zealously had the teenage victim been white.
- AAP
