The DNA was tested twice and did not match the victim, Sollecito, or Rudy Guede, who is currently serving a 16-year prison sentence for the murder.
Prosecutors allege that the kitchen knife was one of the weapons used to kill Kercher in the home she shared with Knox.
Today was the first time co-accused Sollecito attended the trial, which began in September.
He has been on an extended holiday in the Dominican Republic, which has no extradition agreement with Italy, and only landed back in Italy this week.
"He has come to show that he is not running away," Sollecito's lawyer, Luca Maori, told reporters before the start of the hearing.
Sollecito was expected to address the court later on Wednesday.
A ruling is expected later this month or next month but there is in any case one more appeal possible by either the prosecution or the defence.
Knox has lived in the United States ever since an appeals court in the university town of Perugia where the gruesome murder took place acquitted her and Sollecito of the murder in 2011 – after four years in prison.
The supreme court overturned that acquittal earlier this year, sending the case back to the appeals court for a retrial, which has so far focused on re-examining some of the DNA evidence in the case.
Prosecutors say the evidence shows that Guede could not have been the only one to carry out the killing.
Kercher was found half-naked in a pool of blood on the morning of November 2nd 2007 in the university town where she was on an education exchange.
Prosecutors believe that the murder may have been the result of a sexual assault on Kercher involving Knox, Sollecito and Guede.
All three deny involvement in the crime, although Guede has admitted he was in the house at the time of the killing.