Prosecutors at the trial also requested that her ex-boyfriend Raffaele Sollecito be given 26 years, saying the punishment should be harsher for Knox because she initially accused someone else of the crime.
Knox and Sollecito have already served four years in prison for the murder of British student Meredith Kercher, who was found half-naked in a pool of blood on November 2, 2006 in the house she shared with Knox.
The retrial is expected to reach a verdict in January.
Knox is now back in the United States and experts say it is highly unlikely that she could ever be extradited even if she is convicted and even if that conviction is upheld in another appeal to the supreme court.
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Rudy Guede, a local petty thief and drug dealer, has been convicted separately and is serving a 16-year sentence for the gruesome murder which shocked the university town of Perugia and has divided opinions.
Crini said he believed Knox had used a large kitchen knife that was later found by investigators in Sollecito's home, pointing to DNA traces of Kercher on the blade and of Knox on the handle.
He said the sexual element became "marginal" as the drug-fuelled violence increased. "They were trying to get rid of someone who had to be shut up," Crini said.