Prosecutors seeking to uphold Amanda Knox's murder conviction have asked for her sentence to be increased to life – and for her to spend six months in daytime solitary confinement.

Giancarlo Costagliola said the court should also raise the sentence passed on her former boyfriend, Italian Raffaele Sollecito, to life with two months in solitary. Exploiting what has always been regarded as a weakness of the case against them, he said the lack of a motive in the killing of British student Meredith Kercher justified the harshest sentence available under Italian law.

His colleague, Manuela Comodi, earlier said that Knox, 24, and Sollecito, 27, had "killed for nothing". They were given 26 and 25 years respectively at their trial in 2009 but with good behaviour could expect to get out of jail much sooner. Life imprisonment in Italy is intended to keep prisoners inside for at least 26 years, though some are released earlier.

Knox sat motionless as the prosecution request was read out, pressing her lips against her hands. Her father, Curt Knox, said his daughter had been prepared for the development, which the prosecution had earlier signalled in a document submitted to the court.

"It's never easy when you're on trial for your life," he said. Knox, who has looked tense and drawn since her appeal's final arguments began, was holding up according to her father. "She's strong and she'll be ready," he added.

Prosecutors have spent the two-day summing up bristling with indignation over criticism of their evidence and alleged that it reflected a systematic plot to denigrate the Italian judicial system. In June, their case was severely dented when two independent, court-appointed experts found that the key forensic evidence used to convict Knox and Sollecito was unreliable.

Comodi yesterday attacked the independent experts, noting they were both professors of forensic science, rather than practising investigators. She asked the jury of five women and one man: "Would you entrust the wedding reception of your only daughter to someone who knew all the recipes by heart but had never actually cooked?"

Comodi said the independent experts had put on an "embarrassing performance". She told the two judges and the jurors (technically, lay judges) that the two Rome university professors had been given an assignment "that they did not know how to fulfil, betraying your trust".

A third defendant, Rudy Guede, was also convicted of the 2007 murder. The prosecution maintains that Guede, a small-time drugs peddler from the Ivory Coast, joined the others in a narcotics-fuelled sex game that ended in tragedy when Kercher resisted.

Important evidence at the trial of Knox and Sollecito included a trace of his DNA on Kercher's bra clip and a knife, which the prosecution claimed was the murder weapon, bearing the DNA of both defendants and their alleged victim. The experts said that Sollecito's DNA could have reached the bra clip, which was only identified and bagged 46 days after the discovery of the body, by a process of contamination. They also said the third trace of DNA on the knife, which was in Sollecito's kitchen, was too faint to be ascribed confidently to Kercher.

According to Comodi, the original analysis had been carried out by police forensic experts whose competence was internationally recognised and the defence had failed to show how the contamination of the bra clip might have occurred.