Amanda Knox investigation, DNA challenged

Amanda Knox is back in court in Perugia, Italy, challenging her murder conviction and her 26-year prison sentence. Now, two DNA experts are telling judges why they believe some of the key evidence in this case is worthless.

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CBS News correspondent Elizabeth Palmer reports the defense will try to show the quality of the police work in general has been shoddy, not nearly reliable enough to convict a young woman of murder.

Knox, 24, recently celebrated her fourth birthday in jail, but as she entered the courtroom Monday morning, she and her family had reason to hope that by this time next year, she'll be free.

Edda Mellas, Amanda Knox's mother, told CBS News: "She's still locked up, and so that's horrible, and it's hard every day in prison, but I think she's feeling more hopeful, like all of us. She feels like she can breathe. That's what she described to us. She feels like the choke-hold is off of her and that there's definitely more hope."

Knox was convicted of the 2007 murder of her roommate, Meredith Kircher, on circumstantial evidence.

The appeal hearing is taking a fresh look at that evidence, especially Knox's and Kircher's DNA found on the murder weapon, a knife. An independent forensic report argues the sample on the knife was simply too small to be reliable.

Another piece of evidence, Knox's DNA on Kircher's bra strap, is also under question. The prosecution originally argued there was DNA on it, too, from Knox's boyfriend Raffaele Sollecito, which proved he and Knox together were at the murder scene. Not so, says the independent forensic report, which points out that the clasp was left at the crime scene for six weeks and eventually was collected by police using dirty gloves.

"They talked about all the ways that you're supposed to collect evidence -- clean gloves, you don't pass it around, and the fact that none of those procedures were followed is just amazing to me," said Mellas. "Dirty gloves were used. Yeah. It's amazing."

The new forensic report, Palmer notes, maintains the Italian police mishandled evidence or failed to follow proper procedure a total of 54 times after the murder investigation began.

One statistic which may give Knox's family some additional hope, adds Palmer, is that 50 percent of decisions in Italian criminal courts are overturned on appeal.