Tessa Majors scratched and struggled like hell against the teen with a knife who took her life during a botched robbery in Morningside Park last year — and now his DNA, recovered from under her fingernail, has helped implicate the boy who investigators are “confident” is her murderer.

Rashaun Weaver, 14, was indicted by a grand jury and taken into custody without incident Friday at around 10:30 p.m. in the lobby of his home at the Taft Houses project in East Harlem.

He was charged, as an adult, Saturday morning with robbery and second-degree murder for fatally stabbing the 18-year-old Barnard College student steps from her campus, a senseless loss of life that stunned the city.

“What we can do is say that we are confident that we have the person in custody who stabbed her,” NYPD Commissioner Dermot Shea said in announcing the arrest at a press conference Saturday at police headquarters in lower Manhattan.

“And that person will face justice in a court of law.”

Shea and Manhattan DA Cyrus Vance cited a trove of hundreds of pieces of evidence linking Weaver to the stabbing, including not just the DNA forensics, but surveillance footage and at least one pivotal recording.

In the recording, Weaver is caught telling someone that Majors was “hanging onto her phone” as she struggled, according to the criminal court complaint.

“The defendant stated in substance that he was in the park and tried to take the girl’s phone and ‘she was hanging onto her phone’ and that he hit her with a knife,” according to the criminal complaint released after his arraignment.

The boy, who turns 15 in two months, spent much of the brief proceeding looking down at his hands, which were cuffed in front of him.

He will be held in a secure juvenile facility and faces a maximum of life in prison in the Dec. 11 death of Majors, an aspiring journalist and alt-rock composer and bass player from Charlottesville, Virginia.

Investigators say the boy — who is the son of an incarcerated father and a mother who also has a stabbing on her rap sheet from 2006 — is the most violent among the trio of middle school-age muggers that attacked Majors in the park just four months into her freshman year.

The first boy arrested is also the youngest — Zyairr Davis, 13.

Busted the day after the murder — allegedly wearing clothes matching what cops saw on surveillance footage, and with a knife in his backpack. He told cops that Majors was stabbed with such force, feathers flew from her down jacket, officials said.

Davis is being held in a Brooklyn juvenile jail on murder and robbery charges.

Officials have a third suspect, aged 14, in their sights, and while he has been interviewed by police, and a search warrant has been executed at his home, he remains under investigation but uncharged, sources have said.

Weaver’s own arrest was delayed by his going into hiding for several weeks last year, including in The Bronx, Shea said. Investigators also had to get a court order to obtain Weaver’s DNA, which was done in late December.

Prosecutors Saturday also released a chilling new description of Majors’ final struggle.

It was a quarter to 7 on a Wednesday night, and Majors had just walked across the park.

Sources have told The Post that she had just purchased a small amount of marijuana — which would be stolen from her by the young muggers.

As she was about to climb the steep stone stairs leading out of the park, near 116th Street, she was attacked by a trio of young muggers — including, prosecutors say, the knife-wielding Weaver.

“Run your s–t! Gimme your phone!” one mugger yelled, according to an ear-witness cited in the complaint against Weaver.

“You got some weed, gimme that, too!” he yelled at Majors.

The same witness then heard Majors scream, “Help me! I’m being robbed!”

A minute later, at 6:48 p.m., surveillance video shows Majors struggling with her three muggers on the lower landing of the stairs.

“Ms. Majors was able to break free and slowly stagger up the stairs,” Weaver’s criminal complaint says.

As she slowly pulled herself upward, her blood stained the landing and the stone stairs.

She was near death when she reached the top of the stairs.

She had been stabbed multiple times.

One of the thrusts of Weaver’s knife pierced her heart, the criminal complaint alleges.

“After Ms. Majors collapsed but while she was still conscious, she stated that she had been stabbed in the Park and robbed,” the complaint says.

These were the last words she would speak — though she’d continue to bear witness in death.

“The DNA recovered from one of Tessa major’s fingernail clippings matches the Y-STR DNA profile of Rashaun Weaver,” the criminal complaint says.

Y-STR analysis traces paternal lineage DNA.

The pot stolen from Majors that night would, like the DNA evidence, prove critical, sources have told The Post.

An empty baggie was recovered from Weaver’s apartment during a police search, sources said. And a local pot peddler has been confirmed it was the bag he’s sold to Majors right before she died.

Despite his young age, Weaver was already a successful knifepoint predator in Morningside Park when he and his two pals attacked Majors, prosecutors contend.

Four days before her death, Weaver and his two pals allegedly flashed a knife and stole an iPhone XR from a man who’d been walking through the park.

Weaver was caught on video surveillance fleeing that earlier robbery, prosecutors allege.

Later that night, he allegedly used that victim’s stolen iPhone to access his own email.

Then, on Dec. 11, moments before attacking Majors, the three followed another man up the stone stairs after tracking him through the park, but gave up when a second man walked past them.

“Nice kid,” Weaver’s grandfather, Clifford, 82, insisted of him later Saturday at his home in Jamaica, Queens.

“Do I feel that he did that?” he said of Majors’ murder. “No.”

“He’s not a violent person,” agreed Keith Weaver, 32, an uncle. “He’s a jokester. He likes to joke around. He was never violent.”

Investigators said the latest arrest is no cause for celebration.

“Sadly, [this] cannot bring back this young woman,” Shea said. “That is something even the best, most impartial investigation simply cannot do.”

Additional reporting by Olivia Bensimon, Joe Marino, Georgett Roberts, Tina Moore and Laura Italiano