"It's a tough loss, but we can't let that affect us," said Gonzalez, whose third-inning homer was his 21st, but first since July 23 -- a drought of 79 at-bats. "We've been doing a great job. It was a bad outing for Matt, and that stuff is going to happen. We just need to continue the way we've been playing."
The Rockies entered the eighth with a 3-2 lead. After some decent work from starter Drew Pomeranz (five innings, two runs on homers by Alfonso Soriano and Welington Castillo) and reliever Josh Roenicke, the Rockies put the game in the hands of the normally reliable Belisle.
However, Jackson, the Cubs' first-round pick in the 2009 First-Year Player Draft, tied the game by clubbing Belisle's 1-2 breaking ball for his second home run since being called up from Triple-A Iowa on Aug. 5.
"Today it's a matter of choice," manager Jim Tracy said. "Matty chose the right ball but hung it. Then we made a couple of errant throws that allowed them to get the fifth run."
Luis Valbuena doubled two batters later and Joe Mather singled him home for the go-ahead run. The Rockies were in line for at least one out on Josh Vitters' bouncer to third, but former Cub DJ LeMahieu dropped Nelson's throw at second base.
Matt Reynolds replaced Belisle and promptly threw wildly to first trying to retire Anthony Rizzo, which allowed the Cubs to plate an insurance run for a 5-3 lead.
"That's just the way it goes sometimes," said Nelson, whose second-inning home run was his sixth this season. "We've been playing good defense. We just happened to have a blowup in the eighth inning. We'll be back again tomorrow."
In a matchup of teams in the midst of youth movements, the Rockies showed up with 10 rookies on their roster and the Cubs had eight. The Cubs lead the Rockies by one in number of rookies used this year, 16-15. The Cubs were a little better Friday.
"We talk about coming up with the big hit and looking to do that, looking to be a game-changer," Jackson said. "When you get a lineup that has a bunch of guys that want to be game-changers, then you're looking at some big moments."
Lately, the Rockies have been able to overcome their youth through decent pitching and execution. The latter escaped them in the eighth on Friday, which made a winner out of Cubs reliever James Russell (6-1). Carlos Marmol pitched the ninth -- overcoming Nelson's one-out bloop double -- for his 16th save.
The collapse robbed Pomeranz of an overdue chance to celebrate. He has now gone eight starts without a win, but at least he was in line for one Friday for the first time since he beat Washington on July 6.
The fastball and proper mechanics that have come and gone during his rookie year -- and second full professional season -- arrived Friday. He walked just one and struck out one. He also showed a good curveball -- a pitch he hasn't mastered in the Mile High atmosphere of Coors Field.
The fifth-inning home runs two batters apart were the only hits against him. He threw strikes on 43 of his 72 pitches.
Pomeranz had been working inside all game and thought he had Soriano jammed, but the famed out-blowing winds at Wrigley taught him a lesson.
"It's a bad park to be in against a guy like Soriano; I've got to locate a little more," said Pomeranz, who said Castillo hit his ball much harder. "I've got to learn from it."
Jeff Samardzija yielded a run in a first inning in which the Rockies were aided by shortstop Starlin Castro's error on Jonathan Herrera's grounder, and Nelson and Gonzalez led off the next two innings with homers. However, Samardzija struck out seven in seven innings and held the Rockies to six hits.
It was the type of game the Rockies have been winning lately, almost surprisingly so. Shortstop Troy Tulowitzki hasn't played in the Majors since late May and began his rehab assignment for left groin surgery Friday night. First baseman Todd Helton is out for the year after left hip surgery and outfielder/first baseman Michael Cuddyer also could be gone for the season because of a right oblique strain.
"It's not like I've been getting five RBIs every day and Tulowitzki is back in the lineup," Gonzalez said. "But everybody is taking advantage of opportunities."