Kuranyi, hoping for a Germany recall ahead of the World Cup, fired the visitors ahead in the 11th minute from close range and notched his 17th goal, his best tally in a season, by heading in a pinpoint cross from Peruvian Jefferson Farfan in the 27th.
Bayern Munich had earlier lost 2-1 at home to VfB Stuttgart, dropping to second place on 56 points from 28 matches.
Lacklustre Leverkusen saw their title hopes severely dented following a second straight league loss that left them in third place on 53 points.
Schalke did not seem affected by the extra-time defeat by Bayern Munich in the German Cup semi-final in midweek and were the better side from the start.
"This was a beautiful win and a strong performance after 120 minutes of Cup football (against Bayern)," said Schalke coach Felix Magath.
Schalke face Bayern next week and Magath, who won the Bundesliga with VfL Wolfsburg last season, said a only a win against the visiting Munich side next Saturday would do if Schalke were to have a shot at the title.
"If we beat Bayern next week then we could have a chance at winning the title," he told reporters.
KURANYI DOUBLE
With Germany coach Joachim Low watching from the stands, Kuranyi silenced the home crowd by firing into the net from 10 metres after a low Heiko Westermann cross sailed across the box into his path.
Kuranyi, who stormed off from the national team in 2008 after being left out of the squad for a game and was told he would never be picked again by Loew, headed home his second goal from a Farfan cross.
Leverkusen, badly missing injured striker Stefan Kiessling, lacked their usual pace and any attacking moves were quickly stifled by Schalke's gruelling pressing game.
Bayern Munich paid the price for their busy schedule in three competitions, suffering their second successive league loss, this time to visitors VfB Stuttgart, with Romanian Ciprian Marica scoring the winner.
Bayern, who host Manchester United in a Champions League quarter-final on Tuesday, took the lead through Ivica Olic in the 32nd minute before the livelier Stuttgart pulled level nine minutes later through a deflected shot from Christian Traesch.
Unmarked Marica secured the points for Stuttgart with a header five minutes into the second half.
