(Reuters) - San Francisco's Colin Kaepernick tamed the fierce Chicago defense and dominated the battle of back-up quarterbacks as the 49ers scored a commanding 32-7 win over the Bears on Monday.

With both Alex Smith (49ers) and Jay Cutler (Bears) sitting out due to concussions, the second-year Kaepernick rose to the occasion in his first NFL start by tossing two touchdown passes.

The young quarterback led the 49ers to a 20-0 lead at half-time, while Bears' back-up Jason Campbell was sacked six times and intercepted twice in the game at San Francisco's Candlestick Park.

The Bears trailed 27-0 before Brandon Marshall caught a 13-yard touchdown pass in the third quarter for the team's only score. Chicago's (7-3) second straight loss dropped them into a tie with Green Bay for first in the NFC North.

Kaepernick finished with 243 yards passing and no turnovers while San Francisco (7-2-1) added 123 yards on the ground to extend their lead in the NFC West.

(Writing by Jahmal Corner in Los Angeles; Editing by Peter Rutherford)