A microfluidic thermalization chip, a system using such a chip and a PCR method for detecting DNA sequences. The chip contains a block of material in which a cavity is configured to contain at least one fluid. The cavity includes at least one inlet orifice and at least one outlet orifice. The inlet orifice for the fluid is connected to at least one, preferably at least two, fluid-injecting channels. Further, the chip includes at least one microfluidic channel for bypassing the cavity. The channel is connected by a first end to at least one of the fluid-injecting channels. The junction between the bypassing channel and the fluid-injecting channel is located at a distance L from the inlet orifice of the fluid-injecting channel. The distance L is preferably smaller than 2 cm.