Hi Kisia, there are quite a few ways of halting/synchronising cells in mitosis and although I have never used them you might want to consider an RNA binding fluorophore such as Flare probes:
Obviously you could always just park the cells at G2/M and extract the total RNA from the cells and perform a qPCR (this is the cheaper route although unlike the flare probes it won't give any cellular positional information about the RNA).