cDNA product separation

[Yang Y.L.]

Dear netter:
1.my PCR product is 200, 240, 280 bp...
what GEL formula can I separate them clearly ?

2.My target mRNA is so few in quantity 
Dose Northern blot analysis obtained better results if I purified
my mRNA by polyT-collum ?

3.DOSE DNase contaminate serious in labs ?
  where did them usually exist ? , how can I get rid of that ?


THANKS

===================================================================
Yang Yu-Lin: Kaohsiung medical College,Taiwan, ROC   TEL:07-3121101 ext 2138
Lovely Biologist on Cellular Biochemistry :Major in Endocrynology and Signal
Transduction
Working on : MAP kinase, PKC, PKA and cell cycle regulation in Kidney cells
E mail: u8251012 at cc.kmc.edu.tw  

===============Use Well of Every chance to learn===========================