I'm confused by my cloning PCR results.I have used the TOPO TA Cloning kit. I got colonies after blue-white screening. I grew them in the LB liquid culture in presence of ampicillin drug and then isolated plasmid DNA.now I want to do PCR the plasmid DNA with the M13 primers to sequence.However, the PCR product from the M13 PCR should be around 470bp as my insert is around 300bp but it is coming around 300bp.Then I repeat pcr using the pcr product as a template with gene specific primer and I got the right band. now I am confused about M13 primer amplification result.why the amplified product not showing right amplification.