Hi, check your reagents (water, buffer, etc.) and your primers! Cheers, Nadine

thanks Nadine but was just wandering why only the negative control has a smear while the samples and positive control have clear bands

Hellen. it might be because of non specific PCR amplification.That generally results in smears being observed in negative control

Please refer to this book .Principles and Technical Aspects of PCR Amplification

By Elizabeth van Pelt-Verkuil, Alex van Belkum, John P. Hays in section 10.2.4

hope it helps you.

Could be primer dimers, too high or too low magnesium levels can also cause smearing.