hi

If PCR product used for RFLP purpose , you can reduce Time of RE incubation or reduce amount of Restriction Enzyme . usually Star activity or DNA degradation occurred by high amount of enzyme applied for digestion  .

good luck

Hello Maryam

This smear looks as nuclease contamination.

You can use fresh and clean running buffer, use a fresh agarose gel and clean up the cDNA.

Regards

Looks like contamination. Control the conditions (time, the amount of enzyme, concentration. Use clean reagents and tips)

I would carry out  control digests with buffer only to make sure, if its a double digest then incubate with single RE also as controls. There could be  a number of reasons why: these are the common ones that I have encountered

1. Nuclease activity: the metal ions in the buffer activate nucleases in your digest resulting in smearing

2. Star activity: your buffer conditions are not ideal and RE is cutting at non specific sites