Are you using one plasmid primer and one insert primer?

I am wondering if ,in the absence of an insert, the plasmid primer is annealing and extending until the enzyme falls off giving random length smears but in the positive band case the reverse primer mops up the forward primer product giving a band but no smear

Are you using one plasmid primer and one insert primer?

I am wondering if ,in the absence of an insert, the plasmid primer is annealing and extending until the enzyme falls off giving random length smears but in the positive band case the reverse primer mops up the forward primer product giving a band but no smear

I agree with Paul; just compare the pcr result of your positive colony and others. When insert is there primers are used to amplify it when there is not sth crazy happens there. I guess last two positive control lanes are belonging to pcr using plasmid dna and others bacterial colonies?? Don't use excessive amount of primer, or bacterial colony (if you directly put it in the pcr reaction). And also increasing the annealing temperature can help you with the nonspecific bands.

A successful ligation is achieved when all the conditions required are provided. But sth that I can highlight is the total DNA concentration used in ligation, vector-insert ratio and ligation buffer quality, the volume of ligation reaction used for transformation and ofcourse there are other factors.

I think Paul Rutland is correct.

Besides, what you really wanted was some successful colonies to work with. The odds that you have discovered a new way to get non-specific gene amplification are low.

Congratulations on the positive colony PCR! Get it sequenced to verify your insert, make your freezer stocks, & keep making good progress with your experiment.