Hi Alexandra
'A picture is worth thousand words'
It was nice to see the photograph...than actually visualizing the problem you are facing....First of all there are no bands on the lower side of the gel....only smear is visible as pointed by researchers above....I have few suggestions
1) First of all your DNA looks fine for most of the routine applications.....there is some amount of smear...but it should not be much of a problem
please note that the smear is visible mostly in the lanes where 'loaded sample' is in large amout......low quantity bands looks 'clean''....But I am sures if you load more amount of DNA.........in low loaded sample...smear will be visible even in those
2) some amount of smear will be always there with routine methods of DNA isolation....if you really need high quality DNA you will have to use some special protocols
3) If you still want to reduce the smear in your protocol.....take care of few things
a) Use liquid Nitrogen during extraction if possible.....as at low temp...DNAses will be inactive
Genomic DNA being physically fragile shears due to pipetting or vortexing. DNA molecules> 150 kb are prone to breakage by forces generated during isolation.
Solution- Cut the base of the tips and use them. Do NOT vortex.
Another possible source of error could be DNase contamination in tips or tubes.
Solution- Sterilize them properly before use.