How does formaldehyde cause protein-DNA crosslinking?
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2866014/

In this work they find that formaldehyde crosslinking happens by formation of a methylol adduct (due to nucleophilic attack by N or S in case of proteins) in protein which then attacks the DNA or vice-versa.
The final crosslink is by a methylene bridge

Formaldehyde can react to amino groups in nucleotides and proteins and form a schiff's base, but i dont have a clue how this is involved in crosslinking