The results of record-long (1 microsecond) molecular dynamics simulations of DNA fragment wrapped around a whole histone core (a nucleosome) including double-strand breaks are presented. A recently developed technique allows to calculate the atomic-level stress in such a big, heterogeneous system. In the second part, the molecular model of DNA melting is reviewed, and recent results obtained on mismatch defects on short (25-bp) sequences from microarray techniques are discussed.