In some embodiments, the present teachings provide compositions, systems, methods and kits for generating a population of nucleic acid fragments. In some embodiments, nucleic acids can be fragmented enzymatically. For example, methods for generating a population of nucleic acid fragments can include a nucleic acid nicking reaction. In one embodiment, the methods can include a nick translation reaction. A nicking reaction can introduce nicks at random positions on either strand of a double-stranded nucleic acid. A nick translation reaction can move the position of nicks to a new position so that the new positions of two of the nicks are aligned to create a double-stranded break. In some embodiments, methods for generating a population of nucleic acid fragments can include joining at least one end of a fragmented nucleic acid to one or more oligonucleotide adaptors.