The disclosure provides DNA library preparation methods that do not require a purification between adapter ligation and PCR amplification. Adaptors are added to DNA fragments to form oligonucleotide extension products and the oligonucleotide extension products are amplified without stopping or interruption for a cleanup step. Excess materials, such as enzymes, adaptors, or co-factors, from the adaptor addition step do not interfere with the amplification step and the amplification step proceeds without regards to the presence of reagents from the ligation step. In preferred embodiments, the ligation and amplification step make use of a common priming sequence e.g., in the form of one of the adaptor oligos.