DNA library preparation for sequencing. The DNA sample is sheared into fragments and repaired. An optional A-tailing step corresponding to the addition of a unique dATP to the fragment’s 3′-end can increase adaptor ligation efficiency. Alternatively shearing and adaptor ligation can be combined within a single step using a transposase-based method. Fragment bearing adaptors (blue) at their ends can be called libraries and are ready for sequencing. Adaptors or PCR primers can contain a barcode (red) allowing several samples to be sequenced together. Illumina’s adaptors contain the linker (cyan) able to hybridize onto Illumina’s sequencing flow cells. PacBio SMRTbell adaptors complement the primer sequence (cyan) allowing the polymerase to bind prior PacBio’s flow cell loading. ONT adaptors are bound to a helicase motor protein (dark blue) which will regulate the speed at which DNA translocates through the pores.