EXPERIMENTAL FIGURE 6-21 Generation of clusters of identical DNA fragments attached to a solid support. A large collection of DNA fragments to be sequenced is ligated to double-stranded linkers, which become attached to each end of each fragment. The DNA is then amplified by PCR using primers matching the sequences of the linkers that are covalently attached to a solid substratum. Ten cycles of amplification yield about a thousand identical copies of each DNA fragment localized in a small cluster and attached at both ends to the solid substratum. These reactions are optimized to produce as many as 3 × 10⁹ discrete, non-overlapping clusters that are ready to be sequenced.