The Illumina HiSeq 2000 Sequencing Technology
The workflow of high throughput sequencing with the HiSeq 2000 can be divided into three major steps:
Library preparation
Samples consisting of longer fragments are first sheared into a random library of 100-300 base-pair long fragments. After fragmentation the ends of the obtained DNA-fragments are repaired and an A-overhang is added at the 3'-end of each strand. Afterwards, adaptors which are necessary for amplification and sequencing are ligated to both ends of the DNA-fragments. These fragments are then size selected and purified.
Cluster Generation
The Cluster Generation is performed on the Illumina cBot. Single DNA-fragments are attached to the flow cell by hybridizing to oligos on its surface that are complementary to the ligated adaptors. The DNA-molecules are then amplified by a so called bridge amplification which results in a hundred of millions of unique clusters. Finally, the reverse strands are cleaved and washed away and the sequencing primer is hybridized to the DNA-templates.
Sequencing
During sequencing the huge amount of generated clusters are sequenced simultaneously. The DNA-templates are copied base by base using the four nucleotides (ACGT) which are fluorescently-labeled and reversibly terminated. After each synthesis step, the clusters are excited by a laser which causes fluorescence of the last incorporated base. After that, the fluorescence label and the blocking group are removed allowing the addition of the next base. The flourescence signal after each incorporation step is captured by a built-in camera, producing images of the flow cell.
The figures above are provided by Illumina Inc.
