Next-generation Sequencing

Next-generation sequencing is a collective name for technologies that rapidly generate sequence data from many species or individuals. These methods are faster and more cost-efficient than traditional sequencing approaches, enabling parallel sequencing of millions of DNA fragments simultaneously. Today, sequencing costs have dropped over 100-fold compared to the original Human Genome Project.

Reversible terminator sequencing begins by fragmenting genomic DNA into 100-1000 base pair pieces and attaching adapters with primer sites. Fragments bind to a flow cell chip, then undergo bridge amplification, where DNA polymerase synthesizes complementary strands that bend and form bridges, creating clonal clusters. Modified dNTPs with fluorescent labels and reversible terminators enable one base addition per cycle, with imaging after each addition to determine the DNA sequence.

The main alternatives to reversible terminator sequencing are pyrosequencing and sequencing by ligation. Pyrosequencing detects pyrophosphate released during nucleotide addition through a light-generating reaction using firefly luciferase enzyme. Sequencing by ligation relies on DNA ligase specificity to identify mismatched base-pairing and determine nucleotide sequences.

Bridge amplification is a process where denatured single-stranded DNA bends over and uses its second adapter to form a bridge-like structure on the flow cell chip. DNA polymerase then synthesizes a complementary strand of the bent DNA. This cycle repeats through multiple rounds of denaturation and synthesis, generating thousands of copies of each DNA fragment and creating clonal clusters for detection.

Next-generation sequencing technologies support applications in clinical diagnostics, epigenetics, metagenomics, epidemiology, and transcriptomics. These methods enable high-throughput sequencing for transcriptome studies and have potential in personalized medicine for early detection and intervention of disorders including cancer, accelerating diagnosis and treatment planning.

Library preparation involves fragmenting pure genomic DNA into smaller pieces and ligating special adapters with primer sites to both ends of each fragment. These adapter sequences contain complementary sites that allow DNA fragments to bind to oligonucleotide sequences pre-coated on the flow cell chip, enabling subsequent amplification and sequencing reactions.

During sequencing, modified dNTPs tagged with four different fluorescent labels are added one at a time to the growing DNA strand. After each dNTP addition, the flow cell chip is imaged and the emission wavelength from each DNA cluster is recorded. The sequence of fluorescence images captured after each nucleotide addition is analyzed to determine the complete DNA sequence of the fragments.