Genome DNA is a polymer molecule constructed from two interlocked chains/strands of four types of nearly identical chemical groups called nucleotides, labeled by letters A, C, G, and T. The two strands of DNA are complementary to each other: A in one strand always pairs to T in the other, whereas G always pairs to C. The goal of genetic sequencing is determining the precise sequence of the nucleotides in the genome. While knowing the genetic sequence already has massive scientific value, it is by comparing the genomes of different individuals that we can identify the small differences – sometimes consisting of a difference in just one nucleotide base – responsible for genetic diseases or triggering cancer. In the same way, small changes in viral sequences allow tracing the path taken by a virus when spreading from a country to another.

The Human Genome Project – sequencing the first complete human genome – took thirteen years and cost three billion dollars.

Using a strand of the six billion nucleotides long genome, the complementary strand was synthesized nucleotide by nucleotide, checking what type of a nucleotide was being added at every step (e.g., one can use modified fluorescent nucleotides with the colors encoding for A, C, G, and T). The decade that followed saw the successful completion of the 1000 Genomes Project, and the 100,000 Genomes Project is currently underway. Moreover, thousands of full viral, bacterial, plant, and animal genomes are being sequenced and made available to researchers and medical professionals every year. This scaling up and the associated reduction in costs (sequencing a human genome now costs a few thousand dollars) has become possible by the on-chip sequencing technology, referred to as Massive Parallel Sequencing or Next Generation Sequencing (NGS).