DNA sequencing means a process of determining the arrangement of adenine, guanine, cytosine and thymine in a DNA molecule.
In the mid-70s, a scientist called Fred Sanger developed a sequencing method called Sanger sequencing. This discovery of DNA code allowed more basic scientific applications to take place such as translational applications, diagnostic testing and drug therapy.
Many years of scientists improvements enabled Sanger to sequence sections of DNA up to approximately 600 bases in length. It was difficult to sequence one small section of DNA at once because of the more time and high cost required. Massively parallel sequences approaches, which are the Next Generation Sequencing (NGS) method enabled scientists to sequence hundreds of thousands of fragments of DNA at the same time.
“Next Generation Sequencing” (NGS) connotes various approaches use to sequence DNA.
- Smaller fragments for sequencing are derived from a sample of DNA.
- The NGS technology is used to read at the same time sequence of bases in many fragments of DNA. The fragments sequenced at the same vary from hundreds to millions and that depends on the type of sequencing taking place.
- The base sequences taken from DNA are generated in the form of a computer file. Each individual length generated from the original DNA fragment measures between 50-300 bases long, it is called, “read”.
- There is a specific program analyses the reads and matches specifically to the genome they arose from. This is what is called “alignment” or “mapping”.
- Variation between the sample DNA and the reference DNA are checked.
- Lastly the effect that a genetic variant will have on a protein is assessed. This is called “variant annotation”.
It is possible to sequence the whole of human genome quickly using these approaches. Many applications of NGS have been tried to find a specific genetic variant relevant to a specific disease but it has proved difficult. However, various methods have been devised which allow sequencing smaller regions of the genome. It allows sequencing just the portion of the genome that is likely to yield the relevant variation. This method is known as the “target enrichment” “capture” technique.