Genetic sequencing is the process of determining the precise order of nucleotides (A, C, G, and T) that make up a DNA molecule. This can be done for a small segment of DNA, a whole gene, an entire chromosome, or even an entire genome.

There are different methods for genetic sequencing, but the most common one is called Sanger sequencing, which was developed by Frederick Sanger in the 1970s. In Sanger sequencing, DNA is first amplified through a process called polymerase chain reaction (PCR), which makes many copies of the target DNA sequence. The amplified DNA is then mixed with special chemical reagents that halt the sequencing reaction at specific points in the DNA sequence, allowing the nucleotides to be identified and recorded in order.

More recently, new technologies have been developed that allow for faster and more efficient sequencing of DNA, such as next-generation sequencing (NGS) and third-generation sequencing (TGS). These methods can sequence much larger amounts of DNA at a much faster rate and lower cost than Sanger sequencing. NGS uses high-throughput methods to generate many small fragments of DNA, which are then sequenced in parallel, while TGS technologies read long stretches of DNA in a single pass.

Genetic sequencing has many important applications in biology and medicine. For example, it can be used to identify mutations in genes that cause genetic diseases, to study the genetic basis of complex traits such as cancer, to trace the evolutionary history of species, and to develop personalized treatments for patients based on their individual genetic profiles.