The synthesis of these chains (referred to as oligonucleotides) is usually done by companies that specialize in the process. Most of the time the sequences they synthesize are less than 200 base pairs in length. However, there are a lot of ways to ligate multiple of these fragments together and so you can purchase longer sequences that are a few thousand base pairs in length from these companies.

Depending on the genetic change that is being attempted it can vary what length of synthetic DNA is needed. Often a scientist can use oligonucleotides that are only 20-60 base pairs long along with polymerase chain reactions (PCR) to synthesize larger fragments with the needed modifications. Other times it is necessary to have an entire gene or sequence of genes completely synthesized to be compatible with the host of interest.