Genetic engineering techniques

Genetic engineering techniques allow the modification of animal and plant genomes. Techniques have been devised to insert, delete, and modify DNA at multiple levels, ranging from a specific base pair in a specific gene to entire genes. There are a number of steps that are followed before a genetically modified organism (GMO) is created. Genetic engineers must first choose what gene they wish to insert, modify, or delete. The gene must then be isolated and incorporated, along with other genetic elements, into a suitable vector. This vector is then used to insert the gene into the host genome, creating a transgenic or edited organism. The ability to genetically engineer organisms is built on years of research and discovery on gene function and manipulation. Important advances included the discovery of restriction enzymes, DNA ligases, and the development of polymerase chain reaction and sequencing. Added genes are often accompanied by promoter and terminator regions as well as a selectable marker gene. The added gene may itself be modified to make it express more efficiently. This vector is then inserted into the host organism's genome. For animals, the gene is typically inserted into embryonic stem cells, while in plants it can be inserted into any tissue that can be cultured into a fully developed plant. Tests are carried out on the modified organism to ensure stable integration, inheritance and expression. First generation offspring are heterozygous, requiring them to be inbred to create the homozygous pattern necessary for stable inheritance. Homozygosity must be confirmed in second generation specimens. Early techniques randomly inserted the genes into the genome. Advances allow targeting specific locations, which reduces unintended side effects. Early techniques relied on meganucleases and zinc finger nucleases. Since 2009 more accurate and easier systems to implement have been developed. Transcription activator-like effector nucleases (TALENs) and the Cas9-guideRNA system (adapted from CRISPR) are the two most common.