The term genome editing refers to modern molecular biological techniques which facilitate precisely targeted changes in the genome of an organism. This marks significant progress in comparison to classical techniques of genetic engineering (e.g. chemicals or radiation), through which mutations are difficult to control. Moreover, due to their precision, these new techniques also stand out in other respects. On the one hand, mutations caused by modern techniques cannot or only barely be distinguished from natural mutations. On the other hand, for the most part, modern genetic engineering techniques do not introduce gene sequences from the outside but rather change the existing DNA material in one particular place.

Most modern techniques have roughly three steps in common: (1) At first, a specific place of the genome has to be identified with the means of “probes” which fit to the target sequence. (2) Next, the DNA double strand is cut at a specific place using so-called restriction enzymes (or restriction endonucleases). (3) Finally, mechanisms of the cell itself then repair the DNA double strand cuts. The type of repair determines the effect of the genome editing. Thus, incorrect repairs can switch off single genes (“knock-out”) and sections can be inserted (“insertion”), deleted (“deletion”) or modified, which can lead to development of new properties. In principle, genetic engineering techniques are applicable to all organisms.

Especially in the fields of animal breeding and crop cultivation as well as human medicine, genome editing is of central importance. Both in basic research and for possible applications the most widespread techniques are the following three:

• CRISPR/Cas (Clustered Regularly Interspaced Short Palindromic Repeats):
  In CRISPR/Cas systems, the integrated RNA (so-called “guide RNA”) serves to identify specific DNA sequences and the protein Cas (often Cas9), linked to the RNA, cuts the genetic material. The broken double strand is precisely (homologous) or randomly (non-homologous) repaired. Occasionally, new sections are inserted or removed. The fast, simple and economical production of these precise “gene-scissors” has achieved breakthroughs within research and ever since also arouses interest outside of basic research, particularly in the fields mentioned above.
• ZFN (Zinc-Finger Nuclease):
  This technique uses a loop-shaped protein structure (the Zinc-Finger Domain) instead of RNA, which is held together through a zinc ion. The zinc-finger domain recognises the particular DNA sequence which is then cut by an enzyme (an endonuclease). Compared with the CRISPR/Cas systems, ZFNs are time-consuming and expensive.
• TALEN (Transcription Activator-Like Effector Nuclease):
  Similar to the Zinc-Finger Nucleases, TALENs consist of two parts: a DNA binding part and an endonuclease, which cuts the DNA at a specific place. This method is also relatively expensive and time-consuming.

Further information on genome editing techniques:

Gaj, T./ Gersbach, C. A. / Barbas, C. F. (2013): ZFN, TALEN and CRISPR/Cas-based methods for genome engineering. In: Trends in Biotechnology 31 (7), 397–405. doi: 10.1016/j.tibtech.2013.04.004 Online Version

Faltus, T. (ed.) (2019): Ethik, Recht und Kommunikation des Genome Editings. Online Version (German)