A Crack in Creation: Gene Editing and the Unthinkable Power to Control Evolution

Cas9 is a protein enzyme that functions as the primary cutting tool in type II CRISPR systems. Originally identified as Csn1 in bacterial immune defense, Cas9 works in conjunction with guide RNA molecules to locate and cut specific DNA sequences. The protein contains two separate nuclease modules that allow it to cut both strands of a DNA double helix at precise locations, making it an effective tool for genetic modification. Cas9’s ability to be programmed to target any DNA sequence through the use of guide RNA made it the foundation for CRISPR-based gene-editing technology.

The central dogma of molecular biology describes the fundamental process by which genetic information flows from DNA to RNA to proteins in living cells. In this process, DNA serves as the master template that gets transcribed into RNA molecules, which then act as messengers carrying genetic instructions to the parts of the cell that produce proteins. These proteins perform most of the critical functions in the body, such as breaking down food, recognizing pathogens, and sensing light. The central dogma represents the basic language of life, explaining how genetic information is stored, transmitted, and expressed in living organisms.

CRISPR (clustered regularly interspaced short palindromic repeats) is a natural defense system found in bacteria that protects against viral infections. The system consists of specialized regions of bacterial DNA that contain repeating sequences separated by unique spacer segments, along with associated proteins called Cas enzymes. CRISPR functions as a form of adaptive immunity by allowing bacteria to store snippets of viral DNA from past infections and use this genetic memory to recognize and destroy those same viruses during future attacks. This bacterial defense system was later adapted by scientists into a powerful gene-editing tool that can make precise changes to DNA.

CRISPR-Cas9 is a revolutionary gene-editing tool that enables scientists to modify DNA with unprecedented precision and simplicity. The system consists of two main components: CRISPR, which guides the system to specific DNA sequences, and Cas9, an enzyme that acts like molecular scissors to cut DNA at targeted locations. In A Crack in Creation, Doudna and Sternberg describe this technology as allowing scientists to insert, edit, or delete genes in virtually any organism’s DNA. The technology originated from studying bacterial immune systems but has been adapted for use in plants, animals, and potentially humans.

Double muscling is a genetic condition that results in significantly increased muscle mass in animals. The condition occurs due to mutations in the myostatin gene, which normally acts as a natural brake on muscle tissue production. In the book, Doudna and Sternberg explain that this trait appears naturally in certain cattle breeds like Belgian Blue and Piedmontese, producing cows with approximately 20% more muscle mass than typical cattle. The authors discuss how scientists have used CRISPR to recreate this mutation in various animals, including pigs, sheep, and goats, demonstrating the technology’s potential to enhance livestock production.

A gene drive is a genetic engineering technology that can rapidly spread specific genetic modifications through populations of organisms. Doudna and Sternberg explain that gene drives work by overriding normal inheritance patterns, ensuring that a particular genetic modification is passed on to nearly all offspring rather than the usual 50%. The technology combines CRISPR with genetic sequences that can autonomously copy themselves, creating what the authors describe as a self-replicating system that can exponentially increase within a population. The book emphasizes that gene drives represent one of CRISPR’s most powerful and potentially concerning applications, as they could theoretically alter or eliminate entire species in the wild.

The germline refers to the genetic material that can be passed from one generation to the next through reproductive cells such as eggs and sperm. In A Crack in Creation, Doudna and Sternberg explain that modifying the germline has profound implications because these changes would affect not only the immediate offspring but all future generations that inherit that DNA. The authors emphasize that the ability to edit the human germline using CRISPR technology raises significant ethical concerns, as it would allow humanity to direct its own evolution and potentially create inheritable enhancements to the human genome.

GMO stands for “genetically modified organism,” which Doudna and Sternberg explain has varying definitions depending on the context and regulatory body. The US Department of Agriculture defines GMOs broadly as organisms with heritable improvements made through either genetic engineering or traditional methods. However, the authors note that a more common definition includes only organisms modified using recombinant DNA technology, in which foreign genes are integrated into the genome. The book emphasizes that while GMOs have been extensively studied and deemed safe by scientific consensus, they face significant public opposition. This disconnect between scientific evidence and public perception serves as an important case study for the authors in examining how new genetic technologies might be received by society.

Homologous recombination is a natural cellular process that allows the precise exchange of genetic material between matching DNA sequences. This process occurs most notably during the formation of egg and sperm cells, when genetic material from both parents is mixed to create genetic diversity in offspring. Scientists discovered that cells also use homologous recombination to repair damaged DNA by using a matching chromosome as a template. This discovery led to the development of early gene-editing techniques, as researchers realized they could potentially harness this natural repair mechanism to make precise changes to genes.

Phages, also called bacteriophages, are viruses that specifically infect and replicate within bacteria. These viruses are the most abundant biological entities on Earth, with scientists estimating their number at 10^31 (10 million trillion trillion). Phages attack bacteria by injecting their genetic material through the cell wall, then hijacking the bacterial cell’s machinery to produce more viruses until the cell bursts. While phages posed significant challenges for the dairy industry and other bacterial applications, their study led to important discoveries in genetics and molecular biology, including insights that contributed to the development of gene therapy and the understanding of CRISPR.

TALENs (transcription activator-like effector nucleases) are engineered proteins designed to cut DNA at specific sequences for gene editing. These molecular tools were developed after ZFNs and were derived from proteins found in Xanthomonas bacteria that naturally recognize and bind to specific DNA sequences. TALENs improved upon ZFNs by using protein segments that each recognize a single DNA letter, making them more straightforward to design than ZFNs. However, while TALENs offered more reliable DNA targeting than their predecessors, they still remained complex to implement and were quickly superseded by CRISPR technology.

TracrRNA (trans-activating CRISPR RNA) is a type of RNA molecule that plays an essential role in the CRISPR-Cas9 system. Initially discovered by Emmanuelle Charpentier’s laboratory, tracrRNA works as a helper molecule that enables CRISPR RNA to function properly with the Cas9 protein. The molecule’s importance became evident when researchers found that both tracrRNA and CRISPR RNA were necessary for Cas9 to successfully cut DNA. TracrRNA was later combined with CRISPR RNA to create a simplified single guide RNA molecule, making the CRISPR system easier to program for gene-editing purposes.