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1. Students should be able to investigate the structures of animal and plant cells and relate them to their functions

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The Nobel Prize in Physiology or Medicine 2024

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The Nobel Prize in Physiology or Medicine 2024 was awarded jointly to Victor Ambros and Gary Ruvkun "for the discovery of microRNA and its role in post-transcriptional gene regulation."

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How to prepare a cheek cell microscope slide

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Science | Planning Documents | Junior Cycle for Teachers (JCT)

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Table of Contents

gene editing, the ability to make highly specific changes in the DNA sequence of a living organism, essentially customizing its genetic makeup. Gene editing is performed using enzymes, particularly nucleases that have been engineered to target a specific DNA sequence, where they introduce cuts into the DNA strands, enabling the removal of existing DNA and the insertion of replacement DNA. Key among gene-editing technologies is a molecular tool known as CRISPR-Cas9, a powerful technology discovered in 2012 by American scientist Jennifer Doudna, French scientist Emmanuelle Charpentier, and colleagues and refined by American scientist Feng Zhang and colleagues. CRISPR-Cas9 functioned with precision, allowing researchers to remove and insert DNA in the desired locations.

The significant leap in gene-editing tools brought new urgency to long-standing discussions about the ethical and social implications surrounding the genetic engineering of humans. Many questions, such as whether genetic engineering should be used to treat human disease or to alter traits such as beauty or intelligence, had been asked in one form or another for decades. With the introduction of facile and efficient gene-editing technologies, particularly CRISPR-Cas9, however, those questions were no longer theoretical, and the answers to them stood to have very real impacts on medicine and society.

Early attempts to correct genetic mistakes

The idea of using gene editing to treat disease or alter traits dates to at least the 1950s and the discovery of the double-helix structure of DNA. In the mid-20th-century era of genetic discovery, researchers realized that the sequence of bases in DNA is passed (mostly) faithfully from parent to offspring and that small changes in the sequence can mean the difference between health and disease. Recognition of the latter led to the inescapable conjecture that with the identification of "molecular mistakes" that cause genetic diseases would come the means to fix those mistakes and thereby enable the prevention or reversal of disease. That notion was the fundamental idea behind gene therapy and from the 1980s was seen as a holy grail in molecular genetics.

The development of gene-editing technology for gene therapy, however, proved difficult. Much early progress focused not on correcting genetic mistakes in the DNA but rather on attempting to minimize their consequence by providing a functional copy of the mutated gene, either inserted into the genome or maintained as an extrachromosomal unit (outside the genome). While that approach was effective for some conditions, it was complicated and limited in scope.

In order to truly correct genetic mistakes, researchers needed to be able to create a double-stranded break in DNA at precisely the desired location in the more than three billion base pairs that constitute the human genome. Once created, the double-stranded break could be efficiently repaired by the cell using a template that directed replacement of the "bad" sequence with the "good" sequence. However, making the initial break at precisely the desired location-and nowhere else-within the genome was not easy.

Breaking DNA at desired locations

Before the advent of CRISPR-Cas9, two approaches were used to make site-specific double-stranded breaks in DNA: one based on zinc finger nucleases (ZFNs) and the other based on transcription activator-like effector nucleases (TALENs). ZFNs are fusion proteins composed of DNA-binding domains that recognize and bind to specific three- to four-base-pair-long sequences. Conferring specificity to a nine-base-pair target sequence, for example, would require three ZFN domains fused in tandem. The desired arrangement of DNA-binding domains is also fused to a sequence that encodes one subunit of the bacterial nuclease Fok1. Facilitating a double-stranded cut at a specific site requires the engineering of two ZFN fusion proteins-one to bind on each side of the target site, on opposite DNA strands. When both ZFNs are bound, the Fok1 subunits, being in proximity, bind to each other to form an active dimer that cuts the target DNA on both strands.

TALEN fusion proteins are designed to bind to specific DNA sequences that flank a target site. But instead of using zinc finger domains, TALENs utilize DNA-binding domains derived from proteins from a group of plant pathogens. For technical reasons TALENs are easier to engineer than ZFNs, especially for longer recognition sites. Similar to ZFNs, TALENs encode a Fok1 domain fused to the engineered DNA-binding region, so, once the target site is bound on both sides, the dimerized Fok1 nuclease can introduce a double-stranded break at the desired DNA location.

Unlike ZFNs and TALENs, CRISPR-Cas9 uses RNA-DNA binding, rather than protein-DNA binding, to guide nuclease activity, which simplifies the design and enables application to a broad range of target sequences. CRISPR-Cas9 was derived from the adaptive immune systems of bacteria. The acronym CRISPR refers to clustered regularly interspaced short palindromic r epeats, which are found in most bacterial genomes. Between the short palindromic repeats are stretches of sequence clearly derived from the genomes of bacterial pathogens. "Older" spacers are found at the distal end of the cluster, and "newer" spacers, representing more recently encountered pathogens, are found near the proximal end of the cluster.

Transcription of the CRISPR region results in the production of small "guide RNAs" that include hairpin formations from the palindromic repeats linked to sequences derived from the spacers, allowing each to attach to its corresponding target. The RNA-DNA heteroduplex formed then binds to a nuclease called Cas9 and directs it to catalyze the cleavage of double-stranded DNA at a position near the junction of the target-specific sequence and the palindromic repeat in the guide RNA. Because RNA-DNA heteroduplexes are stable and because designing an RNA sequence that binds specifically to a unique target DNA sequence requires only knowledge of the Watson-Crick base-pairing rules (adenine binds to thymine [or uracil in RNA], and cytosine binds to guanine), the CRISPR-Cas9 system was preferable to the fusion protein designs required for using ZFNs or TALENs.

A further technical advance came in 2015, when Zhang and colleagues reported the application of Cpf-1, rather than Cas9, as the nuclease paired with CRISPR to achieve gene editing. Cpf-1 is a microbial nuclease that offers potential advantages over Cas9, including requiring only one CRISPR guide RNA for specificity and making staggered (rather than blunt) double-stranded DNA cuts. The altered nuclease properties gave potentially greater control over the insertion of replacement DNA sequences than was possible with Cas9, at least in some circumstances. Researchers suspect that bacteria house other genome-editing proteins as well, the evolutionary diversity of which could prove valuable in further refining the precision and versatility of gene-editing technologies.

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Gene editing | Definition, History, & CRISPR-Cas9

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gene editing, the ability to make highly specific changes in the DNA sequence of a living organism, essentially customizing its genetic makeup. Gene editing is performed using enzymes, particularly nucleases that have been engineered to target a specific DNA sequence, where they introduce cuts into the DNA strands, enabling the removal of existing DNA and the insertion of replacement DNA.

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Genome editing (also called gene editing) is a group of technologies that give scientists the ability to change an organism's DNA. These technologies allow genetic material to be added, removed, or altered at particular locations in the genome. Several approaches to genome editing have been developed. A well-known one is called CRISPR-Cas9, which is short for clustered regularly interspaced short palindromic repeats and CRISPR-associated protein 9. The CRISPR-Cas9 system has generated a lot of excitement in the scientific community because it is faster, cheaper, more accurate, and more efficient than other genome editing methods.

CRISPR-Cas9 was adapted from a naturally occurring genome editing system that bacteria use as an immune defense. When infected with viruses, bacteria capture small pieces of the viruses' DNA and insert them into their own DNA in a particular pattern to create segments known as CRISPR arrays. The CRISPR arrays allow the bacteria to "remember" the viruses (or closely related ones). If the viruses attack again, the bacteria produce RNA segments from the CRISPR arrays that recognize and attach to specific regions of the viruses' DNA. The bacteria then use Cas9 or a similar enzyme to cut the DNA apart, which disables the virus.

Researchers adapted this immune defense system to edit DNA. They create a small piece of RNA with a short "guide" sequence that attaches (binds) to a specific target sequence in a cell's DNA, much like the RNA segments bacteria produce from the CRISPR array. This guide RNA also attaches to the Cas9 enzyme. When introduced into cells, the guide RNA recognizes the intended DNA sequence, and the Cas9 enzyme cuts the DNA at the targeted location, mirroring the process in bacteria. Although Cas9 is the enzyme that is used most often, other enzymes (for example Cpf1) can also be used. Once the DNA is cut, researchers use the cell's own DNA repair machinery to add or delete pieces of genetic material, or to make changes to the DNA by replacing an existing segment with a customized DNA sequence.

Genome editing is of great interest in the prevention and treatment of human diseases. Currently, genome editing is used in cells and animal models in research labs to understand diseases. Scientists are still working to determine whether this approach is safe and effective for use in people. It is being explored in research and clinical trials for a wide variety of diseases, including single-gene disorders such as cystic fibrosis, hemophilia, and sickle cell disease. It also holds promise for the treatment and prevention of more complex diseases, such as cancer, heart disease, mental illness, and human immunodeficiency virus (HIV) infection.

Ethical concerns arise when genome editing, using technologies such as CRISPR-Cas9, is used to alter human genomes. Most of the changes introduced with genome editing are limited to somatic cells, which are cells other than egg and sperm cells (germline cells). These changes are isolated to only certain tissues and are not passed from one generation to the next. However, changes made to genes in egg or sperm cells or to the genes of an embryo could be passed to future generations. Germline cell and embryo genome editing bring up a number of ethical challenges, including whether it would be permissible to use this technology to enhance normal human traits (such as height or intelligence). Based on concerns about ethics and safety, germline cell and embryo genome editing are currently illegal in the United States and many other countries.

Scientific journal articles for further reading

Ormond KE(1), Mortlock DP(2), Scholes DT(3), Bombard Y(4), Brody LC(5), Faucett WA(6), Garrison NA(7), Hercher L(8), Isasi R(9), Middleton A(10), Musunuru K(11), Shriner D(12), Virani A(13), Young CE(3). Human Germline Genome Editing. Am J Hum Genet. 2017 Aug 3;101(2):167-176. PubMed: 28777929. Free full-text available from PubMed Central: PMC5544380.

Gupta RM, Musunuru K. Expanding the genetic editing tool kit: ZFNs, TALENs, and CRISPR-Cas9. J Clin Invest. 2014 Oct;124(10):4154-61. doi: 10.1172/JCI72992. Epub 2014 Oct 1. Review. PubMed: 25271723. Free full-text available from PubMed Central: PMC4191047.

Hsu PD, Lander ES, Zhang F. Development and applications of CRISPR-Cas9 for genome engineering. Cell. 2014 Jun 5;157(6):1262-78. doi:10.1016/j.cell.2014.05.010. Review. PubMed: 24906146. Free full-text available from PubMed Central: PMC4343198.

Komor AC, Badran AH, Liu DR. CRISPR-Based Technologies for the Manipulation of Eukaryotic Genomes. Cell. 2017 Apr 20;169(3):559. doi:10.1016/j.cell.2017.04.005. PubMed: 28431253.

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What are genome editing and CRISPR-Cas9?: MedlinePlus Genetics

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Gene editing occurs when scientists change the DNA of an organism. Learn more about this process and the different ways it can be done.

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Using debates in the classroom provide students the opportunity to work in a collaborative and cooperative group setting. By having students discuss and organize their points of view for one side of an argument they are able to discover new information and put knowledge into action. Classroom debates help students learn through friendly competition, examine controversial topics and "strengthen skills in the areas of leadership, interpersonal influence, teambuilding, group problem solving, and oral presentation" (Leuser, n.d., para. 1).

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Classroom Debates | Center for Innovative Teaching and Learning | Northern Illinois University

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Using debates in the classroom provide students the opportunity to work in a collaborative and cooperative group setting.