Gene editing is a technology where changes are made to DNA. This could lead to changes in physical traits such as eye colour, or reduce disease risks such as diabetes. CRISPR is also a form of gene editing, programmed to target specific stretches of genetic code for editing DNA at precise locations.
Genetic modification
Different technologies can genetically modify DNA, and they act like scissors cutting the DNA at specific spots. Modifications can include removals, additions, or replacements of DNA where it was cut. A purpose of genetic modification is to modify bacteria in order to produce human insulin for the medical treatments of diabetic patients.
For the production of insulin, the process of genetic modification looks like this:
The gene for human insulin is isolated using a restriction enzyme.
A bacterial plasmid is cut by the same restriction enzyme, leaving it with a corresponding section of unpaired bases.
The gene is inserted into a vector, the bacterial plasmid or virus (because they reproduce quickly).
The plasmid and isolated insulin genes are joined together by a DNA ligase enzyme (imagine that the ligase enzymes are tweezers used to put the two together).
The new bacterial plasmid containing the human insulin gene is called a “recombinant DNA", and the recombinant DNA is inserted into a bacterial cell.
The bacteria goes on to reproduce offspring that contain the human gene for insulin, where it can be extracted and used medically.
Gene editing and gene therapy
You may be confused and wonder about the difference between gene editing and genetic modification. In short, gene editing involves changing the DNA of an organism by making alterations to its genetic code, while genetic modification is the process of changing the DNA of an organism by adding elements of DNA from a different organism.
Gene therapy involves the addition of new genes to human cells, and gene editing (genome editing) is the precise manipulation of the sequence of the human genome for disease management. Genome editing is a form of gene therapy, and can be classified into:
Somatic therapy: Targets non-reproductive cells, only affects the person receiving the gene therapy (genome editing)
Germline therapy: Change in DNA in reproductive cells (genetic modification)
The CRISPR-Cas9 is a technology used to perform gene editing, and has largely improved upon older genome editing technologies, proving to be cheaper and more effective.
CRISPR-Cas9: Clustered Regularly Interspaced Short Palindromic Repeats
CRISPR-Cas9 is a method of finding then altering a specific part of DNA inside a cell. However, CRISPR has also been adapted to perform other functions, such as turning genes on or off without altering their sequence. It is cheaper and easier in comparison to older gene editing methods (eg. nuclease systems), while the latter can take years and cost hundreds of thousands of dollars.
Cas9 is a bacterial RNA-guided endonuclease that recognizes and cleaves target DNAs through base pairing with the complementary guide RNA. As mentioned before, it is programmed to target specific stretches of genetic code for precise editing of DNA.
The Cas9 protein is most widely used by scientists, because it is easily programmable for finding and binding with almost any desired target sequence. When it is added to a cell along with a piece of guide RNA, the Cas9 protein hooks up with the guide RNA, then moves along the strands of DNA, until it finds and binds to a 20-DNA-letter long sequence matching part of the guide RNA sequence.
Modified versions of Cas9 allows researchers to study the gene’s function and simply activate gene expression without editing. With the emergence of more efficient and customisable technologies, CRISPR-Cas9 is a promising alternative for older genome editing tools.
Works Cited
Anders, C. and Jinek, M. (2014). In Vitro Enzymology of Cas9. Methods in Enzymology, 546, pp.1–20. doi:https://doi.org/10.1016/b978-0-12-801185-0.00001-5.
BBC Bitesize. (n.d.). The process of genetic engineering - Higher - Variation - AQA - GCSE Biology (Single Science) Revision - AQA. [online] Available at: https://www.bbc.co.uk/bitesize/guides/zsg6v9q/revision/6.
Broad Institute (2018). Questions And Answers About CRISPR. [online] Broad Institute. Available at: https://www.broadinstitute.org/what-broad/areas-focus/project-spotlight/questions-and-answers-about-crispr.
Karassellos, L. (n.d.). 6.2.4 Genetic Engineering | AQA GCSE Biology Revision Notes 2018. [online] Save My Exams. Available at: https://savemyexams.com/gcse/biology/aqa/18/revision-notes/6-inheritance-variation--evolution/6-2-variation--evolution/6-2-4-genetic-engineering/.
Le Page, M. (2020). What is CRISPR? [online] New Scientist. Available at: https://www.newscientist.com/definition/what-is-crispr/.
National Human Genome Research Institute (2019). What is genome editing? [online] Genome.gov. Available at: https://www.genome.gov/about-genomics/policy-issues/what-is-Genome-Editing.
The University of Edinburgh. (2021). Genetic modification FAQs. [online] Available at: https://www.ed.ac.uk/roslin/about/dolly/facts/genetic-modification.
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