Cut And Paste Genes Callum Sharma (Year 11, Churchill)
This article will present methods used by scientists to find cures for diseases through genetic editing using CRISPR. CRISPR technology is a simple yet powerful tool for editing genomes. It allows researchers to easily alter DNA sequences and modify gene function. Its many potential applications include correcting genetic defects, treating and preventing the spread of diseases and improving crops. However, its promise also raises ethical concerns.
1 BACKGROUND AND PROCEDURE The genomes of organisms encode a series of messages and instructions within their DNA sequences. Genome editing involves changing those sequences, thereby changing the messages. CRISPR is a genetic engineering technique that targets genetic codes in order to edit the DNA. For example, it can be used to treat diseases such as HIV. When the target DNA is found, Cas9 – one of the enzymes that is part of the CRISPR system – binds to the DNA and cuts it, shutting the targeted gene off. The protein Cas9 is an enzyme that acts like a pair of molecular scissors, capable of cutting strands of DNA.1 CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats This means a series of ‘short’ ‘clustered’ repeating DNA sequences with “spacers” sitting in between them, and which mirror the next sequence like a ‘palindrome.’ These are the sequences that are used as a guide by the Cas-9 enzyme. CRISPR technology was originally developed from the natural defence mechanisms of bacteria and archaea. These organisms use CRISPR with RNA and various Cas proteins, including Cas9, to stop attacks by viruses and other foreign bodies. 8 The Cas9 enzyme involves two RNA molecules which move the protein Cas9 to the targeted site, where it will make its cut, cutting both strands of the DNA double helix. In bacteria, once the Cas9 has been guided to the CRISPR sequence, it can insert ‘spacers’. In the case of bacteria, the spacers are taken from viruses that previously attacked the organism. They serve as a bank of memories, which enables bacteria to recognize the viruses and fight off future attacks [1]. Once DNA is cut, the cell's natural repair mechanisms insert changes to the genome. This can happen in two different ways. One way is to join the two cuts back together. This method, also known as "non-homologous end joining," can cause flaws. Nucleotides can accidentally be inserted creating mutations, which could affect a gene. The second method is that the break is fixed by filling in the gap with nucleotides. To create this, the cell makes a short strand of DNA as a template. Scientists can supply the DNA template of their choosing to fix a mutation or to change a gene [8].
2 SCIENTIFIC HISTORY The discovery of clustered DNA repeats was made by Yoshizumi Ashino in Osaka University in 1987 [1]. He accidentally discovered them by cloning a CRISPR sequence along with another gene that was the original target. The repeats were an unexpected finding, because repeats would usually be consecutive rather than in this case clustered. The CRISPR CAS-9 system was discovered by Jennifer Doudna, a professor at the University of California at Berkeley, and Emmanuel Charpentier. They discovered the CRISPR to create the CAS-9; which could locate and target the DNA specified by the guide RNA. Fusing two RNA molecules would create a single guide RNA molecule. They manipulated the nucleotide sequence of the RNA to program the CAS-9 [2]. In 2013, Feng Zhang, of the Broad Institute of MIT and Harvard, was the first scientist to adapt the CRISPR CAS-9 for editing eukaryotic cells. Zhang’s lab focused on Synthetic Biology and he had a central role in the development of CRISPR technologies. In 2011, Doctor Zhang started using the CRISPR system 100
