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CRISPR-Cas9 and Variants for Genome Editing

CRISPR gene editing is a powerful genetic engineering technique that allows scientists to modify the genomes of living organisms with high precision and efficiency. Based on a natural defense system found in bacteria, CRISPR-Cas9 uses a guide RNA to direct the Cas9 enzyme to a specific location in the DNA, where it can cut the DNA strand. This enables the removal or addition of genetic material, making it a valuable tool in biotechnology and medicine. CRISPR is considered highly significant because it is cost-effective, precise, and versatile, with applications ranging from creating new medicines and genetically modified organisms to treating genetic diseases. Its potential in treating inherited conditions and diseases like cancer has sparked widespread interest. However, its use in human germline modification remains controversial. The development of CRISPR-Cas9 earned Jennifer Doudna and Emmanuelle Charpentier the Nobel Prize in Chemistry in 2020. The system works by creating targeted DNA breaks, which can be repaired in two main ways: homology-directed repair, which allows for precise gene insertion, and non-homologous end joining, which often results in random mutations.

Figures (10)

The different generations of restriction endonucleases used for genome editing and the DNA repair pathways used to modify target DNA.
dsDNA-break repair pathways and genome editing using CRISPR-Cas nucleases
CRISPR-Cas9
Overview of CRISPR-Cas9 plasmid construction
Different CRISPR DNA nucleases with their PAM and size
Simplified flowchart for CRISPR based diagnostics like SHERLOCK[251]
General overview of the TALEN process
Synthetic DNA is repeatedly introduced at multiple targeted areas of the chromosome and/or loci and then is replicated producing cells with/without mutations.
Also known as the “CRISPR RNA”, a piece of RNA transcribed from the CRISPR locus that contains one spacer from that locus. The crRNA therefore contains the “targeting sequence” that allows the Cas9 nuclease complex to identify a complementary piece of DNA at which it should make a cut.
DNA repair after a double-stranded break

It uses a guide RNA to find a specific DNA spot and a Cas9 enzyme to cut the DNA there. This cut lets scientists add, remove, or change genetic material. The process can fix broken genes or study how genes work.

Two main ways DNA is fixed after a cut are homology-directed repair, which adds new DNA precisely, and non-homologous end joining, which often causes random changes. CRISPR is faster, cheaper, and more accurate than older methods. It has been used in research, medicine, and agriculture.

Variants like Cas12a and Cas13 can target RNA or DNA differently, expanding its uses. CRISPR can be delivered into cells using viruses or other methods. Scientists also use it to study diseases and create models to test treatments.

It has already helped treat some genetic disorders in humans. However, using it to change human embryos is controversial. CRISPR is a powerful tool, but it must be used carefully to avoid unintended effects.

Key Points

  • CRISPR-Cas9 is a genetic engineering technique in molecular biology by which the genomes of living organisms may be modified, based on a simplified version of the bacterial CRISPR-Cas9 antiviral defense system.
  • A guide RNA is a synthetic RNA molecule that, when complexed with the Cas9 nuclease, directs the Cas9 to a specific location in the cell's genome for modification.
  • The protospacer adjacent motif is a specific DNA sequence that the Cas9 nuclease recognizes and requires to be present near the target site in the genome for successful genome editing.
  • Cas12 is a Class II Type V CRISPR-associated endonuclease-exonuclease that generates staggered cuts in double-stranded DNA and relies on a 'T-rich' protospacer adjacent motif for targeting.
  • Cas13 is a family of CRISPR-associated endonucleases that target RNA, performing two distinct ribonuclease functions: processing its own CRISPR RNA and catalyzing target RNA cleavage.

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