If the Cas9 complex doesn't see a PAM next to its target DNA sequence, it won't cut. Short DNA sequences known as "protospacer adjacent motifs," or PAMs, serve as tags and sit adjacent to the target DNA sequence. There is a built-in safety mechanism that ensures that Cas9 doesn't just cut just anywhere in a genome. Using two separate regions or "domains" on its structure, Cas9 cuts both strands of the DNA double helix, making what is known as a "double-stranded break," according to the 2014 Science article. The target DNA that the Cas9 will cut through is complementary to a 20-nucleotide stretch of the crRNA, where a "nucleotide" is a building block of DNA that contains one base. The protein binds to crRNA and tracrRNA, which together guide Cas9 to a target site on the virus's DNA strand where the protein will make its cut. The crRNA interacts with the Cas9 protein and another kind of RNA, called "trans-activating crRNA" or tracrRNA, in order to help bacteria fend off viruses.Ĭas9: The Cas9 protein is an enzyme that cuts foreign DNA. For example, a G in the DNA molecule would get transcribed as a C in the RNA.Įach snippet of CRISPR RNA contains a copy of a repeat and a spacer from a CRISPR region of DNA, according to a 2014 review by Jennifer Doudna and Emmanuelle Charpentier, published in the journal Science. To build an RNA molecule, one part of the CRISPR acts as a template and proteins called polymerases swoop in to construct an RNA molecule that is "complementary" to that template, meaning the two strands' bases would fit together like puzzle pieces. RNA also differs from DNA in that it's only one strand, rather than two, meaning it looks like just a half of a ladder. Unlike DNA sequences, which remain lodged inside the DNA molecule, this CRISPR RNA (crRNA) can roam about the cell and team up with proteins - namely the molecular scissors that snip viruses to bits. In this way, the researchers were able to alter the bacteria's resistance to an attack by a specific virus, confirming CRISPRs' role in regulating bacterial immunity.ĬRISPR RNA (crRNA): CRISPR regions of DNA act as a kind of bank of viral memories but for that stored information to be useful elsewhere in the cell, it must be copied, or "transcribed," into a different genetic molecule called RNA. The team also manipulated the spacers by removing them and inserting new viral DNA sequences in their place. Moreover, the DNA sequence of these spacers was identical to parts of the virus genome. They observed that after a viral attack, the bacteria incorporated new spacers into their CRISPR regions. In a 2007 paper published in the journal Science, the researchers used Streptococcus thermophilus bacteria, which are commonly found in yogurt and other dairy cultures, as their model, according to the Joint Genome Institute, part of the U.S. Rodolphe Barrangou and a team of researchers at Danisco, a food ingredients company, first demonstrated this process experimentally. Related: Going viral: 6 new findings about viruses You can also think of spacers like "Wanted" posters, providing a snapshot of the bad guys so they can be easily spotted and brought to justice. These spacers serve as a bank of memories, which enables the bacteria to recognize the viruses if they should ever attack again. Short palindromic repeats appear throughout CRISPR regions of DNA, with each repeat bookended by "spacers." Bacteria swipe such spacers from viruses that have attacked them, meaning they incorporate a bit of viral DNA into their own genome. A palindrome, like the word "racecar," reads the same forward as it does backward similarly, in a palindromic sequence, bases on one side of the DNA ladder match those on the opposing side when you read them in opposite directions.įor example, a super simple palindromic sequence might look like this: In a CRISPR region, these bases appear in the same order several times, and in these repeated segments, they form what's known as "palindromic" sequences, according to the Max Planck Institute. Each rung contains two chemical bases bound together: A base called adenine (A) links up to another called thymine (T), and the base guanine (G) pairs with cytosine (C). When we talk about repeats in the genetic code, we're talking about the ordering of rungs within the spiral ladder of a DNA molecule. (Image credit: Shutterstock)ĬRISPRs: The term "CRISPR" stands for "clusters of regularly interspaced short palindromic repeats" and describes a region of DNA made up of short, repeated sequences with so-called "spacers" sandwiched between each repeat. DNA is a double-stranded molecule whose "rungs" are made up of one of two base pairs: adenine paired with thymine or cytosine paired with guanine.
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