What exactly distinguishes CRISPR CAS9 from transposons


CRISPR stands for "clustered regularly interspaced shoard palindromic repeats ”and are DNA sequences in prokaryotes (bacteria and archaea) that act as an adaptive immune system against viruses. It is a sequence of repeating palindromes and an array of DNA sequences from viruses (“spacers”). The palindromes form so-called "direct repeats".

After infections of bacteria and archaea with viruses, microorganisms can develop a kind of memory against viral infections. This happens at the molecular level with the help of nucleases and interactions between nucleic acids (formation of DNA: RNA hybrids), which specifically recognize and break down the DNA of intruders.

The CRISPR system is associated with proteins or genes that are designated with Cas (for "C.RISPR associated "). Cas proteins have both helicase and nuclease activity.

After a virus infection, surviving prokaryotes can build the viral DNA in their own genome as a “spacer”. The palindromes are necessary to add the virus's DNA using Cas1 and Cas2 proteins. This creates a new “spacer” and another palindrome repeat.

The CRISPR-DNA is transcribed (as pre-crRNA) and subsequent modification (post-transcriptional modification) creates RNA molecules (crRNA) that are partially complementary to the viral DNA and also bind to certain Cas proteins. The recognition of the target DNA by the crRNA leads to the Cas proteins finding and cutting the “right” point. This creates a double strand break in the DNA (Eng .: “double strand break”).

The three phases of the process are thus in Acquisition of foreign DNA (as a new "spacer" in the CRISPR-DNA), Machining the crRNA and interference (specific attack on the target DNA).

The bacterium or the archaeon must be able to differentiate its own genetic material from that of the virus in order to avoid undesired degradation of its own DNA. This is done using recognition sequences that are only in the target DNA. Without a recognition sequence, the Cas9-RNA complex can bind to the viral DNA, but it cannot cut it.

There are three different CRISPR systems, which are classified based on the Cas proteins involved and the recognition features on the target DNA. The Type II CRISPR / Cas system is particularly interesting because only a single protein (Cas9) is needed to cut the target DNA.

Two studies in 2012 showed how the CRISPR / Cas9 system is from the model organism Streptococcus pyogenes can be reprogrammed to bind and cut at any location in DNA. It is therefore widely used in biotechnology and genome editing.

The CRISPR-DNA and the cas-genes as well as the tracrRNA are called CRISPR Locus. These DNA segments are not far from each other. Since the pre-crRNA (and also the tracrRNA) performs its function without being translated as a protein, it is also referred to as non-coding RNA.

In the specialist literature, the CRISPR-DNA is sometimes called the CRISPR array.

CRISPR DNA can vary widely between species, or even within the same species. The palindromes or “repeats” are between 24 and 47 base pairs long, while the “spacers” are between 26 and 72 base pairs long. A CRISPR-DNA can contain two to approx. 250 palindromes. Six to 20 cas genes are associated with CRISPR-DNA. A procaroyte can also contain several different CRISPR loci.