Some of the biggest discoveries in science often hide away in plain sight for many years before their importance is fully realised. This is certainly true of Crispr (pronounced crisper), which has taken the world of genetics by storm.
Crispr stands for clustered regularly interspaced short palindromic repeats, a devilishly contrived acronym which just about sums up why it was ignored for so long. For nearly two decades after Japanese researchers first discovered Crispr in bacteria in 1987, scientists mostly dismissed it as junk DNA.
In fact, the apparently nonsensical sequences within Crispr, which were repeated in palindromic order (the same backwards as forwards), did have a purpose and were far from junk. About six years ago, scientists discovered that these DNA sequences matched the genetic sequences of various viruses that attack bacteria, which led to the discovery of a sophisticated bacterial immune system.
Far from being junk, Crispr was actually a way of storing the genetic information of an invading virus in the form of a palindromic DNA sequence. The bacteria used this genetic memory to target the viral invader by chopping it up with powerful Crispr-associated (CAS) enzymes capable of cleaving its DNA molecule, just like a pair of molecular scissors.
The mystery of Crispr was finally resolved by Jennifer Doudna of the University of California, Berkeley, a specialist in RNA, the smaller molecular cousin of DNA. About seven years ago, she was asked by a university colleague to look into this genetic peculiarity of bacteria and quickly became fascinated.
The more we looked into it, the more it seemed extremely interesting, Professor Doudna said. Then, in 2011, she met Emmanuelle Charpentier of Umea University in Sweden at a scientific conference. Professor Charpentier told Professor Doudna of another kind of Crispr system that seemed to rely on a single gene, now called CAS9.
Both professors collaborated on the project and in August last year published what is now considered the seminal paper showing that CAS9 was an enzyme capable of cutting both strands of a DNA double helix at precisely the point dictated by a programmable RNA sequence in other words, an RNA molecule that could be made to order. We found that CAS9 has the ability to make a double-stranded break in DNA at sites that are programmed by a small RNA molecule. What was so important was that we could really show how the CAS9 protein worked, Professor Doudna said.
Not only were we able to work out how it worked, we were able to reprogramme it to recognise new DNA sequences. If it could be made to work in eukaryote systems plants and animals then youd have a system where you could effectively decide where to produce a double-stranded break in that cells genome.
It not only worked on plants and animals, it worked beautifully. Professors Doudna and Charpentier had found the holy grail of genetic engineering a method of cutting and stitching DNA accurately and simply anywhere in a complex genome. Until now, this was carried out by modified viruses, which inserted their DNA at random, or by elaborately cumbersome techniques known as zinc fingers or Talens.
You can actually introduce new genetic information at the site of cleavage. So it has become a powerful way of doing genetic engineering. Its a fundamentally different way of recognising DNA target sites, Professor Doudna said.
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Biochemist Jennifer Doudna hailed for discovery of ‘holy grail’ of genetic engineering