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August 31, 2019 02:03 am

CRISPR Now Cuts and Splices Whole Chromosomes

Researchers report they've adapted CRISPR and combined it with other tools to cut and splice large genome fragments with ease. The study, conducted by researchers at the Medical Research Council (MRC) Laboratory of Molecular Biology in Cambridge, U.K., has been published in this week's issue of Science. Science Magazine reports: The tried and true tools of genetic engineering simply can't handle long stretches of DNA. Restriction enzymes, the standard tool for cutting DNA, can snip chunks of genetic material and join the ends to form small circular segments that can be moved out of one cell and into another. (Stretches of linear DNA don't survive long before other enzymes, called endonucleases, destroy them.) But the circles can accommodate at most a couple of hundred thousand bases, and synthetic biologists often want to move large segments of chromosomes containing multiple genes, which can be millions of bases long or more. "You can't get very large pieces of DNA in and out of cells," says Jason Chin, a synthetic biologist at the Medical Research Council (MRC) Laboratory of Molecular Biology in Cambridge, U.K. What's more, those cutting and pasting tools can't be targeted precisely, and they leave unwanted DNA at the splicing sites -- the equivalent of genetic scars. The errors build up as more changes are made. Another problem is that traditional editing tools can't faithfully glue large segments together. These issues can be a deal-breaker when biologists want to make hundreds or thousands of changes to an organism's genome, says Chang Liu, a synthetic biologist at the University of California, Irvine. Now, Chin and his MRC colleagues report they have solved these problems. First, the team adapted CRISPR to precisely excise long stretches of DNA without leaving scars. They then altered another well-known tool, an enzyme called lambda red recombinase, so it could glue the ends of the original chromosome -- minus the removed portion -- back together, as well as fuse the ends of the removed portion. Both circular strands of DNA are protected from endonucleases. The technique can create different circular chromosome pairs in other cells, and researchers can then swap chromosomes at will, eventually inserting whatever chunk they choose into the original genome. "Now, I can make a series of changes in one segment and then another and combine them together. That's a big deal," Liu says.

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