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Nature

A Genetic Jigsaw: Single-Celled Organism Breaks Up Its Genome And Rearranges DNA Pieces

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Justine Alford

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clockSep 9 2014, 10:51 UTC
2063 A Genetic Jigsaw: Single-Celled Organism Breaks Up Its Genome And Rearranges DNA Pieces
John Bracht, American University, and Robert Hammersmith, Ball State University

You may think of unicellular organisms as the simpletons of life on Earth, but Oxytricha trifallax is a prime example of just how remarkably complex single life can be.  This unassuming microorganism performs genetic gymnastics during development, unravelling its jigsaw genome and rearranging some quarter-million pieces of DNA. What’s more, it manages the whole process in just 60 hours.

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O. trifallax is a single-celled, algae gobbling eukaryote (organism with a defined nucleus) that is widespread on Earth. Princeton scientists, headed by Laura Landweber, have been studying the impressive genetics of this pond-dwelling organism for several years now, and its complexity has not ceased to amaze them.

Alongside its unique genome, this organism has several unusual features that make it stand out from the crowd. O. trifallax cells are large, around 10 times the size of your “average” human cell, and they contain two nuclei instead of one. The nucleus is a membrane-bound organelle that stores genetic information in the form of DNA.

O. trifallax keeps its active DNA in one working nucleus and uses the other as a storage box for an archive of scrambled genetic material that can be passed on to the next generation. It is the DNA within the latter—known as the germline nucleus—which is disassembled and subsequently rearranged in order to produce a new working, or somatic, nucleus in the offspring.

But this organism’s peculiarity doesn’t end there. Offspring spawn from a single organism, so rather than using sex to reproduce, O. trifallax cells do the deed solely to swap genetic material between one another. This allows them to trade in aging genes for new ones, diversifying their genome. “It’s kind of like science fiction—they stop aging by trading in their old parts,” Landweber said in a news release.

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With the assistance of RNA molecules, the organisms then chop out “junk” DNA sequences in the germline nucleus that don’t encode proteins and rearrange the remaining pieces, some 225,000 segments, into functional genes. The end result is a new, somatic (working) genome that is passed on to offspring. The researchers describe this impressive reorganization in the journal Cell.

Alongside being an extremely interesting organism, Oxytricha may help us understand more about chromosome disassembly and reorganization in multicellular life forms, perhaps even in humans.

“It’s basically bad when human chromosomes break apart and reassemble in a different order,” said Landweber. “The process in Oxytricha recruits some of the same biological mechanisms that normally protect chromosomes from falling apart and uses them to do something creative and constructive instead.”

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Remarkably, the somatic nucleus was found to contain at least 16,000 tiny chromosomes; human somatic cells contain just 46 (germ cells, which develop into sperm and eggs, contain half). The majority of these chromosomes only contain one gene, but some of the larger chromosomes produce ginormous proteins.

According to Landweber, Oxytricha’s unusual jigsaw genetics is probably one of nature’s early attempts to become more complex while remaining unicellular. Its intricate genome rearranging system showcases the “true diversity of life on our planet,” she said

Read this: Less Than 1% Of Sweden's Trash Ends Up In Landfills


Nature
  • genetics,

  • DNA,

  • genome,

  • Oxytricha trifallax

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