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The Cells That Changed The Earth
Some of the oldest cells on Earth are single-cell organisms called bacteria. Fossil records indicate that mounds of bacteria once covered young Earth. Some began making their own food using carbon dioxide in the atmosphere and energy they harvested from the sun. This process (called photosynthesis) produced enough oxygen to change Earth’s atmosphere. Soon afterward, new oxygen-breathing life forms came onto the scene. With a population of increasingly diverse bacterial life, the stage was set for some amazing things to happen. Bacteria are single-celled organisms with a circular DNA molecule and no organelles.
Cells must control transposons and foreign genetic elements to maintain genomic stability. This is especially important in gametes, where unchecked transposon activity could affect the next generation. We now know that organisms have pathways in which small RNAs guide proteins to actively silence transposons in the genome. These pathways exist in prokaryotes and eukaryotes, with general mechanisms conserved between the two groups. These include mechanisms for recognizing foreign genetic elements, activating the silencing responses, and maintaining a “memory” of the active transposon event. Small RNAs associated with piwi proteins — known as piRNAs — are critical to these processes.
In 2007, Brennecke et al. proposed that the piRNA pathway could act as a molecular adaptive immune system, capable of sensing and reacting to adaptive transposons by silencing them. We begin this Collection looking back at these findings and then taking a broader view to appreciate how much we have learned about the roles of small RNAs in the maintenance of genomic stability.Discrete Small RNA-Generating Loci as Master Regulators of Transposon Activity in Drosophila.
Julius Brennecke, Alexei A. Aravin, Alexander Stark, Monica Dus, Manolis Kellis, Ravi Sachidanandam, Gregory J. Hannon
SnapShot: Mouse piRNAs, PIWI Proteins, and the Ping-Pong Cycle
Jogender S. Tushir, Phillip D. Zamore, Zhao Zhang