Nearly all cells house their DNA inside a nucleus, but Tetrahymena houses different versions of its DNA in each of its two nuclei. The smaller nucleus (called the micronucleus) does nothing more than keep the cell's full genome safe, acting as a sort of "lock box." The larger nucleus (called the macronucleus), on the other hand, uses the DNA to regulate the cell's life functions. This macronucleus houses about 15 percent fewer DNA sequences than the smaller one, and when the cell mates to create a new generation of cells, this large macronucleus withers and dies. Before it completely expires, however, the small nucleus steps in and produces a new large and small nucleus set. To make sure no new viral sequences have sneaked into the lock box, the cell checks the DNA of the small, lock box nucleus against the DNA of the old large nucleus and eliminates any foreign material before allowing the new large nucleus to develop.
Gorovsky's team believes that in evolutionarily ancient times, cells had to fight against a variety of assaults just as they must today: viruses attacked cells, injecting their DNA, disrupting normal cell functions; and transposons, bits of nomadic genetic material, would fit themselves in several places in the cell's genome, copying themselves prodigiously and wreaking havoc. To survive, cells evolved a correction system that recognized the invading DNA and either eliminated or silenced it. Gorovsky and Mochizuki propose that the small nucleus "transcribes" into RNA part or all of its DNA, essentially making RNA photocopies of certain DNA sections. These transcriptions include the sections that may have been corrupted by some genetic marauder like a virus or transposon. The transcribed RNAs then migrate out of the small nucleus, through the cell to the large nucleus where they mingle with the macronucleus' DNA. If an RNA comes across a section
Contact: Jonathan Sherwood
University of Rochester