EXOBIOLOGY:
Finding Life's Limits
Gretchen Vogel
WASHINGTON, D.C.--DNA, RNA, and the ribosomes that help translate the genetic code into proteins have a fixed size, which puts a limit on how small a self-replicating cell can be, according to a group of experts who gathered at the National Academy of Sciences last month to discuss the limits of life at the tiniest level. Assuming that a cell needs DNA and ribosomes to make its proteins, a spherical cell much smaller than about 200 nanometers in diameter--about one-tenth the diameter of an Escherichia coli--is not compatible with life as we know it, they concluded.