A few years ago, researchers discovered the largest viral genome on the planet — that of the Mimivirus, which infects freshwater amoeba. It's genome contains more than 1 million base pairs, and encodes genes that had previously only been found in living cells, according to Ars Technica's John Timmer. Researchers recently discovered an oceanic cousin of the Mimivirus — the Cafeteria roenbergensis virus — that has the second-largest viral genome ever found in a single-celled host that preys on the bacteria and plankton in the water. The C. roenbergensis genome is more than 700,000 base pairs long, Timmer says, adding that its "ends are filled with repetitive DNA, which the authors speculate acts a bit like telomeres do in human cells."
At Byte Size Biology, Iddo Friedberg asks, "How much cellular machinery does a virus need to code in its genome to cross the border between life and non-life?"