The giants of the virus world

Viruses tend to have tiny genomes with only around 3,000 genetic letters. That’s compared to around 3 billion in our own genome. They strip their genetics back to the bare essentials, and borrow everything else from the cells they infect. Even so, there are a few unusual ‘megaviruses’ that buck the trend. Mimivirus has a bloated genetic code containing 1.2 million letters. It’s so enormous that when researchers first saw it they thought it was a bacterium. Unlike most viruses, it carries genes for building proteins, suggesting that it may have evolved from an organism that could once fend for itself. An alternative hypothesis is that it stole the genes from the cells it infects.