Giant viruses

Mimivirus is the largest and most complex virus known to mankind, with a diameter of 750 nanometers. It possesses a genome, truly outsized by viral standards, of 1.2 million base pairs, coding an outlandish 1,018 genes. For comparison, the smallest free-living bacterium, Mycoplasma genitaliurn, is just 450 nanometers in diameter and possesses a genome half the size of that in mimivirus, while coding just 482 proteins. Most giant viruses have only been discovered and characterized in the past few years. There are several reasons why these striking biological entities remained undetected for so long. The mimivirus particles in samples from the Bradford cooling tower were discovered among bacteria with the potential to cause pneumonia, and consequently there has been interest in the question of whether mimivirus might be a human pathogen. Yet mimivirus often challenges the usual rules. It gains entry into phagocytic cells when the scavengers engulf it.