The simplest life-forms on Earth have just one cell. An example of a one-celled life-form is a bacterium. These species of bacteria all cause diseases in humans. But many bacteria are not harmful, or even helpful.
17 Things you should know about DNA (Infographic). It's the basic building block of all life but what do you really know about it? See how you compare to other humans and crack the genetic code with this informative infographic.