The definition of life is a tricky question. The most obvious example is red blood cells dump their DNA. So they can't replicate, and virus can't target them. However, insect drones also can't reproduce and are considered alive so it's a little more open to debate than you might think.
As to life without DNA. The methodology is somewhat in question for this but here is a more in depth link: http://science.sciencemag.org/content/332/6034/1163.full Now, this is really 1:1 with DNA just using slightly different chemistry, but it's hard to call something DNA when you swap out one of the building blocks.
Other examples are hard to locate in large part because you can't direct detect this stuff.
Anyway, my point was while a virus is 'stuck' using DNA as it needs to infect things that use DNA. However, they can use RNA internally.
PS: For a really out there example, prions seem really close to life.
This does not mean that arsenate does not get into the bacteria, he points out. “It just shows that this bacterium has evolved to extract phosphate under almost all circumstances.”
Yes, it really wants phosphorus. But, a strong preference does not create phosphorus when there is none to be found.
Not that single cells really want or try etc, but you get the idea.
I'm not aware of any life which lacks DNA. This is kind of "definitional" - really depends on your definition of life, but I don't include viruses, because they don't have a regulated metabolism.
