[1] https://en.wikipedia.org/wiki/RNA_world [2] https://en.wikipedia.org/wiki/Panspermia
The definition of life itself is not really obvious to me either, some sources count viruses as life other don't.
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.
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.