(The one Issac blew up at the end of dead space 1) He was a senior scientist onboard the USG O'Bannon and was in charge of collecting shards of the red marker from Aegis 7. Nolans story was explained in an animated movie (which you should check if you haven't seen it yet).

Issac was locked away after the events of dead space 1 cause he came into contact with the marker (can't remember if it was the black or a red one he came in contact with) that's why at the beginning of dead space 2 the doctor asks if Isaac came into contact with it. They are most likely red because of the bismuth they used to build it. Red markers are man made recreations of the marker.

Their brains are able to withstand the 'voices' coming from the markers and are the ones who are destined (by the markers) to make more markers for more convergences. People with higher IQ's (like Issac and Nolan) are affected in a different way. I read somewhere (I believe it was one of the text files in one of the games) that people with lower IQ's don't have to be super close to the marker for it to turn them into necromorphs. The infection rate depends on the person.

Dead space 1 what is the marker