Forum:Do Mystical Space Typhoon/Dust Tornado negate the cards they destroy?

Title says all. If my opponent activates, say Tribute to the Doomed, can I activate Mystical Space Typhoon to negate it's effect? Also, if the effect is negated (by something that says "negate the effect") does my opponent still have to discard, since that's a cost? Sorry the first question sounds so dumb, my friends and I have always assumed that's what they do.