Combining PROTOTYPE with onEnterFrame?

Can someone tell me how to cimbine the prototype with onEnterFrame. I’d like to fade in some objects like this:

[AS]

mc._alpha = 0
onEnterFrame = function () {
if (mc._alpha<101) {
mc._alpha = Math.round(mc._alpha+6)
trace(mc._alpha);
}
mc._y = Math.round((mc._y+12)*0.9)
}

[/AS]

or can it be done without onEnterframe with an interval or something like that?

hope someone can help me out