Character Offset to Randomize Specific Pairs of Letters

If you own iExpressions, you can use the Random Letter Wipe iExpression, which contains a customizable alphabet.

You have to create a source text expression that's pulling these characters e.g. from an array. Not sure why you would want double letters, though. DNA base pairs are always specific and the same base type can't pair. Your second callout would merely be a random arrangement of ATs and GCs. anyway, here's something you might use:

mPairs=[AC,GT,AC,AC,GT];

mChange=3;

mSeed=Math.floor(time/framesToTime(mChange));

seedRandom(index+mSeed)

mIndex=Math.round(random(0,mPairs.length);

mPairs[mIndex]

Modify as you need like tying it to a slider or such...

Mylenium