Achtung! Effective this afternoon "until further notice", the TXL bus no longer runs to Alexanderplatz. They seem to be terminating it at Brandenburg Gate, in that the announcements (in German only that I have seen) for each of the destinations between there and Alexanderplatz all suggest taking the 100 bus to Brandenburg Gate.

If you have to change anyway, it is likely faster now to take the two connections route via U and S bahn to pick up the TXL a few stops away from the airport. This info has been posted here in FT recently.

I checked because I am currently in Alexanderplatz and was planning to take TXL on Saturday.