Section A (beat) 0:00-0:31
Section A (horn) 0:31-1:02
Section A (vocals) 1:02-1:31
Coda (independent) 1:31-2:15
Comments: This even beats out  "Love Me Do" as the simplest formal structure of any Beatles song. There is only one section (even "Love Me Do" had two!), which is repeated twice (for a total of three iterations). The chord progressions are identical in each: a 12 bar blues. The biggest difference is the instrumentation: first it's heard just with rhythm section, then with a horn (or, more precisely, a mellotron set to sound like a horn), and finally with vocals.
For good measure, a coda (musically independent from the rest of the song) is tacked on at the very end.