Position rack in lower third of oven.
Combine flour, baking powder, baking soda, and salt in bowl. Gradually add remaining 1 cup sugar and beat at high speed abt. Spoon about 3 quarters of the white batter evenly into bottom of tube pan. Spoon on dollops of chocolate batter, then small dollops of white batter over the chocolate. Draw small knife thru batters in zigzag motion to marbleize.