Design a musical composition incorporating the alphabetic expression of the chosen structure and the provided melodic theme.
X:1
L:1/8
M:6/8
K:G
['D G2 A_BAG d3 d2 c _B>AG']  
X:1
L:1/8
M:6/8
K:G
 D | G2 A_BAG | d3 d2 c | _B>AG F2 G | A2 z3 G | d2 d _B2 G | g3 _e2 c | _B>AGA>GF | G2 z3 |]  