e. The letters are oriented such that if you read an assembled sequence from left to right, you'll be reading from the 5' end to the 3' end of the strand, which is the convention for reporting DNA sequences. 

The OpenSCAD file is included in case you want to see how I modeled this or make changes. It also has an assembly function that allows you to specify a sequence and it will display the corresponding single helix. You'll have to make a complementary sequence as well to see the full double-helix.

These pieces print out so quickly that it doesn't take long to make a good strand (I can do about 30 base pairs in an hour and a half). However, be prepared to wait awhile if you want to print out all 2.9 billion base pairs of the human genome.