This tutorial will teach you how make a double helix using Illustrator, it’s another great example of using the pathfinder.
1. Turn on the grid (view menu > grid) and turn on snap to grid (view menu > snap to grid). Switch to the default color by typing D on the keyboard or clicking the default colors icon at the bottom of the tool bar and then change the fill color to none (red diagonal stripe).
2. In the upper left corner of a larger grid square, drag out a curve point horizontally to the right 4 small grid squares, go to the bottom right of the same larger grid square and drag out a curve point again horizontally to the right 4 smaller grid squares. Continue the line and drag out a third curve point at the upper right corner of the adjacent larger grid square to the right.
3. Click on the selection tool (black arrow) and select the curve you have just drawn and hold the option (alt on windows) key down in order to copy the curve. Drag the copy so the left and right hand sides meet up. Drag out a third copy and line it up as well.
4. Join the points where the copies meet, draw a small rectangular selection over one meeting point with the direct selection tool, go to the object menu > path > join and choose smooth as the join type. do the same for the other meeting point.
5. Give the lines a new stroke weight. Select both lines with the black arrow and enter 20pt in the stroke palette.
Then go to the object menu > path > outline stroke to make shapes out of the lines.
6. Mirror the line horizontally by selecting with the black arrow and double click on the reflect transformation tool. Click on the radio button for horizontal and click COPY.
7. Select both shapes and make them the default colors. Then click on the divide icon on the pathfinder palette.
8. Select the newly divided shape and go to the object menu > ungroup to make them into separate pieces. Go back to the object menu > path > cleanup (this gets rid of the invisible shapes inside the curves) With the selection tool, starting at the upper left hand side of the shape, select the upper curve, the intersecting area and the lower curve in the adjacent larger grid square (do this while holding down the shift key), then click the Add to Shape area icon on the pathfinder palette (while holding down the Option/Alt key)
Continue along the shape until the lines appear to be interweaving.