Create a DNA Helix icon. Sounds complex, but it is quite simple. This is take on ropes, knots and twists that had been posted a while back. Simple twists, over and under lapping of lines and you have a twist!
Start with a line. give a stroke weight of 35 px.
Select the line and add a Zig Zag effect from the Effect Menu> Distort & Transform> Zig Zag. Set the line to Smooth, Size to 50 pt to get a nice wave effect. Click OK.
This is the Zig Zag effect.
Go to the Object Menu and choose Expand Effect to convert this to an editable line.
Use the Direct Selection Tool to delete all but the portion of the line as shown.
Open the Appearance Panel. Click on the lower left corner icon to add an additional stroke to the existing stroke. Make the stroke BEHIND the blue stroke white, make it 60pt.
Select the line and RIGHT + CLICK and choose Transform> Reflect.
Reflect the shape and choose COPY.
This will duplicate and flip the shape.
Using the Appearance Panel set the top stroke to black.
Use the Direct Selection Tool to select a single portion of the line. Copy that portion of the line and paste it onto the end of the chain.
Send the line to the back or to the front to get the effect of the overlapping/ underlapping lines.
Repeat the copying of the lines.
Send the line to the back/ bring to the front to get the overlap.
Add lines and send them to the back to create connection lines in between the helix.