This tutorial shows how to draw a seamless pattern in illustrator. The fence is drawn as an example. Once you create the pattern put it in your swatches panel.
How to draw a fence in illustrator | seamless pattern design
how to draw DNA helix in illustrator | create twisted gradient | gradient pattern brush
This tutorial shows how to create a gradient pattern brush. The video also shows how to create double helix using this brush.
How to create rope vector in adobe illustrator
twisted pair wire design in illustrator.