DNA model
First we set two parameter functions of helix curves.
Since we know DNA is like a bond with supercoil, the two helix may not spin in mid-distance.
So, we definite it with a little difference.
That is, move one upper (or oppose) to make it a bit released
Then, draw down the basic curves.
1. cos(s+π), sin(s+π), s
2. cos(r), sin(r), r-1
3. a dot(x) on 1+ half vector to end point(x) 2, another relative dot(y) on 1+ half vector to end point(y) 2, [ t ] ( [] is floor function)
4. like 3 but inverse vector
3. & 4. are function of bases
then adapt the parameters