Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
Schematic representation of a helical tunnel. (A) Left-handed helix; (B) right-handed helix. The long arrow indicates the axis of the helix: a, direction of rotation; b, helical pitch; c, diameter of the helix; and d, length of the helix. The dark circle indicates the position of the cell.
