The point of contact of a strand of supercoiled DNA where it loops back on itself. A node, and the writhe of the DNA, may be positive or negative; it is positive if, when viewed so that the strand enters the node from below and leaves from above, and the shortest turn from strand entry and strand exit is clockwise. Negative nodes are formed from negatively supercoiled DNA. A knot in the DNA is seen when three nodes all of the same sense, e.g. all negative, occur together; this is seen in electron micrographs as a trefoil structure, a knot from which extend three DNA loops.