This paper gives an elementary and self-contained proof of Conway's Ba
sic Theorem on rational tangles. This theorem states that two rational
tangles are topologically equivalent if and only if they have the sam
e associated rational fraction. Our proof divides into a geometric hal
f that relates the arithmetic of continued fractions to the topology o
f tangles and an algebraic part that defines the fraction of any tangl
e via the bracket model far the Jones polynomial. We present an applic
ation to molecular biology. (C) 1997 Academic Press.