Mitosis recursion for coefficients of Schubert polynomials

Mitosis is a rule for manipulating subsets of the n by n grid. It provides an algorithm that lists the rc-graphs for a permutation w in S_n by downward induction on weak Bruhat order, thereby generating the coefficients of Schubert polynomials inductively. This note provides a short and purely combinatorial proof of these properties of mitosis.