Total revenue is the running sum of marginal revenue (since $MR$ is the addition to $TR$ from each unit and $TR = 0$ at $q = 0$); the demand curve is $AR = TR/q$; and elasticity follows from $e = \dfrac{AR}{AR - MR}$.

Step 1 — Total revenue. Marginal revenue is the change in total revenue from selling one more unit, and $TR = 0$ at $q = 0$, so $TR$ at any quantity is the cumulative sum of the $MR$ values up to that quantity.

Step 2 — Demand curve. The demand curve is the average revenue, $AR = \dfrac{TR}{q}$.

Step 3 — Price elasticity. Using the monopoly relation $MR = AR\left(1 - \dfrac{1}{e}\right)$, we get $e = \dfrac{AR}{AR - MR}$.

The teaching point is clear: while $MR$ is positive demand is elastic ($e > 1$); where $MR = 0$ demand is unitary elastic ($e = 1$); and once $MR$ turns negative demand becomes inelastic ($e < 1$).