Preheat oven to 325 degrees F.
For the crust: In a mixing bowl, stir together the graham cracker crumbs and sugar until combined. Then, stir in the melted butter. Press the mixture firmly into a 9-inch pie plate and set aside.
For the filling: In a medium bowl, mix the condensed milk, lemon juice, zest and egg yolks, reserving the whites for the meringue. Pour the mixture into the crust.
For the meringue: Beat the egg whites and cream of tartar in a medium bowl with an electric mixer until soft peaks form, then add the sugar and whip until peaks are stiff.
Spread the meringue over the pie and seal to the edge of the crust. Bake until the meringue browns slightly, 15 to 20 minutes. Chill for at least 2 hours before serving.
Recipe adapted from Home Cooking with Trisha Yearwood (c) Clarkson Potter 2010