A method of dually labeling a nucleic acid polymer in a cell, comprising: (i) providing a nucleic acid polymer comprising at least one first nucleotide analogue that comprises, carried by the nucleotide base thereof, a first reactive unsaturated group and at least one second nucleotide analogue that comprises, carried by the nucleotide base thereof, a second reactive unsaturated group, wherein the nucleic acid polymer is inside the cell, and wherein the at least one first nucleotide analogue and at least one second nucleotide analogue are incorporated into the nucleic acid polymer during DNA replication; and (ii) contacting the nucleic acid polymer with a first reagent comprising a third reactive unsaturated group attached to a first label, such that a [3+2] cycloaddition occurs between the first and third reactive unsaturated groups, and (iii) contacting the nucleic acid polymer with a second reagent comprising a fourth reactive unsaturated group attached to a second label, such that a [3+2] cycloaddition occurs between the second and fourth reactive unsaturated groups, wherein either: a) the first reactive unsaturated group comprises an azide group and the third reactive unsaturated group comprises an alkynyl group, and the second reactive unsaturated group comprises an alkynyl group and the fourth reactive unsaturated group comprises an azide group; or b) the first reactive unsaturated group comprises an alkynyl group and the third reactive unsaturated group comprises an azide group, and the second reactive unsaturated group comprises an azide group and the fourth reactive unsaturated group comprises an alkynyl group; wherein the method is performed in vitro. (i) providing a nucleic acid polymer comprising at least one first nucleotide analogue that comprises, carried by the nucleotide base thereof, a first reactive unsaturated group and at least one second nucleotide analogue that comprises, carried by the nucleotide base thereof, a second reactive unsaturated group, wherein the nucleic acid polymer is inside the cell, and wherein the at least one first nucleotide analogue and at least one second nucleotide analogue are incorporated into the nucleic acid polymer during DNA replication; and (ii) contacting the nucleic acid polymer with a first reagent comprising a third reactive unsaturated group attached to a first label, such that a [3+2] cycloaddition occurs between the first and third reactive unsaturated groups, and (iii) contacting the nucleic acid polymer with a second reagent comprising a fourth reactive unsaturated group attached to a second label, such that a [3+2] cycloaddition occurs between the second and fourth reactive unsaturated groups, wherein either: a) the first reactive unsaturated group comprises an azide group and the third reactive unsaturated group comprises an alkynyl group, and the second reactive unsaturated group comprises an alkynyl group and the fourth reactive unsaturated group comprises an azide group; or b) the first reactive unsaturated group comprises an alkynyl group and the third reactive unsaturated group comprises an azide group, and the second reactive unsaturated group comprises an azide group and the fourth reactive unsaturated group comprises an alkynyl group; wherein the method is performed in vitro. a) the first reactive unsaturated group comprises an azide group and the third reactive unsaturated group comprises an alkynyl group, and the second reactive unsaturated group comprises an alkynyl group and the fourth reactive unsaturated group comprises an azide group; or b) the first reactive unsaturated group comprises an alkynyl group and the third reactive unsaturated group comprises an azide group, and the second reactive unsaturated group comprises an azide group and the fourth reactive unsaturated group comprises an alkynyl group;