Reverse transcription is the synthesis of DNA using RNA as the template.

The enzyme that catalyzes this reaction is a reverse transcriptase, an RNA-dependent DNA polymerase. The name “reverse” refers to the direction of information transfer relative to ordinary transcription:

ordinary transcription:  DNA → RNA
reverse transcription:   RNA → DNA

The chemistry of strand growth still follows the same polymerase rule: the new DNA strand is extended by adding nucleotides to a free $3'$ hydroxyl, so DNA synthesis proceeds

$$5'\rightarrow3'.$$

Template complementarity still determines sequence

If an RNA template is

3'-A C G U U A-5'

a complementary DNA product is

5'-T G C A A T-3'.

Uracil in RNA pairs with adenine in the DNA product; the newly synthesized DNA itself uses thymine rather than uracil.

RNA can be converted into a double-stranded DNA copy

Many reverse-transcription systems first produce an RNA-DNA hybrid. Additional enzymatic steps can remove or degrade the RNA template and synthesize the complementary DNA strand, yielding double-stranded DNA.

A DNA copy synthesized from an RNA molecule is often called complementary DNA (cDNA).

Why reverse transcription matters

Reverse transcription appears in several biological contexts.

• Retrotransposons use an RNA intermediate to create new DNA copies that can insert elsewhere in a genome.
• Retroviruses copy their RNA genomes into DNA during infection.
• Telomerase contains a specialized reverse-transcriptase activity that copies a short internal RNA template to extend chromosome ends.
• In molecular biology, reverse transcriptase is used experimentally to convert RNA populations into cDNA for analysis.

Reverse transcription therefore does not violate template-directed polymerization. It changes the kind of template being read—from DNA to RNA—while preserving complementary base selection and $5'\rightarrow3'$ synthesis of the new strand.