RNA-dependent DNA polymerase, or reverse transcriptase, is an enzyme that generates DNA by reading an RNA template. This process, called reverse transcription, reverses the typical flow of genetic information from DNA to RNA. By creating DNA from an RNA blueprint, this enzyme has a unique function in molecular biology.
The Mechanism of Reverse Transcription
The process begins with the reverse transcriptase enzyme binding to an RNA molecule, which serves as the template. A short nucleic acid sequence, called a primer, is required to initiate synthesis, as polymerases cannot build a new strand from scratch. In many viruses, this primer is a transfer RNA (tRNA) captured from the host cell.
Once the primer is in place, the enzyme assembles a new DNA strand complementary to the RNA sequence. It adds deoxynucleoside triphosphates (dNTPs), the building blocks of DNA, one by one. This results in a hybrid molecule where one strand is the original RNA and the new strand is DNA.
After the first DNA strand is synthesized, the original RNA template is removed. The enzyme has a secondary function, RNase H activity, that degrades the RNA portion of the hybrid. With the RNA template gone, the new single strand of DNA, called complementary DNA (cDNA), is used as a template to create a second DNA strand. This process results in a stable, double-stranded DNA molecule.
Natural Roles in Biology
RNA-dependent DNA polymerase is used by retroviruses, such as the Human Immunodeficiency Virus (HIV). When HIV infects a human cell, it injects its RNA genome and the reverse transcriptase enzyme. The enzyme converts the viral RNA into double-stranded DNA, which is then inserted into the host cell’s DNA. The host cell’s machinery is then used to produce new viral components, assembling and releasing new virus particles.
The enzyme also functions in eukaryotes, including humans. One role is maintaining chromosome ends, known as telomeres, which are protective caps that shorten with each cell division. An enzyme called telomerase, which contains a reverse transcriptase component, adds repetitive DNA sequences to the chromosome ends. This process prevents the loss of genetic information during replication.
The enzyme is also involved with mobile genetic elements called retrotransposons, or “jumping genes.” These DNA sequences move to new positions within a genome by first being transcribed into an RNA intermediate. A reverse transcriptase then uses this RNA as a template to create a new DNA copy of the element, which is inserted into a new genomic location.
Applications in Science and Medicine
RNA-dependent DNA polymerase has several applications in research and medicine. Its most widespread use is in a technique called Reverse Transcription Polymerase Chain Reaction (RT-PCR), which detects and measures specific RNA molecules. In this method, reverse transcriptase converts RNA from a sample into more stable cDNA. This cDNA is then amplified using standard PCR, allowing researchers to detect minute quantities of RNA, like the genetic material from RNA viruses.
RT-PCR has significant diagnostic applications in public health. It is the standard for diagnosing infections caused by RNA viruses, including influenza, SARS-CoV-2, and HIV. By detecting viral RNA, the test identifies an active infection. The technique’s sensitivity allows for early detection, which is important for treatment and preventing viral transmission.
The enzyme is a target for antiviral drugs because it is used by retroviruses like HIV but is not required for most human cell processes. A major class of these drugs is the nucleoside reverse-transcriptase inhibitors (NRTIs). These drugs are molecular mimics of natural dNTPs. When the enzyme incorporates an NRTI into the growing DNA chain, it terminates synthesis and halts viral replication.