This lesson is about reverse transcriptase, a protein that copies RNA to DNA inside cells.
It defies the idea that DNA is always copied to RNA inside cells. This rebel protein is responsible for the current epidemic of drug resistant HIV.
Definition of Reverse Transcriptase
Normally, DNA is transcribed, or copied, to RNA and then translated to protein.
Reverse transcriptase copies RNA back to DNA.
Think of it as a car moving down a one-way street. Normally traffic only moves in one direction. But then, someone from out of town is driving the opposite way! Reverse transcriptase drives the opposite way in molecular processes in cells, converting RNA back to DNA.Although it is very different from the normal process, reverse transcriptase is an important enzyme. It is needed for function in viruses, eukaryotes and prokaryotes.
Reverse transcriptase enzymes in cells are involved in genetic diversity and in the process of aging in eukaryotic cells. In viruses, reverse transcriptase allows the virus to insert its DNA to the host cell’s DNA, forcing the cell to make more viruses. This is good for the virus but bad for the host.
Reverse Transcriptase in Cells
Even though reverse transcriptase occurs in viruses, our cells have reverse transcriptase enzymes that are helpful and even essential to our well-being! An enzyme called telomerase is an important reverse transcriptase in our body.
It helps to prevent our chromosomes from breaking down over time and controls aging in cells. It’s like a handyman in the house, preventing our appliances or the structure of the house from wear and tear over time. In the image, telomerase is shown copying RNA to DNA to add on to the chromosomes in the cell to prevent them from wearing down.
Both eukaryotic and prokaryotic cells have retrotransposons. These are pieces of DNA in the genome that are converted to RNA through a reverse transcriptase and then back to DNA and eventually inserted somewhere else in the genome.
It’s like cutting and pasting pieces of a sentence into a different order. This process can change the message of DNA, much like cutting and pasting parts of a sentence would change the meaning. As long as the cell controls the process correctly, this can be important for genetic diversity in organisms.
Reverse Transcriptase in Viruses
Viruses also use reverse transcriptase to survive. Viruses called retroviruses have an RNA genome and convert RNA back to DNA before hijacking the cell. There are several viruses that use reverse transcriptase, such as Human T-lymphotropic virus (HTVL) type 1 and 2 and human immunodeficiency virus (HIV). HTLV 1 may cause leukemia, a cancer of the white blood cells, in some infected patients by mutating the white blood cell DNA.
HTVL 2 has been associated with brain disorders, such as neurodegeneration, where parts of the brain and spinal chord break down.HIV is the most well known example of retroviruses. When HIV infects a cell, it brings with it a genome made of RNA, not DNA. The RNA enters the cell, and the reverse transcriptase copies the viral RNA back to DNA. The viral DNA is cut and paste into the host cell DNA. The host cell gets tricked into making lots of proteins for the virus.
It doesn’t notice the instructions have changed and keeps making lots of new viruses! Although this is bad for the host, it is great for the virus, since the goal of a virus is to survive and make more viruses.
Structure of Reverse Transcriptase
The structure of reverse transcriptase allows it to have the functions that viruses and cells need to survive. Cellular reverse transcriptases, like telomerase, are structurally similar to DNA polymerase, which copies DNA during DNA replication. Both enzymes have places to read the template, add nucleotides, cut apart the new strand using a nuclease, and check for errors. The difference is that DNA polymerase reads DNA and adds DNA nucleotides, and reverse transcriptase reads RNA and adds DNA nucleotides.
Telomerase in cells has a high fidelity rate, meaning it doesn’t make many errors. However, viral reverse transcriptase does make many errors during copying. Telomerase is like a student fluent in Spanish.
They can copy notes from Spanish to English with very few mistakes. However, a student new to Spanish might make a lot of mistakes copying notes. This is how viral reverse transcriptase operates, making many mistakes during copying RNA to DNA. Although it has the polymerase part to copy the RNA, add DNA nucleotides, and the nuclease, there is no proofreading section of the enzyme.