On the politics website, AmericaBlog, there was a discussion about the resurgence in interest in the H1N1 swine flu. A question there inspired this post.
What are the differences between RNA and DNA?
Both RNA and DNA are nucleic acids, which store genetic information. DNA is long lived, stable, and is what everything but some viruses use for information storage. The D stands for deoxyribose, a sugar that is part of the backbone of the nucleic acid making DNA deoxyribonucleic acid. The R stands for ribose (ribonucleic acid), another sugar, very similar is structure, but differing slightly enough that it isn’t as stable. The difference between the two sugars is just the presence of an oxygen on ribose, where you would find a hydrogen on deoxyribose.
Everything but RNA viruses use RNA primarily to carry information from DNA storage to structures called ribosomes, which read the information and make proteins according to it. RNA viruses use RNA for storage and to code directly for proteins.
So what is a retrovirus?
It is virus that uses RNA to store its genetic material, but it does something different. Everything but retroviruses follows a pattern called the “Central Dogma.” DNA is transcribed (copied) to RNA. RNA is translated (decoded) to protein. Just about everything except for those RNA viruses replicates its genetic information at step A, including us. RNA viruses, to my knowledge are the only ones that replicate using step B. Never does this go backwards, or at least thats what we thought.
Retroviruses run this system backwards, so we call them retro, as in backwards. They carry an enzyme that can reverse the process of transcription, called reverse transcriptase, taking RNA and making DNA copy, at step C. This DNA copy is then inserted somewhere into the host’s DNA where it can begin producing RNA copies of itself. Some will be used to code for proteins and enzymes that make up the virus, while others will become new viral genomes. As part of this cell’s genome, when it divides, you now have two infected cells. Then four, eight, sixteen and so on.
Because it is in infected cells as part of their chromosomes, they are there forever. We can’t get rid of them. HIV, a retrovirus, infection is forever, but treatable, unless a new conceptual procedure can be repeated in more patients.
Since retroviruses become a permanent part of a cell and its descendants, if that cell is a germ line cell, meaning sperm or egg producing cells, the virus can be passed directly on to the offspring (HIV doesn’t infect germ line cells, so this isn’t one of the routes that children can become exposed to HIV). We call these endogenous retroviruses. If the virus isn’t something that kills the infected individual before they can reproduce, it can become part of a larger population. Over time, those viruses can become mutated to the point that they can’t produce new functional viruses. At that point, the virus is dead, and may become further mutated. If you look in the human genome, you will find six or seven of these dead viruses, silenced over hundreds of thousands of years of evolution.