Viruses are a special little topic indeed. Here we are talking about life in one of its most bizarre, misunderstood and disturbing expressions. Viruses are the stuff of horror movies.
They are tiny microscopic entities that have absolutely no activity whatsoever. In the presence of a larger organism of their specific fit (and there are viruses to target anything), they come alive by hijacking its life tools: nutrients, energy, ribosomes, you name it.
They only carry the genetic information they need to invade and replicate. Invade and replicate. A bit of a glitch of life, or the perfect expression of it?
Both DNA and RNA viruses make use of their host’s transcription and translation machinery such as ribosomes and enzymes to enable protein synthesis. Retroviruses on the other hand bring their own reverse transcriptase enzyme to enable the production of DNA using their RNA template once inside the host cell. Once the RNA is reverse transcribed into DNA (DNA->RNA is transcription, hence RNA->DNA is reverse transcription), the normal protein synthesis pathway can take place.
Bacteriophages invade their host (bacteria) and start multiplying. This disturbs the bacterium and makes it lyse, killing it.
HIV and AIDS
The human immunodeficiency virus (HIV) causes acquired immune deficiency syndrome (AIDS). Upon infection, it replicates in helper T cells. HIV is a lentivirus with the expected viral components such as its genetic material (RNA in this case) and capsid.
Upon infection via transmission of infected bodily fluids such as blood or semen, the virus seeks its host cell, the helper T cell which is a key component of the immune response. As HIV hijacks the cell’s machinery to replicate, it destroys it and thus impedes the immune response, leaving the victim with a compromised immunity and therefore susceptible to opportunistic infections.
Part of the HIV replication strategy is the embedding of its genetic material into the host cell’s DNA. The virus contains RNA which has to first be converted into DNA. This is what the reverse transcriptase enzyme does (transcription is DNA to RNA, so reverse transcription is RNA to DNA).
Some drugs for AIDS take advantage of this DNA embedding step by preventing it, and therefore slowing down the replication process of the virus. The reason antibiotics don’t work on any viruses is quite simple. We’ve seen that prokaryotes like bacteria are a cell of their own with many different structures and organelles such as a cell wall, while viruses are neither a cell nor alive. You can imagine you can’t really target something as vague as a piece of genetic information in a protein capsid.
Bacteria have complex cell walls with many components and various enzymes building them up or performing other metabolic functions within the cell. Any of these parts or processes can be targeted, as long as it is distinct from its host (that is ourselves with our eukaryotic cells that do not, for example, have cell walls and thus won’t be harmed by the antibiotic if that’s the part it targets). Viruses have no cell walls, no ribosomes, nothing really taking place.