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.
Lytic cycle and latency
As if the horror movie wasn’t bad enough as it is, it now has a truly creepy plot twist. Not only does the viruses invade, replicate and kill the host cell, making it burst (lyse) hence the lytic cycle, but it can invade and just lie quietly inside the host cell’s genetic material until a later time. This is latency also known as the lysogenic cycle.
Instead of expressing the virus genes to create more viruses, the genetic material of the virus simply incorporates itself into the host DNA. With it, it replicates with each cell division and spreads until a later time when a genetic separation event (such as a recombination event resulting in excision of the viral DNA) allows the viral DNA, termed prophage because it precedes the virus (phages are viruses that target bacteria) to split from the main host genetic material and activate its lytic cycle all over again.
This results in the synthesis of viral components including viral proteins and viral genetic material, and their assembly (which can be spontaneous) into new viruses. The host cell is compromised and the viruses are free to spread again.
Pathogenicity refers to the qualitative property of pathogens of causing disease, inherently e.g. by their behaviour upon infection. The viral reproduction mode is central to how much the virus can reproduce and spread to host tissues to cause damage. A virus entering the lysogenic cycle upon infection will not raise any symptoms and lurk unnoticed until a later time, if ever. Once the lytic cycle begins, an active and potentially aggressive process of replication occurs. Depending on other factors e.g. viral proteins, pathogenicity is determined, and the symptoms and course of disease are unveiled.