A bacteriophage is a virus which attacks bacteria and consists of a head, tail, and tail fibers all made of protein. The head contains genetic material (usually linear DNA) which is injected into the host bacterium upon infection. The protein parts do not enter the bacterium, and it does not have any ribosomes or organelles of its own.

The three main media of attenuating a virus are infecting birds, horses, or cultured cell lines. West Nile Virus is an interesting virus in that host and susceptible organisms happen to be birds, horses, and humans. This virus also does not seem to replicate in other animal reservoirs such as lizards. It does not replicate in mosquitos. Moreover, recent attempts to culture a neuronal cell line to replicate the virus have been unsuccessful due to the extended length of life cycle of neuronal cells.

Retroviruses employ the enzyme reverse transcriptase to convert their RAN genomes into DNA.

Reverse transcriptase binds RNA and catalyzes the formation of a complementary DNA molecule. Since it needs an RNA template to work, this enzyme is RNA dependent. Since it forms a DNA molecule, this enzyme is a DNA polymerase. Recall the chemical difference between DNA and RNA - DNA lacks a hydroxide group at the 3' position of the ribose sugar, which makes it deoxyribose.

The lysogenic cycle occurs when a virus incorporates its DNA into the host genome. It stays inside the host genome, getting replicated along with the host until conditions are right when it excises itself and beings the process of hijacking the cell to make more virus. Both retroviruses and bacteriophages can undergo this process.

Prions are proteins that are folded in alternate conformations. The molecules can then produce a disease state by catalyzing the misfolding of additional proteins in the cell. Note that vitamins are not one of the four classical biological macromolecules.

The nucleic acids that make up viral genomes are very diverse. Viral genome can consist of both double and single stranded DNA as well as double and single stranded RNA.

Reverse transcriptase is a viral enzyme used by some viruses to transcribe double-stranded DNA from their single-stranded RNA, which is the exact opposite of the normal transcription process of the cell. The double-stranded viral DNA is then able to integrate into the double-stranded DNA of the infected cell’s genome so that whenever the cell divides, all of its daughter cells will carry the viral DNA. This viral DNA can eventually be transcribed by the cell’s own enzymes to produce viral RNA. Some of the viral RNA will be translated into viral proteins, and new viruses will be assembled from these products.

In the lysogenic cycle, a virus replicates its genome but does not kill the host. The host lives and reproduces normally, and the daughter cells can inherit the prophage (the bacterial circular chromosome and the bacteriophage genome)

Viruses are unique organisms in that they have a relatively simple cell structure. They have an outer protein coat, which sets them apart from other organisms. Peptidoglycan is found in bacteria, lipid bilayer is found in eukaryotic cells, and chitin is found in fungi. Membrane-bound orangelles are found in more complex cells, such as eukaryotic cells, and are not unique to viruses.