Viruses come in a variety of shapes and sizes, but they are extremely small and can only be seen with an electron microscope. They are found almost everywhere, and all types of cells can be infected by viruses. A typical virus consists of genetic material—either DNA or RNA—surrounded by a protective protein coat called a capsid. Some viruses also have an outer lipid envelope.

Unlike living cells, viruses are not considered fully living organisms because they cannot carry out life processes on their own. They can only reproduce by infecting a host cell. Once inside, the virus takes over the cell’s machinery to make copies of itself. This process often damages or destroys the host cell, leading to symptoms associated with viral infections.  Because they rely entirely on host cells for reproduction, viruses are completely dependent on living organisms for survival. Some viruses are highly specific in what they infect. For example, bacteriophages are viruses that infect and reproduce within bacteria.

There are two main patterns of viral infection: the lytic cycle and the lysogenic cycle.  In a lytic infection, a virus immediately takes control of a host cell to produce new viruses. First, a bacteriophage attaches to a bacterial cell and injects its DNA. The viral DNA then directs the cell to produce viral proteins and nucleic acids. These parts are assembled into new virus particles. Finally, the host cell bursts, or lyses, releasing many new viruses that can go on to infect other cells. In simple terms, the virus enters the cell, replicates, and causes the cell to burst.

In a lysogenic infection, the process is different. After the bacteriophage injects its DNA, the viral DNA forms a circle and inserts itself into the bacterial chromosome. This embedded viral DNA is called a prophage. Instead of immediately destroying the cell, the prophage is copied along with the host cell’s DNA as the cell reproduces. This means the viral DNA can be passed on for many generations without harming the host.  However, certain environmental factors—such as radiation, heat, or chemicals—can activate the prophage. When this happens, it exits the bacterial chromosome and enters the lytic cycle, where it begins producing new viruses and eventually causes the cell to lyse.

Many viruses contain RNA instead of DNA as their genetic material. RNA viruses are responsible for a wide range of diseases, from the common cold to more serious illnesses such as certain cancers and HIV. AIDS (acquired immunodeficiency syndrome) is caused by an RNA virus called HIV (Human Immunodeficiency Virus).

HIV belongs to a group of viruses known as retroviruses. Retroviruses contain RNA and have the unique ability to produce a DNA copy of their RNA after infecting a host cell. This DNA can then integrate into the host cell’s genetic material, allowing the virus to replicate along with the cell.

Although viruses are not considered living organisms, they do share some characteristics of living things, such as having genetic material and the ability to reproduce, though only inside a host cell.

Review:

1. How do viruses reproduce?
2. Compare a lytic infection to a lysogenic infection.
3. What is a retrovirus?