Viruses

Viruses are different from either eukaryotes or prokaryotes because they are not considered living and are referred to as acellular because they are not formed of cells. Viruses are very simple particles composed of only a few components:

• A strand of genetic material (either DNA or RNA) that contains the genetic code to form new viral particles

• A capsid that is made of protein and protects the genetic material

• Attachment proteins that allow the virus to bind to a target cell

Some viruses also have additional proteins (e.g., vital enzymes for replication), a lipid envelope (derived from the membrane of the host cell), and a matrix (a junction between the capsid and lipid envelope). Viruses do not perform many of the life processes and rely on living cells for replication. To do this, viruses attach to the host cell using attachment proteins and inject their genetic material into the cell. The cell is ‘tricked’ into using the genetic material to synthesize new viruses that then burst out of the cell and destroy it. Different viruses infect cells of different organisms; some infect animals, and others infect plants, fungi, or bacteria.

The structure of a virus.

Replication

As viruses are non-living, they do not undergo cell division. Instead, after injecting their nucleic acid into a host cell, the host replicates the virus particles.

Human immunodeficiency virus (HIV)

The HIV virus has a lipid envelope embedded with attachment proteins. Inside the envelope is a protein layer called the capsid, which encloses two single strands of RNA and several enzymes. One of these enzymes is reverse transcriptase, which catalyses the production of DNA from RNA. This ability to produce DNA from RNA classifies HIV as a retrovirus.

HIV infects the body by attaching to CD4 proteins on the surface of helper T cells. Once bound, the virus’s lipid envelope fuses with the cell membrane, allowing the viral RNA and enzymes to enter the cell. Reverse transcriptase then converts the viral RNA into DNA, which integrates into the host cell's DNA. This viral DNA directs the production of new mRNA, which codes for new HIV proteins. The mRNA exits the nucleus through nuclear pores and utilizes the cell's protein synthesis machinery to produce the viral proteins needed for new virus particles. These new HIV particles exit the cell, taking with them a portion of the cell membrane to form their own lipid envelope.

AIDS (Acquired Immune Deficiency Syndrome) is caused by the HIV virus, which targets and impairs helper T cells. This damage prevents helper T cells from stimulating an effective immune response, making the body vulnerable to other infections and cancers. HIV itself does not cause death directly; instead, it leaves the immune system weakened, so individuals infected with HIV are unable to fight off opportunistic infections, which ultimately leads to death.

Why antibiotics are ineffective against virsuses

Antibiotics are designed to target bacterial structures or functions, such as cell walls or protein synthesis, which are not present in viruses. Viruses lack these cellular structures and instead hijack a host cell’s machinery to replicate. Therefore, antibiotics have no effect on viruses, including HIV, because they cannot disrupt any viral structures or processes directly.

Definitions