Defining a Virus
A virus is a microscopic infectious agent that can only replicate inside the living cells of an organism. Unlike bacteria, viruses are not considered true living organisms because they lack the machinery to carry out life processes, such as metabolism and reproduction, independently. They are obligate intracellular parasites, meaning they absolutely require a host cell to multiply.
Structure and Composition
The basic structure of a virus consists of genetic material, either DNA or RNA, encased in a protein shell called a capsid. Some viruses also have an outer lipid envelope, derived from the host cell membrane, which helps them evade the immune system and attach to new cells. Viruses are significantly smaller than bacteria, often requiring electron microscopes to be observed.
How Viruses Infect and Replicate
Viruses initiate infection by attaching to specific receptors on a host cell's surface and injecting their genetic material into the cell. Once inside, they hijack the host cell's machinery (ribosomes, enzymes, energy) to produce viral proteins and replicate their own genetic material. These newly assembled viral particles then burst out of the cell, often destroying it in the process, to infect more cells.
Impact and Importance in Biology
Viruses are significant in biology as agents of disease, causing illnesses ranging from the common cold to AIDS. However, they also play crucial roles in ecosystems, such as regulating bacterial populations and driving genetic diversity through horizontal gene transfer. In biotechnology, viruses are harnessed as tools in gene therapy and vaccine development due to their ability to deliver genetic material into cells.