An inquiry for “vireses” is often a misspelling for viruses, which are microscopic infectious particles. To multiply, a virus must infect a living host cell and reprogram its functions to produce more viral particles. This dependence on a host for replication is a defining characteristic of all viruses.
Viral Structure and Composition
Every virus contains genetic material that serves as its blueprint. This material is either DNA or RNA, but never both, and can be single-stranded or double-stranded. The genome holds the instructions for the virus to replicate inside a host cell. Some simple viruses encode as few as four proteins, while more complex ones can encode over a hundred.
The genetic material is surrounded by a protective protein shell called a capsid, built from smaller protein subunits called capsomeres. This structure shields the nucleic acid from damage when the virus is outside a host cell. The shape of the capsid, which can be helical or icosahedral, is a primary feature used for classifying different types of viruses.
Some viruses possess an additional outer layer called an envelope, a lipid membrane acquired from the host cell as it exits. Embedded within this envelope are viral proteins that can appear as spikes on the virus’s surface. These proteins are important for the virus to recognize and attach to new host cells. Viruses with this layer are called enveloped viruses, while those with only a capsid are known as naked viruses.
The Viral Replication Process
The function of a virus is to replicate, a process that relies on the machinery of a host cell. This cycle begins with attachment, where the virus binds to specific receptor molecules on a host cell’s surface. This highly specific interaction, known as tropism, determines which types of cells a particular virus can infect.
Following attachment, the virus must get its genetic material inside the host cell, a stage called entry or penetration. Some viruses fuse their envelope with the host’s membrane, releasing their contents directly into the cell. Others may be taken into the cell through endocytosis, where the cell membrane engulfs the viral particle. In some cases, only the viral genetic material is injected into the cell.
Once inside, the viral capsid is degraded in a process called uncoating, releasing the genetic material into the host cell’s cytoplasm. During the replication stage, the host cell’s enzymes and ribosomes are hijacked to copy the viral genome and produce viral proteins. The host cell ceases its normal functions, becoming a factory for viral components.
In the assembly stage, the newly synthesized viral genomes and proteins are put together. Protein capsids are formed, and the genetic material is packaged inside them, creating new, complete viral particles called virions. This process can occur in different parts of the host cell, like the nucleus for most DNA viruses or the cytoplasm for most RNA viruses.
The final step is release, where the newly assembled virions exit the host cell. This can happen through lysis, where the viruses burst out of the cell, destroying it. Alternatively, enveloped viruses are often released through budding, where they push through the cell membrane, taking a piece with them to form their envelope. This budding process does not always kill the host cell immediately.
Distinguishing Viruses from Bacteria
Viruses and bacteria differ in several fundamental ways.
- Structure: Bacteria are single-celled living organisms, possessing a cell wall and all the internal components, like ribosomes, needed to live and reproduce independently. Viruses are not cells. They are much simpler, consisting only of genetic material surrounded by a protein coat, and they lack the organelles necessary for self-sufficiency.
- Size: Viruses are significantly smaller than bacteria. While an average bacterium might be around 2 micrometers long, most viruses are between 0.02 and 0.4 micrometers, making them invisible under a standard light microscope.
- Reproduction: Bacteria reproduce on their own through a process called binary fission, where a single cell divides into two. Viruses are incapable of this self-replication and must invade a living cell and take over its machinery to produce new copies.
- Medical Treatment: Antibiotics are designed to target the specific cellular structures and metabolic pathways of bacteria, so they have no effect on them. Viral infections require antiviral medications, which work by interfering with the viral replication cycle, by blocking attachment or assembly.
Viral Impact on Host Organisms
The viral replication process often causes direct damage to the host organism. The release of new virus particles can cause the host cell to rupture and die (lysis). This cell death leads to many symptoms of viral diseases, such as the tissue damage in the respiratory tract caused by the influenza virus, leading to coughing and a sore throat.
Some viruses, however, can infect an organism without immediately causing harm. Temperate phages, for example, can integrate their genetic material into a host bacterium’s genome and remain dormant, a state known as lysogeny. This allows the viral DNA to be copied and passed on each time the bacterium divides, without producing new virions.
Not all viruses are detrimental to human health. A group of viruses called bacteriophages specifically infect and kill bacteria. These phages play a role in regulating bacterial populations in various ecosystems, including the human gut. There is growing interest in using bacteriophages for therapeutic purposes, as a potential alternative to antibiotics for treating bacterial infections.