10: Viruses - Biology

A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.

  • 10.1: General Characteristics of Viruses
    Viruses are infectious agents with both living and nonliving characteristics. Living characteristics of viruses include the ability to reproduce – but only in living host cells – and the ability to mutate. Nonliving characteristics include the fact that they are not cells, have no cytoplasm or cellular organelles, and carry out no metabolism on their own and therefore must replicate using the host cell's metabolic machinery.
  • 10.2: Size and Shapes of Viruses
    Viruses are usually much smaller than bacteria with the vast majority being submicroscopic, generally ranging in size from 5 to 300 nanometers (nm). Helical viruses consist of nucleic acid surrounded by a hollow protein cylinder or capsid and possessing a helical structure. Polyhedral viruses consist of nucleic acid surrounded by a polyhedral (many-sided) shell or capsid, usually in the form of an icosahedron.
  • 10.3: Viral Structure
    Since viruses are not cells, they are structurally much simpler than bacteria. An intact infectious viral particle - or virion - consists of a genome, a capsid, and maybe an envelope. Viruses possess either DNA or RNA as their genome. The genome is typically surrounded by a protein shell called a capsid composed of protein subunits called capsomeres.
  • 10.4: Classification of Viruses
    Viruses can store their genetic information in six different types of nucleic acid which are named based on how that nucleic acid eventually becomes transcribed to the viral mRNA. (+) and (-) strands of nucleic acid are complementary. Copying a (+) stand gives a (-) strand; copying a (-) stand gives a (+) strand. Only (+) strands of viral RNA can be translated into viral protein. The "dependent" part of the name refers to the nucleic acid is being copied.
  • 10.5: Other Acellular Infectious Agents: Viroids and Prions
    Viroids are small, circular, single-stranded molecules of infectious RNA that cause several plant diseases. Prions are infectious protein particles responsible for a group of transmissible and/or inherited neurodegenerative diseases as a result of prion protein misfolding. Diseases including Creutzfeldt-Jakob disease Gerstmann-Straussler-syndrome, and mad cow disease.
  • 10.6: Animal Virus Life Cycles
    Viruses that infect animal cells replicate by what is called the productive life cycle. The productive life cycle is also often referred to as the lytic life cycle, even though not all viruses cause lysis of their host cell during their replication. Some viruses, such as HIV and the herpes viruses are able to become latent in certain cell types. A few viruses increase the risk of certain cancers. We will now look at the life cycles of viruses that infect animal cells.
    • 10.6A: The Productive Life Cycle of Animal Viruses
    • 10.6B: Productive Life Cycle with Possible Latency
    • 10.6C: The Life Cycle of HIV
    • 10.6D: Natural History of a Typical HIV Infection
    • 10.6E: The Role of Viruses in Tumor Production
  • 10.7: Bacteriophage Life Cycles: An Overview
    bacteriophages are viruses that only infect bacteria (also see Fig. 1C and Fig. 2E). There are two primary types of bacteriophages: lytic bacteriophages and temperate bacteriophages. Bacteriophages that replicate through the lytic life cycle are called lytic bacteriophages, and are so named because they lyse the host bacterium as a normal part of their life cycle. Bacteriophages capable of a lysogenic life cycle are termed temperate phages.
    • 10.7A: The Lytic Life Cycle of Bacteriophages
    • 10.7B: The Lysogenic Life Cycle of Bacteriophages
  • 10.8: Pathogenicity of Animal Viruses
    Alteration of host cell function and/or death of the host cell occurs as a result of viruses using an infected host cell as a factory for manufacturing viruses. The body’s immune defenses recognize infected host cells as foreign and destroy infected cells. The body’s adaptive immune defenses produce antibodies against viruses that block viral adsorption to host cells or result in opsonization of the virus.
  • 10.9: Bacteriophage-Induced Alterations of Bacteria
    Lytic bacteriophages usually cause the host bacterium to lyse. The added genetic information provided by the DNA of a prophage may enable a bacterium to possess new genetic traits. Some bacteria become virulent only when infected themselves with a specific temperate bacteriophage. The added genetic information of the prophage allows for coding of protein exotoxin or other virulence factors.
  • 10.10: Antiviral Agents
    Relatively few antiviral chemotherapeutic agents are currently available and they are only somewhat effective against just a few limited viruses. Many antiviral agents resemble normal DNA nucleosides molecules and work by inhibiting viral DNA synthesis. Some antiviral agents are protease inhibitors that bind to a viral protease and prevent it from cleaving the long polyprotein from polycistronic genes into proteins essential to viral structure and function.
  • 10.11: General Categories of Viral Infections
    Acute infections are of relatively short duration with rapid recovery. Persistent infections are where the viruses are continually present in the body. In a latent viral infection the virus remains in equilibrium with the host for long periods of time before symptoms again appear, but the actual viruses cannot be detected until reactivation of the disease occurs. In a chronic virus infection, the virus can be demonstrated in the body at all times and the disease may be present or absent.
  • 10.E: Viruses (Exercises)
    These are homework exercises to accompany Kaiser's "Microbiology" TextMap. Microbiology is the study of microorganisms, which are defined as any microscopic organism that comprises either a single cell (unicellular), cell clusters or no cell at all (acellular). This includes eukaryotes, such as fungi and protists, and prokaryotes. Viruses and prions, though not strictly classed as living organisms, are also studied.

Ten cool facts about viruses

1. Some parasitic wasps lay eggs in caterpillars, where they mature into adult wasps. The wasp eggs contain a virus, encoded in the wasp genome, which prevents the caterpillar from rejecting the eggs.

2. There are a million virus particles per milliliter of seawater – for a global total of 10 30 virions! Lined up end to end, they would stretch 200 million light years into space.

3. The genetic information of viruses can be DNA or RNA single or double stranded one molecule or in pieces.

4. The name virus was coined from the Latin word meaning slimy liquid or poison.

5. Walter Reed discovered the first human virus, yellow fever virus, in 1901.

6. Viruses are not alive – they are inanimate complex organic matter. They lack any form of energy, carbon metabolism, and cannot replicate or evolve. Viruses are reproduced and evolve only within cells.

7. Over 10 16 human immunodeficiency virus genomes are produced daily on the entire planet. As a consequence, thousands of viral mutants arise by chance every day that are resistant to every combination of antiviral compounds in use or in development.

8. The first human influenza virus was isolated in 1933. In 2005, the 1918 pandemic influenza virus strain was constructed from nucleic acid sequence obtained from victims of the disease.

9. The biggest known viruses are mimiviruses, which are 400 nanometers (0.0004 millimeters) in diameter. The viral genome is 1,200,000 nucleotides in length and codes for over 900 proteins.

10. The smallest known viruses are circoviruses, which are 20 nanometers (0.00002 millimeters) in diameter. The viral genome is 1,700 nucleotides in length and codes for two proteins.

Bonus fact: The HIV-1 genome, which is about 10,000 nucleotides long, can exist as 10 6020 different sequences. To put this number in perspective, there are 10 11 stars in the Milky Way galaxy and 10 80 protons in the universe.

I made up this list a few weeks ago in response to a request from a journalist. The final version, shortened and re-ordered by an editor, was published online at ColumbiaNews.

Types of Viruses and How They Work

Over many centuries and even millennia, infectious diseases such as smallpox and measles have claimed millions of lives. Advances in modern medicine have helped to stop the spread of many viral infections through mass vaccination, and some infections have been completely eradicated.

The Cost of Deadly Virus Infections

Viruses gain their infamy through a combination of large infection rates and death, even when their power has been relatively subdued. Rabies, measles and chickenpox are still notorious, even though vaccines and medications have drastically reduced their deadliness. Some viruses are either no longer a threat or not thought of as threatening: Smallpox has been eradicated, and there hasn’t been a polio case of American origin since 1979.

Other viruses are still active and pose a serious threat to an individual’s health. In addition to coronavirus (COVID-19), hepatitis, HIV and the flu still cause mass infection and noticeable death rates.

The Three Categories of Viruses

There are three different virus types that are made distinct by their shape. The cylindrical helical virus type is associated with the tobacco mosaic virus. Envelope viruses, such as influenza and HIV come covered in a protective lipid envelope. Most animal viruses are classified as icosahedral and are nearly spherical in shape.

The viruses within these categories share similar characteristics. They’re composed of RNA or DNA, and are coated with either a protein, lipid or glycoprotein. They’re also parasitic, meaning they can’t replicate without a host. Viruses are also the most abundant biological form of life on the planet. While they can’t be cured, a vaccination can prevent their spread.

Watch the video: A-Level Biology - 10 things you have to know about viruses (January 2022).