General Properties of Viruses: Structure, Origin, and Life Cycle

Study notes August 10, 2025

 

General Properties of Viruses – Structure, Types, and Life Cycle

Viruses are tiny infectious agents that can only multiply inside the living cells of other organisms. They can infect animals, plants, fungi, bacteria, and archaea. Despite their microscopic size, viruses are among the most abundant biological entities on Earth, present in virtually every ecosystem.

Since the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898, scientists have identified around 5,000 virus types, although millions more likely exist. The scientific study of viruses is known as virology, a branch of microbiology.

What Are Viruses?

Viruses exist as independent particles known as virions when they are outside host cells. A virion typically consists of:

  1. Genetic material – DNA or RNA, carrying the virus’s genetic instructions.

  2. Capsid – A protein shell that protects the genetic material.

  3. Lipid envelope – Found in some viruses, surrounding the capsid and aiding infection.

Viruses can have simple shapes like helical or icosahedral, or more complex structures. Most are so small that they require an electron microscope to be seen.

Origin of Viruses

The evolutionary origin of viruses is still debated. Possible theories include:

  • Evolution from bacteria (reductive evolution)

  • Origin from plasmids – DNA fragments that can move between cells

  • Role in evolution – Viruses contribute to horizontal gene transfer, increasing genetic diversity.

Although viruses can reproduce, mutate, and undergo natural selection, they lack cell structure and independent metabolism, so scientists call them "organisms at the edge of life" or "replicators".

How Viruses Spread

Viruses use different transmission methods, depending on their type:

  • Insects – Aphids transmit plant viruses; mosquitoes spread viruses like dengue.

  • Airborne droplets – Coughing and sneezing spread influenza.

  • Fecal-oral route – Norovirus and rotavirus spread via contaminated food or water.

  • Blood and sexual contact – HIV and hepatitis viruses.

The host range refers to the variety of cells a virus can infect. For example, HIV specifically targets certain white blood cells (CD4+ T-cells).

Immune Response and Treatment

  • The immune system can often clear viral infections naturally.

  • Vaccines provide immunity against specific viruses.

  • Some viruses, like HIV and hepatitis B, can evade immunity and cause chronic infections.

  • Antibiotics do not work against viruses, but antiviral drugs are available for certain infections.

Structure of Viruses

  1. Nucleic Acids – DNA or RNA, single-stranded or double-stranded, linear or circular.

  2. Capsid – Protein coat made of capsomeres.

  3. Envelope – Lipid-protein layer in some viruses.

  4. Spikes – Glycoprotein projections aiding in host recognition and entry.

Morphology (Virus Shapes)

  • Icosahedral – 20 triangular faces, symmetrical arrangement (e.g., adenovirus).

  • Helical – Ribbon-like structure that twists into a helix (e.g., tobacco mosaic virus).

  • Complex – Unique shapes like those of poxvirus or bacteriophages.

Life Cycle of a Virus

Viruses do not reproduce by cell division. Instead, they follow these steps:

  1. Attachment – Virus binds to specific host cell receptors.

  2. Penetration – Entry through membrane fusion or endocytosis.

  3. Uncoating – Viral capsid breaks down, releasing genetic material.

  4. Replication – Genome and viral proteins are synthesized using the host cell machinery.

  5. Assembly – New viral particles are put together.

  6. Release – Viruses exit the host cell via lysis or budding.

Types of Viruses by Genetic Material

  • DNA Viruses – Usually replicate in the nucleus (e.g., herpesviruses).

  • RNA Viruses – Usually replicate in the cytoplasm; classified by strand type (positive or negative sense).

  • Retroviruses – Use reverse transcription to integrate viral DNA into the host genome (e.g., HIV).

Key Takeaways

  • Viruses are acellular, non-living outside a host, but highly adaptable once inside.

  • They play both harmful and evolutionary roles in nature.

  • Prevention through vaccination and good hygiene is essential.

  • Understanding their structure and life cycle is key to developing treatments.

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