Introduction of Viruses

A virus is a small collection of genetic code, either DNA or RNA, surrounded by a protein coat. Viruses are not cells, without cytoplasmic membrane, cytosol, or functional organelles. A virus cannot replicate alone. Therefore, viruses obligate parasites that must infect a cell and use its organelles to reproduce. Despite their simple structure, they are a major cause of disease. Scientists have worked for hundreds of years to understand viruses and find better ways of fighting them.

Virus Structure

The virus consists of a viral capsid (a protective outer protein shell), that encases a nucleic acid (DNA or RNA, but not both), and possibly an envelope. If envelopes are not present, then the virus is considered to have a naked nucleocapsid. Some viruses have a tail piece attached. They have many shapes. In general, the shapes of viruses are classified into four groups: filamentous, isometric (or icosahedral), enveloped, and head and tail. Most viruses vary in diameter from 20 nm to 250-400 nm; the largest, however, measure about 500 nm in diameter and are about 700-1,000 nm in length.

Non-enveloped and enveloped viruses. Fig.1 Non-enveloped and enveloped viruses. (Reddy, 2016)

Viral Life Cycle

For a virus to infect a host organism, the viral genome must be transferred from a virus particle into the cytoplasm of a host cell. The virus life cycle could be divided into six steps: attachment, penetration, uncoating, gene expression, and replication, assembly, and release. The majority of viruses lyse their host cell at the end of replication, allowing all the newly formed virions to be released to the environment. Another possibility, common for enveloped viruses, is budding, where one virus is released from the cell at a time.

A general view of the lytic life cycle of a virus. Fig.2 A general view of the lytic life cycle of a virus. (Munke, 2020)

How Viruses Spread

Virus transmission can occur through multiple pathways. Once a person is infected with a virus, their body becomes a reservoir of virus particles. Some viruses can be transmitted such as by coughing and sneezing or by shedding skin or in some cases even touching surfaces. SARS, tuberculosis, and influenza are examples of diseases spread by airborne droplet transmission. Some viruses that infect animals, including humans, are also spread by vectors, usually blood-sucking insects, but direct transmission is more common. Some viruses can use mother-to-child transmission (vertically transmitted infection), such as HIV, rubella, cytomegalovirus, and herpesviruses.

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  1. Reddy T.; Sansom M S P. Computational virology: from the inside out. Biochimica et Biophysica Acta (BBA)-Biomembranes. 2016, 1858(7): 1610-1618.
  2. Munke, A. Small particles with big impact: structural studies of viruses and toxicological studies of nanodiamonds. Acta Universitatis Upsaliensis, 2020.

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