- Review week 5 course content topics “General Properties of Viruses” and “Viral Replication (Life Cycle)” and week 5 reading assignments in course module 4

Wk. 5 Key Concept General Properties of Viruses.html

General Properties of Viruses

Key concepts: Viruses are not cells and require host cell components for replication. The basic structure of a virion (a virus, also described as a complete virus particle) includes a nucleic acid core of either DNA or RNA and a protein coat called a capsid.

What are viruses?

Viruses and several diseases they cause were introduced in Course Module 1 Introduction to Microbiology and others are described in Course Module 4 Viruses. Recall that viruses are not cells (they are acellular). Unlike bacteria, they don't have cytoplasm or ribosomes, and they don't grow (increase in size), metabolize nutrients, or produce waste products. The viral genome is either DNA or RNA, and there is a lot of diversity in the physical characteristic of the genome (Parker et al., 2016 & UMGC, n.d.). Similar to bacteria and other cellular microbes, viruses are able to mutate and reproduce, but viruses require the structures and molecules of a host cell for replication. Most viruses have a diameter in the 20 nm to 300 nm range. Some viruses are larger than mycoplasma and other small bacteria, and there are the Giant Mimiviruses, which have capsid (the outer protein coat of the virion) diameters of 400 – 500 nm, which when stained are visible with a light microscope (Wessner, 2010). However, as a group, viruses are one of the smallest infectious agents known. There are viruses that infect vertebrates, invertebrates, plants, fungi, and even bacteria. Several hundred different viruses are known to infect humans. This week you will learn more about the structure and classification of viruses, and their productive life cycles (also described as viral replication cycle).

General Structure of a Virion

A virion is a single complete virus particle composed of nucleic acid (either DNA or RNA), a capsid (a protein shell that encloses the DNA or RNA genome), and attachment proteins (used to bind the virus to the surface of host cell) and other proteins. The capsid and genome complex is called a nucleocapsid. Some viral particles are surrounded by a lipid envelope derived from the host cell and some viruses have enzymes or other protein. The structure of a typical virion is illustrated below.

Components of a Typical Virion (Figure 4.2 from UMGC, n.d.)

In the following YouTube video Paul Anderson (2012) describes and illustrates structural components and reproductive features common for all viruses and some differences in structural components (for example, presence or absence of envelope and type of genome) and life cycle that contribute to the wide diversity in the structure and life cycle of viruses.