- 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 Virus Replication.html

General Steps in the Viral Replication Cycle

Key Concept: The general steps in viral replication (life cycle) are similar for enveloped and non-enveloped (naked) animal viruses.  

Although viruses vary in viral replication and transmission strategies, the following general stages of the viral replication cycle (also referred to as the viral life cycle) are similar for all known animal viruses.

Attachment (Adsorption) occurs when a virus binds to specific receptors on the host cell envelope. Some viruses have a specific tropism in that infect certain types of cells, and some viruses have a broad host range (that is they can infect many different host species, for example rabies) and others have a narrow host range (they only infect one or few species, for example HIV).

Penetration (Entry) is when the attached virion (virus particle) is taken into the host cell cytoplasm by fusion of the viral envelope with the host cell membrane (enveloped viruses) or endocytosis or in some cases by direct penetration of the virion across the plasma membrane (enveloped viruses)

Uncoating is the stage in which the virion the separation of the virion nucleic acid from the viral capsid. The viral capsid is degraded and the nucleic acid is available for the biosynthesis stage.

Biosynthesis is the stage in which the viral genome is replicated and viral enzymes and other proteins are produced. 

Assembly occurs when a certain level of newly synthesized viral proteins and nucleic acid genomes has been produced and is the process in which immature viral particles are formed by packaging viral nucleic acids and proteins into a new synthesized capsid.

Maturation is the stage in which a virus becomes infectious. This may require modifications to viral or host cell proteins or other mechanisms.

Release of the infectious virion may occur by budding from the host cell membrane or lysis of the cell allowing the release of the virions.

Note: Assembly, maturation, and release are not always recognized as distinct stages.

The general stages of an animal virus life cycle are illustrated in Figures 4.6 to 4.9 of Course Module 4 Commentary (UMGC, n.d.) and Figure 6.10 in Parker et al (2016).  

Viral Genome Replication and Viral Protein Synthesis

As previously noted viruses differ in the structure and nucleic acid compositions of their genome. The genome of viruses can be circular or linear, segmented or a single molecule of nucleic acid. The genome can be either single stranded (ss) or double stranded (ds) DNA or RNA, and single stranded nucleic acid genomes can be either a positive strand or a negative strand based on how it becomes transcribed to mRNA. Only a positive (+) viral mRNA strand can bind to ribosomes of the host cell and be translated into viral proteins. There are seven different genome groups by which viruses can store their genetic information (see Table 4.1 Viral Genome Groups in Course Module 4).

Table 4.1 from Course Module 4 Viral Genome Groups

Group	DNA or RNA	Nucleic Acid Type and Orientation	Examples of Viruses	Examples of Disease(s) Caused
I	DNA	double-stranded	adenoviruses	respiratory illnesses
II	DNA	(+) single-stranded	parvoviruses	fifth disease (childhood rash)
III	RNA	double-stranded	reoviruses	gastroenteritis
IV	RNA	(+) single-stranded	picornaviruses	polio, foot and mouth disease, common cold, hepatitis A (HAV)
V	RNA	(–) single-stranded	orthomyxoviruses	influenza
VI	RNA	single-stranded retroviral	HIV	AIDS

 

As previously noted, for replication a virus uses the host cell molecules and machinery to make viral proteins. DNA viruses need to make mRNA that is then translated to produce viral proteins. Some DNA viruses enter the host cell nucleus and use the host cell machinery to make mRNA and replicate the viral DNA (e.g., adenoviruses, human herpesviruses, and Human parvovirus B-19). Other DNA viruses do not enter the host cell nucleus, but replicate in the cytoplasm (e.g., vaccinia virus) (Hunt, 2016, Chapter 3). These viruses enter the host cell with their own viral RNA polymerase, which is used for transcription, and other enzymes Some RNA viruses have a DNA phase (i.e., they copy their RNA into DNA) – Group VI and Group VII; these viruses include HIV and other retroviruses. Other RNA viruses do not have a DNA phase – Groups III, IV, and V (Hunt, 2016, Chapter 4).

 Cited Sources

Hunt, M. (2016, May  31) Virology Chapter 3 “DNA Virus Replication Strategies” with additions by Mcllroy, D. in Microbiology and Immunology Online, University of South Carolina Medical School. Retrieved from http://www.microbiologybook.org/book/virol-sta.htm. 

Hunt, M. (2016, May 31) Virology Chapter 4 “RNA Virus Replication Strategies” with additions by Mcllroy, D. in Microbiology and Immunology Online, University of South Carolina Medical School. Retrieved from http://www.microbiologybook.org/book/virol-sta.htm

Parker, N., Schneegurt, M., Thi Tu, A.-H., Lister, P., & Forster, B. M. (2016) 6.2 The Viral Life Cycle in Microbiology. Houston: OpenStax. Online version retrieved from https://openstax.org/details/books/microbiology

UMGC (n.d.) Animal Virus Life Cycle in BIOL 302 Course Module 4 Viruses: Commentary. Document posted in University of Maryland Global Campus (UMGC) BIOL 302 6981 (2205) online classroom archived at: http//:campus.umuc.edu