Viruses: Structures and their Functions


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Virus Structures and Their Functions

All viruses are made of two main constituents, the nucleic acid genome, and the protein capsid. These structures create up the nucleocapsid but in animal viruses, the nucleocapsid has an additional structure called the lipid envelope (Zimmer, 2015). The genome can either be DNA or RNA but not both which may be single or double stranded. The function of the genome is to encode for proteins present on the protein coat/capsid.

The genome is also responsible for multiplication of the virus particle. The protein capsid covers the viral genome protecting it from nucleases and attaches the virus particle to specific receptors on prospective host cells during infection. The lipid envelope found in animal viruses, and it enables the virus to enter or exit host cells through endosomal membranes and protects the virus from the extracellular environment.

Diversity in Animal Viruses

The diversity of animal viruses is as a result of structural differences present in different viruses including the structure of the genome. Some viruses have DNA both single or double stranded as their genome; papillomavirus is a DNA virus which causes warts while others have RNA either single or double stranded an example is Influenza virus which causes the flu. Other viruses differ in their virus particle structure; some are icosahedral such as Adenovirus, others such as the rabies virus are helical or rod-shaped, while others are pleomorphic or irregular. Viruses can also be differentiated by the presence or absence of the lipid envelope, and based on this criteria animal viruses are classified into different families, for instance, the herpes simplex virus has an envelope while others do not (Villarreal, 2005).

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HIV Replication Cycle

The first stage of HIV replication is binding, where the virus enters the host immune cells by attaching its glycoproteins to the receptors on target cells (Murphy et al., 2012). After this attachment, the viral casing/envelope then fuses with the membrane of the host cell and releases its capsid into the cell in the process. Once the virus has bound to the target cell, its genome and enzymes, such as reverse transcriptase, integrase, rib nucleases, and protease, are injected into the cell (Murphy et al., 2012).

During the reverse transcription stage, the single-stranded viral RNA is then transcribed into complementary DNA by the enzyme reverse transcriptase. The cDNA and its complement are transported to the cell nucleus as double-stranded DNA and is integrated into a host chromosome by the enzyme integrase. This is referred to as the integration stage. The fourth stage is the transcription step where provirus is transcribed into RNA, which undergoes RNA splicing to form mRNAs (Zimmer, 2015). The mRNAs are the translated into regulatory proteins, and this takes place in the cytoplasm. These proteins are packaged into new virus particles.

Structural Differences and Similarities between PRPc and PrPSc

            The main difference between the two prion forms is that PRPc has various amino acids and one disulphide bond giving them an alpha helical structure while PrPSc has a higher β-sheet structure. They are also different in solubility, fibril formation, and other features. There are no obvious similarities in structure between the two forms (Zimmer 2015). The function of the normal prion is controversial because present information indicates many different roles. Normal Function of Pro-Oncogenes

Proto-oncogenes code for proteins that control cell development and differentiation. They also play a role in signal transduction and passage of mitogenic signals (Berns, 2013). Viruses are known to cause tumor development when they integrate close to proto-oncogenes. The nearby integration of viral sequences genetically alters the proto-oncogene causing its RNA to cause cancerous tumor growth (Berns, 2013). Some oncogenic viruses include hepatitis c virus that causes 22% of liver cancers, human papilloma virus that causes cervical and vulvar cancers (Zimmer, 2015).

How Viruses Cause Disease

Viruses can reside in cells without causing any harm. However, when they start dividing inside the host cell, they breach its walls, and this triggers the activity of the immune cells. They invade the host cells and takes over the activities taking part inside the host cells. The virus continues to replicate until it fills up the host cell. At this stage, the host lyses releasing the viral particles which start infecting other cells repeating the replication pattern (Villarrealal, 2005).By this tyme, the host immune cells recognise the antigen and starts fighting and blocking the virus and establishes a disease.


Berns, K. I. (Ed.). (2013). The parvoviruses. Springer Science & Business Media.

Murphy, F. A., Fauquet, C. M., Bishop, D. H., Ghabrial, S. A., Jarvis, A., Martelli, G. P., … & Summers, M. D. (Eds.). (2012). Virus taxonomy: classification and nomenclature of viruses (Vol. 10). Springer Science & Business Media.

Villarreal, L. P. (2005). Viruses and the Evolution of Life. American Society of Microbiology.

Zimmer, C. (2015). A planet of viruses. University of Chicago Press.

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