
Concept explainers
To review:
The comparison and differences between icosahedral and filamentous symmetry of viruses with examples.
Introduction:
The viral capsid protects and encloses the genetic material of a virus. It is a proteinaceous structure that can be either symmetric or asymmetric. During replication or reproduction of a virus, the genetic material is inserted into the host cytoplasm and the capsid remains outside.

Explanation of Solution
The symmetrical capsid of virus can be of two types: icosahedral and filamentous. This symmetrical geometry proves to be advantageous in packaging the repeating protein units that are produced from the genetic material of the virus. The differences between these shapes of viral capsid are given below:
1. The icosahedral capsid contains 20 triangular faces and each face is formed by three identical proteins that are asymmetrical. The filamentous capsid is composed of monomers that spiral around the genome of the virus.
2. The size of an icosahedral capsid is fixed, however, the length of a helical (filamentous) capsid can be increased to include larger sized
3. The filamentous viruses contain tail fibers at their end that mediate attachment to host cell surface. The icosahedral virus contains glycoprotein spikes in their membrane envelope to aid in attachment.
4. The example of viruses having icosahedral capsid are human immunodeficiency virus (HIV) and herpes simplex virus. The examples of filamentous virus are Ebola virus, influenza virus, and tobacco mosaic virus.
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