Microbiology & Phycology - Lecture -2

 Morphological Features of Viruses

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Introduction

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Welcome back to Nucleotides biology discuss. Today we will continue from the lecture number two of microbiology and phycology. In the last class, we started discussing the general characteristic features of viruses. In this class, we will focus on the morphological features of viruses.

Size and Shapes of Viruses

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One of the key morphological features of viruses is their size and shape. Different viruses exhibit different sizes and shapes. The size of viruses can vary from virus to virus. However, viruses are generally very small in size. Most viruses have a size ranging from 20 nanometers to 350 nanometers. The smallest known virus is the adeno-associated virus (AAV), measuring approximately 20 nanometers. On the other hand, the largest known virus is the mimivirus, with a size of 1.5 μm.

Shapes of Viruses

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Viruses also come in different shapes. There are four main categories of virus shapes: helical virus, polyhedral virus, enveloped virus, and complex virus.

### Helical Virus

A helical virus is characterized by its rod-shaped structure. The nucleic acid of the virus is helically coiled, resembling a spring. An example of a helical virus is the tobacco mosaic virus (TMV).

### Polyhedral Virus

A polyhedral virus has a multi-sided shape, similar to a 3D geometrical shape. The polyhedral structure is made up of protein subunits called capsomers. One common polyhedral shape is the icosahedral shape, which has 20 triangular faces. Examples of polyhedral viruses include poliovirus and adenovirus.

### Enveloped Virus

An enveloped virus has an additional outer covering called an envelope. The envelope is derived from the host cell's membrane and surrounds the nucleocapsid (the nucleic acid and capsid). Enveloped viruses are commonly found in animal viruses.

### Complex Virus

A complex virus has a more intricate structure compared to other viruses. One example of a complex virus is the bacteriophage, which infects bacteria. Bacteriophages have a head region containing the nucleic acid, a tail region, and tail fibers. The structure of a bacteriophage resembles a tadpole.

Composition of Viruses

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Viruses have two main components: the covering and the central core part.

### Covering of Viruses

The covering of viruses is called the capsid, which is a proteinaceous structure that encloses the nucleic acid. The capsid is made up of protein subunits called capsomers.

### Central Core Part

The central core part of the virus contains the nucleic acid, which can be either DNA or RNA. Only one type of nucleic acid is present in a single virus.

Tobacco Mosaic Virus (TMV)

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The tobacco mosaic virus (TMV) is a plant virus that infects tobacco plants. It is an RNA virus with a helically coiled RNA surrounded by protein subunits. The TMV has a rod-shaped structure with a width of 180 angstroms and a length of 3000 angstroms. It is composed of 2130 capsomers, with each capsomer consisting of 158 amino acids.