GENERAL CHARACTERISTICS OF VIRUSES

 INTRODUCTION

Viruses are acellular entities. They are genetic elements that cannot replicate independently of a living cell called the host cell. Viruses have extracellular forms which enable them to exist outside the host for long periods. But to multiply, they have to enter a cell in which they can replicate causing infection. Viruses are the most numerous microorganisms on earth and infect all types of cellular organisms. The study of viruses is known as virology. This unit examines the general characteristics of viruses, their structures, genomes, symmetry, replication in hosts and purification. 

Definition

Viruses are simple acellular entities that can only reproduce within living cells.

 General Characteristics of Viruses

1. They are the smallest microorganisms. They range in size from 10 to 400mm in diameter and can only be viewed under an electronmicroscope. 
2. They are acellular, i.e. not cellular and non living. 
3.  They only reproduce when present within living cells. 
4. They are infectious agents. 
5. A complex virus particle or virion consists of one or more molecules of DNA or RNA enclosed in a coat of protein. 
6.  Viruses can exist in two phases: extracellular and intracellular. vii. The extracellular phase known as virion possesses few if any enzymes and cannot reproduce independent of living cells. It is metabolically inert and does not carry out respiration. 
7.  In the intracellular phase, viruses exist primarily as replicating nucleic acids in the host cells that induce host metabolism to synthesise virion components which are later released. 

Viruses differ from living cells in three ways:

1. They have simple acellular organisation. 
2.  The presence of either DNA or RNA but not both in almost all virions. 
3.  They do not have the ability to reproduce independent of cells and carry out cell division as procaryotes and eukaryotes do. 

Virion Size

Virions range in size from about 10 to 400µm in diameter. The smallest viruses are a little larger than ribosomes whereas the pox viruses which include vaccinia are about the same size as the smallest bacteria and can be seen in the light microscope. Most viruses however, are too small to be visible in the light microscope and must be viewed with scanning and transmission electron microscope

The Structure of Viruses

A virus is made up of a central genetic nucleic acid molecule surrounded by a protein coat called a capsid. The combination of both is called the nucleocapsid. The capsid surrounds and protects the viral nucleic acid. The capsid also gives the virus a characteristic shape and help to establish the specificity of the virus for a particular host cells. Capsids are large macromolecular structures that self assemble from many copies of one or a few types of proteins. The protein used to build the capsids is called protomers. The simplest virus is a naked virus (nucleocapsid) consisting of a geometric capsid assembled around a nucleic acid. On the other hand, we can have a virus made up of a nucleocapsid surrounded by a flexible membrane called an envelope. This type of virus is called an envelope virus.

The various morphology types of viruses results from the combination of a particular type of capsid symmetry with the presence or absence of an envelope which is a lipid layer external to the nucleocapsid.

Fig. 1: Generalised Structures of Viruses 

Source: triroc.com 

 Viral Genomes

All cells contain double stranded DNA genomes. By contrast, viruses have either DNA or RNA genomes (one group of viruses does use both DNA and RNA as their genetic material but at different stages of the replication cycle). Hence, we have RNA viruses or DNA viruses.

Virus genomes can be classified based on whether the nucleic acid in the virion is DNA or RNA and further subdivided to whether the nucleic acid is single or double stranded. Linear or circular, some viral genomes are circular but most are linear. We can have single stranded DNA, double stranded DNA, single stranded RNA and double stranded RNA. All four types are found in animal viruses. Most plant viruses have single stranded RNA genomes and most bacteria viruses contain double stranded RNA.

Capsids Symmetry

There are three types of capsid symmetry: helical, icosahedral and complex.

Helical Capsids

They are shaped like hollow tubes with protein walls. The tobacco mosaic virus is an example of this virus. In this virus, the self assembly of protomer in a helical or spiral arrangement produces a long rigid tube, 15 to 18 nm in diameter by 300nm long.
The capsid encloses an RNA genome, which is wound in a spiral and lies within a groove formed by the protein molecule. The size of a helical capsid is influenced by both its protomers and nucleic
acid enclosed within the capsid.

 Icosahedral Capsid

The icosahedral is a regular polyhedron with 20 equilateral triangular faces and 12 vertices and is roughly spherical in shape. It is one of the nature’s favourite shapes. A few genes sometimes

only one can code for protein that self-assemble to form the capsid. These capsids are constructed from ringo-krob-shaped into caller capsomers each usually made up of five or six
protomers. Pentamers (pentons) have five subunits hexamers (hexons) have six.

3. Viruses with Capsids of Complex Symmetry (Complex viruses) Some virons are more complex than the helical and icosahedral capsid being composed of several parts, each with separate
shapes and symmetries. The most complex viruses in terms of structures are some of the bacterial viruses which possess icosahedral heads plus helical tails. In some bacterial viruses such
as bacteriophage T4 of Escherichia coli the tail itself has a complex structure. The complete T4 tail has 20 different proteins and the T4 head has several more protein.

Virus Reproduction

Viruses need a host cell in which to reproduce; hence the first step in the life cycle of a virus is attached to a host. This is followed by entry of either the nucleocapsid or the viral nucleic acid into the host. If the nucleocapsid enters uncoating of the genome usually occurs before further steps can occur.

Fig. 1: Generalised Illustration of Virus Reproduction 

Source: goldiesroom.org 

Once free in the cytoplasm, genes encoded by the viral genome are expressed, i.e. the viral genes are transcribed and translated. This allows the virus to control the host cell’s biosynthetic machinery so that new virions can be made. The viral genome is then replicated and viral proteins are synthesised. New virions are constructed by self assembly of coat proteins with the nucleic acids and finally, the matured virions are released from the host. Summarily, the steps involved in viral replication or reproduction are: 

1.  attachment of the virion to a susceptible host 
2. penetration or entry of the virion or its nucleic acid into the host · synthesis of virus nucleic acid and protein by cell metabolism as directed by the virus 
3.  assembly of capsids and packaging of viral genomes into new virions 
4. release of mature virions from the cell. 

However, there is great variation in the details of virus reproduction for individual virus species.

The Cultivation of Viruses

Because viruses are unable to reproduce independent of living cells, they cannot be cultured in the same way as prokaryotic and eukaryotic microorganisms. Animal viruses are cultivated by inoculating suitable host animals or embryonated egg – fertilised chicken eggs incubated about 6 to 8 days after laying. More recently, animal viruses have been grown in tissue (cell) culture on monolayers of animal cells. Bacterial and Archea viruses are cultivated in either broth or agar cultures of young, actively growing cells. Plant viruses are cultivated in a variety of ways which include plant tissue cultures, cultures of separated cells, or cultures of protoplasts (cells lacking cell wall) and growing of the viruses in whole plants.

Virus Purification and Assay

Viral purification and Assays are necessary so as to accurately study virus structure, reproduction and other aspects of their biology.

Virus Purification

This involves getting or isolating the viral particle in its pure state, purification makes use of several virus properties.


Four of the most widely used methods to isolate and purify viruses are:

1. differential and density gradient centrifugation. This is often used in the initial purification steps to separate virus particles from host cells. 
2.  precipitation of viruses particles. 
3.  denaturation of contaminants. 
4.  enzymatic digestion of host cells constituents. 

Virus Assays

The quantity of viruses in a sample can be determined either directly by counting particle numbers using the electron microscope or indirectly by measurement of an observable effect of the virus using techniques such as the hemaglutination assay.