General characteristics of viruses

General characteristics of viruses

 

Virology: is the study of viruses – submicroscopic, parasitic particles of

genetic material contained in a protein coat and virus-like agents

A Virus: is a small infectious agent that replicates only inside the living

cells of other organisms. Viruses can infect all types of life forms, from

animals and plants to microorganisms, including bacteria and archaea.

The following properties distinguish viruses from other

microorganisms:

1- They are very small in size.

2- They contain one kind of nucleic acid (RNA or DNA) as their

genome

3- They are metabolically inert as they don’t possess ribosomes or

protein synthesizing apparatus.

4- They are obligate intracellular parasites.

Size of viruses:

They vary in size from 20-300nm. Due to their small size:

1- They can pass through bacterial filters

2- They require high speeds (ultracentrifugation) for their

sedimentation

3- They are only seen by electron microscope, except poxviruses.

Virus characteristics:

-Genetic material (Nucleic acid)

 RNA (single strand RNA, double strand RNA, segmented)

 DNA (ssDNA,dsDNA, liner or circular)

-Protein coat (subunit structure)

 Nucleoprotein

 Capsid

Structure of viruses:

 Each virus particle or virion is composed of a protein coat or

capsid and a nucleic acid core.

 The capsid with its enclosed nucleic acid is called nucleocapsid.

 Many viruses are naked but some viruses are enveloped.

-Viral capsid :

1- It is composed of small protein subunits called capsomers.

2- The arrangement of capsomers determines the viral symmetry.

3- Functions of capsomers are:

A. It protects the viral genome against inactivation by

nucleases(enzymes that inactivate nucleic acid)

B.It is responsible for the structural symmetry of the virion

C.It is participate in attachment of the virion to susceptible cells.

D. It determines the antigenicity of the virion (thus Ab formed against

protein coat Ag neutralize virus infectivity).

-viral nucleic acid (genome):

 Viruses contain either DNA or RNA but not both.

 Most DNA viruses are double strand.

 While most RNA viruses are single strand.

 The molecular weight and type of nucleic acid are specific for each

virus group.

 Functions of nucleic acid are:

A. It is the infectious part of the virus, coreless particles are noninfectious.

B. It carries the genetic information for:

- virus replication

- virulence or ability to parasite cells

- Antigenic specificity of the protein coat.

-viral envelop:

Many viruses are surrounded by lipids or lipoprotein envelop

which may be covered by glycoprotein spike-like projections ,

which attach to the host receptors during the entry of the virus into

the cell.

Due to their lipid content such viruses are sensitive.

Loss of lipids result in disruption of virus and loss of infectivity.

The surface proteins (whether the capsid proteins or envelop

glycoproteins) are the principle antigens against which the host

mounts its immune response.

Some viruses produce proteins that act as superantigens.

Envelop functions:

- Help viruses avoid the host immune system.

- Glycoproteins serve to bind to receptor sites on the host’s

membrane.

- The envelop fuses with the host’s membrane allowing capsid and

the viral genome to enter and infect the host.

Virus symmetry:

Viruses have 3 types of symmetry

1- Cubical symmetry: these viruses resemble a crystal and are called

icosahedral viruses.

2- Helical symmetry: at which the particle is elongated, most helical

viruses are enveloped such as influenza virus.

3- Complex symmetry: at which the viruses are complicated in

structure such as poxvirus.