General Characteristics of Viruses

 General Characteristics of Viruses

Viral structure: Typical viral components are shown in Fig. 2. These components are a nucleic acid core and a surrounding protein coat called a capsid. In addition some viruses have a surrounding lipid bilayer membrane called an envelope.

Fig. 2. The components of helical virus

A.  Nucleic acid

• Viral genomes are either DNA or RNA (not both)

• Nucleic acid may be single- or double-stranded

Fig. 3. Types of virus genomes 

B.  Capsid

• Protein coat

• Protection of Nucleic Acid

• Provides Specificity for Attachment

• Capsomeres are subunits of the capsid

Fig. 4. Capsid structure

C.  Envelope

• Outer covering of some viruses

• Envelope is derived from the host cell plasma membrane when the virus buds out

• Some enveloped viruses have spikes, which are viral glycoproteins that project from the envelope

• Naked (non-enveloped) viruses are protected by their capsid alone

Fig. 5. Enveloped helical virus

 

 

2.  Size of viruses:

• Determined by electron microscopy

• Ranges from 20 to 14000 nm in length

Fig. 6. Size of different viruses

 

3.  Shape of viruses:

 

Four basic morphologies

• Icosahedral - efficient means to conserve and enclose space; form capsomers (planar faces formed by association of proteins) 

• Helical - capsid is shaped like a hollow protein tube 

• Enveloped - outer covering derived from the host cell's nuclear or plasma membrane and often possessing spikes or peplomer projections involved in attachment and entry into a host cell sometimes via their enzymatic activity 

• Complex symmetry - viruses that fit neither of the above categories or which may employ portions in combination, e.g., bacteriophage

Fig.7. Types of viral symmetry

 

4.  Host Range: The specific types of cells a virus can infect in its host species represent the host range of the virus.

 

• Animal virus

• Plant virus

• Bacterial virus (bacteriophage)

Host range is determined by attachment sites (receptors)   

 

Important points to remember: 

• VIRION – a complete single viral particle

• Obligatory intracellular parasites

• Contain DNA or RNA

• Do not undergo binary fission

• Sensitive to interferon

• Contain a protein coat

• Some are enclosed by an envelope

• Some viruses have spikes

• Most viruses infect only specific types of cells in one host

• Host range is determined by specific host attachment sites and cellular factors (receptors)

• Viruses replicate through replication of their nucleic acid and synthesis of the viral protein.

• Viruses do not multiply in chemically defined media

• All ss-RNA viruses with negative polarity have the enzyme transcriptase (RNA dependent RNA polymerase) inside virions.

• Retroviruses and hepatitis B virus contain the enzyme reverse transcriptase.