Introduction to Viruses

‘Virus’ is the latin word which means poison. Originally, the name was used for infectious agents that could pass a membrane filter (which has pores too fine to be passed by normal bacteria).

              Fig: Electron micrographs of  bacterial viruses (Brock Biology of Microorganisms, 13th ed.)

Viruses are known as obligate intracellular parasites – they have no own metabolism but are dependent on the metabolism, energy and protein synthesis of the host cell which is modified after infection to suit the virus requirements.

Viruses never reproduce by division, but by production of separate part of the virions followed by self-assembly.

Viral structure

Virion means the virus particle. It contains nucleic acid (viral chromosome), which is either DNA or RNA, but never both. It surrounded by a protein capsule (the capsid). Both nucleic acid and capsid together are called nucleocapsid. The nucleocapsid may be additionally surrounded by a phospholipid membrane envelope (rare for bacterial viruses, bacteriophages). The capsid is built up by one or a few identical subunits, the capsomers. Almost all viruses fall into one of three groups according to the shape of their capsule:

1. Helical viruses:  The capsid is rod-shaped, and the capsomers form a helix with the viral chromosome in the centre.

2. Spherical (polyhedral) viruses: The capsid is of polyhedral shape.

3. Composed viruses (viruses with binal structure):  Having a helical part (the tail and a polyhedral part (the head), a common type for bacteriophages)

The replication cycle of bacteriophages

For the replication of a virus it is essential to get touch with a living host cell to synthesize all the essential components that is needed to make more virions. After that all these essential particles must then be assembled to convert into new virions which are eventually comes out through lysis from the cell. The whole viral replication cycle can be divided into the following six steps:

1. Entry into the host cell

• By utilizing specific receptor molecules on the surface of the host cell (see table)
• the capsid often keeps attached to the cell wall (a difference to animal and plant viruses, the exception are the filamentous bacteriophages, which enter the host cell completely)
• apart from the viral chromosome, additional viral enzymes often enter the host cell as well

2. Replication of the viral chromosome

• DNA viruses often utilize the host replication machinery (after some modification)
• For RNA viruses, the chromosome serves at the same time as messenger, and a viral encoded replicase (RNA-dependent RNA polymerase is required) for replication

3. The viral genome

• The number of gene products vary considerably in different bacteriophages
•  from 4 (in simple RNA phages) up to >100 (e.g. in even number T phages)

4. Production of virus gene products and its effect on the host cell

• virus infection has always a negative effect on the host cell
• in most cases the host cell is irreversibly damaged during infection and is eventually destroyed
• the host cell metabolism is efficiently redirected to serve the phage reproduction

5. Assembly of the virions

• the assembly of the new virions begin when capsid proteins and viral nucleic acid accumulates in the host cell
• the assembly occurs spontaneously without assistance of the host cell

6. Escape / Release

• after assembly, the new phage particles have to get out of the host cell
• filamentous phages escape from the host cell as part of the final assembly of the phage particle
• most phages escape by lysis (mechanical distruction) of the host cell (often assisted by viral enzymes like lysozyme)

                                              Reference Book: Brock Biology of Microorganisms, 13th edhttp://www.microbesworld.com/2018/07/introduction-to-viruses.html