A2.3 Viruses

Features of a virus structure:

Fixed size

Nucleic acid genetic material – all viruses have DNA or RNA

Protein capsid

No cytoplasm, few enzymes

Diversity of virus structure:

Diversity of genetic material:

Enveloped & non-enveloped viruses:

Enveloped vs Non-Enveloped

(2) Enveloped & non-enveloped viruses:

     - viruses burst host cell (lysis) to be released

     - some viruses, especially animal viruses, become

       membrane covered  (membranes contain both phospholipids & proteins)

     - membrane phospholipids are derived from host’s

     - the proteins (mostly glycoprotein) are the virus’

     - membrane helps virus make contact with and infect host

     - most bacteria or plant viruses are non-enveloped.  

Suggest why…….   

Lytic Cycle

Lysogenic cycle

Problems with the lytic cycle:

• Virus can spread to more and more host cells but as it kills them it must find new host cells. 

• If they kill an entire population, they risk dying out.

Origins of Viruses

1. Viruses may have originated from ancient RNA or DNA molecules that became encapsulated in a protective protein coat. The encapsulated genetic material or ‘proto-virus’ may have been able to replicate and evolve, eventually forming the viruses that we know today.

2. Viruses may have evolved from viroids, which are small infectious agents that consist only of a short strand of RNA and infect angiosperms (flowering plants).

3. Viruses may have originated from transposons, which are genetic elements that can move around within an organism’s genome.

4. Viruses may have evolved from ancient cells that lost their ability to live independently and became dependent on other cells for reproduction.

Reasons for rapid evolution in viruses…

1. Generation cycles short - hours / High reproduction rates

2. High mutation rates especially RNA based virus

3. Virus that are co-infecting a cell can share genetic info with one another