Viruses

Viruses

Shared features of viruses:

• Viruses are Non-cellular (Not made of or contains cells)

• Small Size: between 20 and 300 nanometres in diameter.

→ Viruses have to be smaller than their host cells to enter them

→ Viruses lack cytoplasm and other structural features

• Fixed Size: Viruses Don't grow. 

→ Composed of a fixed number of components that determines the overall size

• Nucleic Acid as genetic material: have genes made of DNA or RNA, act as universal genetic code. 

→ Their viral proteins are synthesized by the nucleic acid-to-polypeptide translation mechanisms of their host cell

• Capsid made of protein: Capsid packs virus’ genetic material into a protein coat before release.

→ Made of repeating protein subunits, Capsomeres

→ Gives viruses a symmetrical structure unlike shape of living cells

• No cytoplasm and few/no enzymes: viruses rely on the metabolism of their host. 

→ few enzymes produced by some viruses are required to replicate the Virus’ genetic material to infect host cells, or lysis (bursting host cells to release the new viruses)

Diverse structure of Viruses:

Viruses are very diverse, suggesting that they have multiple evolutionary origins.

1. Genetic Diversity: 

• no single gene is shared by all viruses

→ The genetic material can be DNA or RNA and Single-stranded or Double-stranded. 

DNA: can be either Linear or  Circular

RNA: genes can be ‘positive-sense’ and used directly as mRNA or ‘negative-sense’ and need to be transcribed before translation

2. Enveloped and non-enveloped viruses: 

viruses may become enveloped in membranes during lysis, with phospholipids from the plasma membrane of the host cell and proteins, mostly glycoproteins, from the virus itself.

→ Membrane helps the enveloped virus make contact with the host cell to infect it.

Animal viruses are usually enveloped. Plant viruses and bacteriophages mostly not.

Lytic and Lysogenic cycle

The bacteriophage virus lambda uses a bacterium (E.coli) as a host. When it infects it, the bacteriophage can follow one of two infection paths:

1. The lytic cycle

2. The lysogenic cycle

Other viruses show similar lifecycles

Lytic Cycle:

• The virus reproduces without integrating its DNA into the host cell DNA. Instead it immediately produces viral particles and then bursts out of the host cell, killing it. To infect other near host cells.

→  Viruses that infect cells in plants or animals often follow a lytic cycle.

Lysogenic Cycle:

• Alternative to the lytic cycle. 

• The virus attaches to a host cell and injects its DNA (lytic cycle)

• Instead of replication, the Virus’ DNA becomes Integrated into the host cell’s DNA molecule. 

• It stays undetected and inactive.

• Every time the host replicates its DNA, the daughter cells inherit the viral DNA (Prophage DNA) but don't produce viral proteins

• The virus is Temperate in this state and does not kill the host and causes minimal harm. 

• It could change into the lytic state and cause lysis. Stimulus for this can come from inner or outside of the bacterial cell.

Evidence for several origins of viruses

• Viruses are Obligate parasites. (an organism that can’t complete its life cycle without depending completely on a living host)

→ They need a host cell to replicate: suggesting that cells evolved before viruses

• Viruses (regarded as not living) use same universal genetic code as living organisms: suggesting that viruses evolved from cells

• Extremely diverse in structure and genetic constitution: multiple origins from living cells, than evolving from one common viral ancestor, convergent evolution

Viruses rapidly evolve

Viruses can show extremely rapid rates of evolution.

1. Very short generation times of <1 hour in the lytic cycle

2. High mutation rates, especially in RNA viruses

3. Intense natural selection due to host organisms evolving defences such as antibodies for destroying viruses