A2.3 Viruses

Virus (+draw)

• non living

• non cellular

• fixed size → do not grow

• no cytoplasm, no enzyme → no metabolism

Diversity in Virus

Genetic material - DNA or RNA; single- or double-stranded

Membrane envelope - present or absent

• envelope: outer lipid membrane is not coded for by the viral genes, as it forms from the host cell membrane when the virus bursts out of the host cell, but it may have viral proteins embedded in it.

Genome size

Comparison of Viruses

Obligate Intracellular Parasite → Virus

• restricted to one function

• Inside a cell

• uses host to survive, at the expense of host

virus rely on host cell to

• replicate

• metabolism : site of energy

• Nutrition : obtain amino acid, nucleotide for replication

• reproduction : obtain ribosome, enzyme for replication

• movement : move around the cell / spread to other cells

Lytic cycle

Attachment : Bacteriophage attach on host cell after recognising the antigen

Penetration : Bacteriophage penetrates host cell, inject DNA

Biosynthesis : destroys host dna, Virus replicate its genetic material, synthesis viral protein

Assembly : new phage assembles into virions

Release : Host cell lyses, new virions are released

Lysogenic cycle

Attachment : Bacteriophage attach on host cell after recognising the antigen

Penetration : Bacteriophage penetrates host cell, inject DNA

Integration : phage DNA integrates with the bacteria chromosome by recombination→ Prophage

Cell division : lysogenic bacteria reproduce normally

Excision: prophage may excise from bacteria chromosome → enter lytic cycle

Evidence for virus evolving from cells

1. Viruses are obligate parasites; they need a host cell to survive

2. Viruses use the same genetic code as living organisms, suggesting that viruses evolved from cells

Origin of virus

1. Genetic material that ‘escaped’ from a cell, becoming a virus

2. A cell that lost the ability to carry out the functions of life and became a virus
   

3. From basic self-replicating molecules, which became encased in protein coats

Convergent evolution

The process by which similar features evolve separately due to similar selective pressures

Rapid evolution of virus

• short & high replication times → many new viruses produced in a short time span

• High mutation rate → mechanism used to replicate

• (mutation → change in genetic material)

• Horizontal gene transfer (the non-parent-to-offspring movement of genetic material between organisms) → viruses exchange genetic information to acquire new traits

Reassortment : Influenza and its mechanisms in mutation

• Antigenic shift - reassortment of genes, where two viruses infect the same cell and exchange genetic information, resulting in a new virus.

• Antigenic drift - where the error-prone methods of replication cause changes to the shape of the surface proteins (antigens)

(virus with segmented genome swap genome)

Recombination : HIV and its mechanisms in mutation

• HIV has highest known mutation rate of any virus.

• HIV reproduces very quickly, using error-prone reverse transcriptase. 

• High replication rate + error-prone methods = high mutation rate.

• Not all mutations will be beneficial to the virus, but because of the high rate, some will be.

HIV can also exchange genetic material with other viruses or its host cell in a process called recombination.

• This leads to even greater variability in the virus.

(gene pair together, exchange sequences)

How does mutation affect treatment

Our immune system, and vaccines, rely on identifying the antigens on the surface of pathogens.

higher mutation rate of pathogen → difficult to treat