HL Bio - Cellular Bio (I): A2.3 Viruses

Explain the common features of a virus

1. Small - diameter of 20 - 300 nm

2. Fixed size as they do not grow

3. Nucleic acid core - DNA or RNA, circular or linear, single or double stranded

4. No cytoplasm - non-living organisms

5. No enzymes - relies on host cells enzymes

Enveloped viruses are typically...

Animal viruses

Describe the structure of HIV

2 RNA strands

Transmitted by exchange of bodily fluids

Contains reverse transcriptase which allows it to produce DNA from viral RNA

Has a viral envelope with a lipid bilayer and glycoproteins as attachment proteins

Describe the structure of COVID-19

Spherical

Single stranded RNA

Envelope outside capsid

Glycoproteins

Describe the structure of bacteriophage lambda

Infects E. coli

Double stranded DNA genome

Has tails and fibrils which allows it to insert DNA into host cell

Describe the lytic cycle

1. Virus attaches itself to host cell via attachment proteins

2. Secretes lysozyme to digest cell wall

3. Tail tube injects phage DNA into host cell, capsid remains outside cell

4. Host cell macromolecular synthesising machinery used to produce phage proteins

5. Phage DNA and capsid assembles to form virions

6. Cell lyses, releasing virions

Describe the lysogenic cycle

1. Virus attaches itself to host cell via attachment proteins

2. Linear phage circularises and inserts itself into host cell genome via enzyme integrase, forming a prophase

3. Prophage replicates with chromosomes, producing daughter cells with prophase

4. Lytic cycle - Host cell macromolecular synthesising machinery used to produce phage proteins

5. Phage DNA and capsid assembles to form virions

6. Cell lyses, releasing virions

How do the common features of viruses come about?

Viruses come from different ancestors, convergent evolution results in common structural features which adapt to similar environmental pressures to serve functions

Describe the 3 hypotheses of the origins of viruses

1. Virus first hypothesis - suggests viruses evolved before cells, originating from bits of self replicating RNA infecting other cells, forming viruses

2. Regressive/Reduction hypothesis - suggests viruses were once small cells that were parastic in bigger cells. Over time, the smaller cells lost cell components and became viruses

3. Escape/Progressive hypothesis - suggests DNA escaped from cells and membrane formed around it, forming a virus

Explain the 3 reasons for the rapid evolution of viruses

1. High replication rate - the number of generations produced in viruses are fast, less than 1 hours

2. High mutation rate - basis of evolution

3. Natural selection - defence mechanisms from host cells - selection pressures - evolution

What are the two types of mutations a virus can go through?

Antigenic shift, antigenic drift

Describe the process of antigenic drift

1. Two or more different strains of the same virus or two or more different viruses combine to form new sub-type

2. Results in genetic reassortment with new surface antigens

3. Antibodies in host cell cannot recognise new surface antigens - host cell is susceptible to virus

Describe the process of antigenic shift

1. Accumulation of mutations in viral RNA genome

2. Results in rapid mutation of new surface antigens

3. Antibodies in host cell cannot recognise new surface antigens - host cell is susceptible to virus

Because of the rapid evolution of viruses, ...

Annual vaccinations are needed to give immunity to the new strains

Explain why HIV is incurable due to rapid mutation rates

1. HIV uses reverse transcriptase to conver ssDNA into DNA, which has no proofreading ability - high mutation rates

2. Host cells convert cytosine into RNA uracil by cytidine deaminase - higher mutation rates

3. High mutation rates - easily evade immune system; mutated surface antigens binds to different cell types - resistant to combination drug therapy - incurable and chronic

Why are viruses not considered living organisms?

Viruses cannot carry out the functions of life independently (e.g. reproduction)