BIO HL THEME A2.3 [VIRUS]

viruses

Structural Features Common to Viruses: [SIZE]

• Small size (20-300 nm)

• Fixed size (do not grow over time)

  • Small and fixed size is the result of chemical simplicity. Contains relatively small numbers of organic molecules → composition of those molecules prevents the formation of bonds to produce larger entity

Structural Features Common to Viruses: [Nucleic acid/DNA, cel ]

• Nucleic acid [either RNA or DNA] as the genetic material

  • Contain fewer than 100 genes within capsid 

  • Genetic material can demonstrate variation by being linear/circular 
    Protein capsid - where genetic material is stored, made of repeating protein subunits. 

    • Capsid of each virus is unique due to presence of different specific amino acids + structures they produce when bonding.

• No cytoplasm, few enzymes

Diversity of Structures [diversity of genetic material]

1. Viruses have genes made of either DNA or RNA (could be single or double stranded)

Nucleic acid molecule length varies - can either be circular or linear (affects how viruses perform protein synthesis). 10-400nm in size,

Diversity of Structures [single stranded RNA viruses]

1. Positive-sense RNA viruses use their genes as messenger RNA

2. Negative-sense RNA transcribe their genes to make messenger RNA

Retroviruses: make double-stranded DNA copies of their RNA → transcribe the negative-sense strand of the DNA to make mRNA

Different types of VIRUS

1. There are 3 types of viruses including Bacteriophage lambda, coronaviruses, and human immunodeficiency virus (HIV). 

important features of bacteriophage lambda 

1. Capsid head that protects double stranded DNA core 

2. Tail fibres that attach the virus to the host cell

3. A tail sheath that consists of proteins that contract to drive the tail tube through the host cells outer membrane

4. DNA  that is injected through the tail into the host cell

Features of Corona Virus 

1. Spherical shape 

2. Single stranded RNA as genetic material 

3. Envelope outside capsid 

4. Various projections of spike proteins on the envelope → creating corona 

Features of HIV

• Envelope outside capsid 

• Two identical strands of RNA, protected by capsid 

• Within viral RNA, reverse transcriptase is encoded, allowing for production of DNA using viral RNA as model 

• Known as retrovirus because makes DNA copy of its RNA code 

• Envelope spikes of HIV are made of protein and carbohydrate 

Enveloped vs Non-enveloped Virus 

1. Some viruses, during lysis, become covered in a membrane (common in viruses affecting animal cells) → “enveloped viruses”

   1. The phospholipid membrane around the virus is derived from the host cell’s phospholipid bilayer

   2. This membrane aids in contact + infection of future host cells

Lytic cycle

1. Attachment

   1. Proteins in the tip of the bacteriophage cell bind to maltoporin - protein in the outer membrane of E. coli for carbohydrate absorption 

2. DNA Entry

   1. Viral DNA molecule enters host cell via penetrated pores in the inner membrane

3. DNA Replication

   1. Ends of the linear DNA molecule join to form circular DNA - this DNA is replicated around 100 times via ‘rolling circle’ method

4. DNA Transcription

   1. Messenger RNA copies of the viral DNA is made → translated to the production of viral proteins

5. Protein Synthesis

   1. Viral proteins are made using host cell ribosomes → large quantities of proteins self-assemble into capsids

6. Lysis 

   1. Viral proteins make holes through the walls/membrane of the host cell

7. Spread

   1. Cell contents burst - with around 100 viruses. These viruses spread to infect other host cells

Lysogenic Cycle 

1. Attachment

   1. Like lysis, proteins in the tail tip bind to membrane proteins

2. DNA Entry

   1. Like lysis, viral DNA molecule enters through inner membrane pores

3. Integration

   1. The viral DNA becomes circular → viral enzyme integrase inserts it into the host DNA → virus only exists from this point on as a length of DNA (a temperature virus existing as a prophage)

4. Cell Division

   1. When the host DNA is replicated it also replicates the prophage

• Prophages are activated in response to interior/exterior stimuli → virus switches to the lytic stage

Progressive Hypothesis

• Viruses arose from genetic elements, such as DNA and RNA, that gained the ability to move between cells

• These genetic elements became surrounded by an outer boundary forming a virus particle

Regressive Hypotheses

• Viruses are remnants of cellular organisms or were once small cells that became parasites of larger cells

• Over time the cellular structures that were no longer needed were shed , leaving behind just viral structures

Rapid Evolution of Viruses

Viruses can evolve extremely rapidly for three reasons: 

1. Evolutionary change is usually limited by generation time - but viral generation times are less than an hour

2. Mutation rates are high in viruses - RNA viruses such as coronavirus do not perform error checks while replicating genetic material

3. Strong natural selection pressures (i.e. host cell’s mechanisms for detecting/destroying viruses → antibodies, etc) encourages rapid evolution; viruses with favourable mutations evade immune systems

Example of rapid evolution of Virus (Influenza) 

• Influenza: an enveloped virus using negative-sense single-stranded RNA as genetic material

  • RNA is replicated using RNA replicase → does not check for errors (high mutation rate)

• Genome consists of 8 separate molecules → combining of RNA molecules can occur if one host cell is infected with different strains of viral RNA → rapid creation of new strains

• 2 proteins in the enveloping membrane of the virus:

  • Haemagglutinin: binds to a host cell

  • Neuraminidase: helps with release from host cell

    • These proteins can be put together in new combinations, creating new virus strains

      • H1N1: Spanish Flu

      • H3N2: Hong Kong Flu

• Vaccination (containing several new strains of influenza) are useful to get every year

Example 2: HIV Virus

• HIV: a retrovirus using reverse transcriptase to convert its single-stranded RNA genome to DNA

  • Reverse transcriptase does not check for errors → many mutations

  • Cytidine deaminase (converts cytosine to uracil) also causes many mutations

    • These 2 factors give HIV the highest known mutation rate of any virus

  • New strains are constructed within a person, evading the immune system → most infections are chronic and incurable

• HIV has a surface protein allowing it to bind/enter host cells 

  • Mutations in the env gene for this protein allow HIV to use different types of cells in the body as hosts

• HIV easily becomes resistant to antiretroviral drugs → drug combinations are necessary for treatment

Virus-First Theory

• Viruses predate their current cellular hosts

• During evolution we expect simpler organisms to give rise to more complex organisms, so the simple nature of virus particles could indicate that viruses evolved first