Viruses

Question 1

What are viruses?

Answer 1

• Acellular microorganisms (biological) that cannot survive without a host: no metabolic abilities of their own - “a borrowed life”
• Rely completely on biosynthetic machinery of infected cell to multiply
• Infect all types of cells (anima, plant, bacterial - obligate intracellular parasites
• Viruses are the most abundant biological entities on earth

Question 2

What is the composition of a virus?

Answer 2

1. Genetic material - made from either DNA or RNA
2. Capsid - a protein coat that surrounds and protects the genetic material
   - And in some cases a third part…
3. Envelope of lipids that surrounds the protein coat when they are outside a cell

Naked virus = no envelope
Enveloped virus = envelope

Question 3

Describe the viral capsid

Answer 3

• Capsids are made of multiple units of the same protein building block known as Capsomers
• Capsomers = subunit of the capsid arranged in a precise and highly repetitive pattern around the nucleic acid
• Capsid/capsomers can be arranged in three types of symmetry:
  
  1. Helical (lots of capsomers built together to make the capsids that form this structure)
  2. Icosahedral (has 20 faces, there are lots of axes of symmetry so it is an efficient way of packing lots of proteins)
  3. Complex (head is icosahedral symmetry and tail is helical)

Note: capsomers an also be called capsomeres

Question 4

Describe the viral genome

Answer 4

Made of up nucleic acid:
- Ribonucleic idic (RNA)
- Deoxyribonucleic acid (DNA)

Could be:
- Linear
- Circular
- Segmented

Sizes can vary from 4,000 t0 >1 million nucleotides (3 genes to 100-1,000)

All four possible forms of RNA and DNA are found in viruses:
- Single stranded
- Double stranded
- RNA
- DNA

Question 5

What do viruses infect?

Answer 5

• Viruses infect all cell types ie. eukaryotic and prokaryotic
• Viruses infect all forms of life
• An organism a virus infects is called a “Host organism”
• A cell a virus multiplies in is called a “Host cell”

Question 6

Describe how some viruses infect bacteria

Answer 6

• Bacteriophages = viruses that infect and replicate in bacteria
• Bacteriophages were heralded as a potential treatment for diseases such as typhoid and cholera
• IN 1940, the first electron micrograph of a bacteriophage was published. This silenced sceptics who had argued that bacteriophages were relatively simple enzymes and not viruses.

Question 7

Describe the Lytic (virulent) cycle of bacteriophage infection (this is also how viruses infect us)

Answer 7

Cycle takes around 20-30 minutes
100-200 particles (burst size

1. Attack (to host cell)
2. Penetrate (virus inserts its genome into the host cell)
3. Uncoat
4. Genome replication & Gene expression
5. Assembly (of newly replicated viruses inside the host cell
6. Release (the viruses exit the host cell, opening holes in it to get out)

These steps can happen at the same time or in defined steps

Steps 1/2/3 and 5/6 can occur at the cell surface for some viruses

Question 8

Describe the SARS-CoV-2 virus

Answer 8

It is enveloped
+ssRNA
Linear
Non-segmented
Has spikes
Genome is ~30,000bp’s long (16 protein in the replication module, the rest are structural/accessory proteins)

• Includes RNA polymerase to copy the genome
  
  • Limited proof-reading so makes some errors: generates variants
    - Allows tracking
    - Leads to new strains

> 700m cases
7m deaths

Question 9

Describe the spike

Answer 9

• Critical for attachment and cell entry
• A major target for neutralising immunity
  Viruses must bind to a receptor protein to infect a cell. Vaccines target the spike.
• In humans a common receptor is called ACE2 (found on epithelial cells)

Question 10

Describe the SARS-CoV-2 replication cycle

Answer 10

(SARS-CoV-2 replicates in the cytoplasm)
1. Spike binding to ACE2 receptor
2/3. Cell entry/Fusion
4. - Genome translation. Replication Molecule only
- Viral RNA synthesis
- Viral mRNAs & genome
- Translation
5. Assembly
6. Exocytosis

Question 11

Describe replication of an RNA Enveloped Virus (HIV): Retrovirus that causes AIDS

Answer 11

Host cell: CD4 T cell
Receptor: CD4 receptor

HIV Virus: RNA genome (9 genes) - two identical single strand copies of each

1/2/3. HIV fuses with host cell membrane and digestion of capsid
4. Reverse transcriptase (the dsDNA incorporates itself into the host genome using an enzyme called integridase = provirus)
- Viral proteins and envelope glycoproteins are made from this process as well as vesicles of the usual proteins
5. ?? The replicated stuff combines?
6. New viruses, with viral envelope glycoproteins, bud from the host cell and find another cell to infect.