Viruses

Part 1 of 5What a virus is

• A virus is an extremely small infectious particle that is not made of cells.
• It has no cell membrane and no cytoplasm.
• A virus is much smaller than one of the body's cells.
• Viruses cannot be seen with a school light microscope.
  • An electron microscope is needed, and it shows the picture on a screen.
  • Viruses in an electron microscope photograph can look 100 000 times bigger than they really are.

Part 2 of 5Structure of a virus

1Protein coat

— the outer layer, made of protein.

2Pegs (spike proteins)1 point

— small projections on the outside of the coat.

• They help the virus stick onto a cell and get inside it.

3RNA1 point

— the substance inside the coat, made of little threads.

• It carries a set of coded instructions for making more viruses.

Feature	Cell	Virus
Cell membrane	Yes	No
Cytoplasm	Yes	No
Nucleus	Most have one	No
Genetic instructions	Yes	Yes, as RNA
Outer layer	Membrane (and a wall in plants)	A coat of protein
Size	Seen with a school microscope	Needs an electron microscope

Part 3 of 5How viruses replicate

• 01Replicate = make copies of itself.
• 02A virus cannot replicate on its own, so it must get inside a living cell first.
• 03Steps of replication:

1. The virus reaches the body, for example influenza viruses go up the nose when a person breathes in.
2. The pegs on the coat stick onto one of the body's cells.
3. The virus gets inside the cell.
4. The virus bursts open inside the cell.
5. The virus forces the cell to copy the instructions on its RNA and make many new viruses.
6. This kills the cell, and the new viruses burst out, ready to infect more cells.

• Damage to many cells makes the infected animal feel ill.

Part 4 of 5Examples of viruses

• Influenza (flu) is caused by influenza viruses.
  • H₃N₂ influenza viruses can invade the cells of birds, humans and other mammals.
  • In 1968–1969 these viruses killed approximately one million people.
• There are thousands of different kinds of virus.
  • Each kind infects a particular kind of cell.
  • Some viruses infect plant cells.
• SARS-CoV-2 appeared in 2019 and causes Covid-19 (coronavirus disease 2019).
  • Scientists think it developed in a wild animal and then spread to humans.
  • Many people have mild symptoms or none, but in some people it causes dangerous illness and death.
  • Vaccination and drugs are used to prevent and treat the illness.
• Antibiotics do not kill viruses, because they only work on bacteria.

Part 5 of 5Are viruses alive?

• Some scientists think viruses are living organisms and others think they are not.
• Reasons for non-living:
  • A virus does nothing on its own: it does not respire, feed, excrete, grow, move or respond to changes.
  • A virus is not made of cells.
• Reasons for living:
  • A virus carries coded genetic instructions in its RNA.
  • A virus reproduces, but only inside a living cell.
• Most biologists classify viruses as non-living, although they sit on the boundary.