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Unit 4: Living organisms and classification

Viruses — HIA International - Grade 6 Revision Notes & Topical Questions

5 min read 6 parts Lesson 4 of 8 2 quizzes

📚 HIA International - Grade 6 Hub / Topical Revision
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Lesson summary

4.2 Viruses — HIA INTERNATIONAL notes: Describe the structure of a virus, how viruses replicate and why scientists disagree about whether they are alive.Size — a virus is much smaller than a cell and can only be seen with an electron microscopeA photograph from an electron micros…

  • Not made of cells
  • Protein coat
  • Needs a living cell
  • H3N2 influenza virus
  • SARS-CoV-2
  • Host cells

Section 1 of 6Structure of a virus

Structure of a virus
Size
  • Size a virus is much smaller than a cell and can only be seen with an electron microscope
  • A photograph from an electron microscope can show viruses 100 000 times bigger than they really are.
Not made of cells
  • Not made of cells a virus has no cell membrane or cytoplasm
Protein coat
  • Protein coat the outer layer of the virus is a coat made of protein
  • Little pegs on the outside of the coat help the virus stick to a cell.
RNA
  • RNA threads of RNA inside the coat carry a set of coded instructions for making more viruses

Section 2 of 6How viruses replicate

Alone
  • Alone a virus cannot respire, feed, excrete, grow, move or respond on its own
Needs a living cell
  • Needs a living cell a virus must get inside a living cell before it can make copies of itself
1
The virus sticks to a cell using the pegs on its coat and gets inside.
2
The virus forces the cell to copy the instructions on its RNA.
3
The cell makes many new viruses, which is called replication.
4
The cell is killed and the new viruses burst out, ready to infect more cells.

Section 3 of 6Examples of viruses

H3N2 influenza virus
  • H3N2 influenza virus invades cells of birds, humans and other mammals and enters the body when breathed in through the nose
  • In 1968–1969 it killed approximately one million people.
SARS-CoV-2
  • SARS-CoV-2 appeared in 2019 and causes Covid-19
    • (coronavirus disease 2019)
  • Spike proteins on the outside help it attach to cells and get inside.
  • Many people get the virus without being ill, but in some it causes dangerous illness and death.
Host cells
  • Host cells each kind of virus infects a particular kind of cell, and some viruses infect plant cells

Section 4 of 6Are viruses alive?

ArgumentReason
Non-livingnot made of cells, and show no life processes of their own
Livingcontain genetic material (RNA) and can reproduce, but only inside a living cell
  • Scientists disagree, so viruses are described as living or non-living depending on the reasons given.

Section 5 of 6Key terms

TermMeaning
VirusA tiny particle with RNA inside a protein coat that can only replicate inside a living cell.
Electron microscopeA special microscope needed to see viruses.
ProteinThe substance that makes the outer coat of a virus.
RNAThreads inside a virus carrying coded instructions for making more viruses.
ReplicateTo make copies.
InfluenzaA disease (flu) caused by a virus.

Section 6 of 6Exam technique

1
Saying viruses are made of cells — they have no cell membrane or cytoplasm
2
Saying a virus can replicate on its own — it can only replicate inside a living cell
3
Saying viruses are clearly living — scientists disagree, since viruses show no life processes alone
4
Saying a light microscope can show a virus — an electron microscope is needed

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