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Chapter 5: Viruses, Bacteria and Protoctists
Study the first subtopic below. Unlock Kingdom Monera, Protoctista (Amoeba, Euglena, Plasmodium, Trypanosoma) and full assessment in the app.
Key Terms and Concepts
Virus — An extremely small fragment of nucleic acid (DNA or RNA) surrounded by a protein coat. It is not a cell and can only reproduce inside a living host cell.
Capsid — The protein coat that surrounds the genetic material of a virus.
Obligate parasite — An organism that can only survive and reproduce by living inside a host cell (viruses are obligate parasites).
Bacteriophage — A virus that specifically infects and attacks bacteria.
Prokaryotic — Cells that lack a true nucleus and membrane-bound organelles (e.g. bacteria).
Binary fission — A form of asexual reproduction in which a single cell divides into two identical daughter cells.
Protoctista (Protists) — A kingdom of mostly unicellular eukaryotic organisms that are not animals, plants or fungi.
Pseudopodium — Temporary cytoplasmic projections used by Amoeba for movement and feeding (“false feet”).
1. Viruses (Free full subtopic)
What is a Virus?
- A virus is an extremely small fragment of nucleic acid (DNA or RNA) surrounded by a protein coat.
- It is smaller than a living cell and is not a cell itself.
- Viruses lie on the borderline between living and non-living things.
- The study of viruses is called Virology.
Characteristics of Viruses
- They do not have a nucleus, cytoplasm or cell organelles.
- Simple structure: genetic material (DNA or RNA) enclosed in a protein coat called a capsid.
- Some viruses have an outer viral envelope made from the host cell membrane.
- They cannot reproduce on their own — they must invade a host cell and use its materials (obligate parasitism).
- Outside a host cell they are dormant (no growth, feeding, excretion or respiration).
- They can crystallize when outside a living host.
- They are host-specific — a particular virus usually attacks only one type of host.
- Most viruses are infectious and can cause disease.
Spotlight
Viruses are so small that most can only be seen with an electron microscope. Some, like the tobacco mosaic virus, can even form crystals that look like ordinary chemicals — yet they become “alive” once inside a suitable host cell!
How viruses reproduce (simplified lytic cycle)
- Attachment – virus attaches to specific receptors on the host cell.
- Entry – genetic material is injected into the host cell.
- Replication – host cell is forced to make viral nucleic acid and proteins.
- Assembly – new virus particles are put together.
- Release – host cell bursts (lysis) releasing hundreds of new viruses.
Continue this chapter in the app
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What you unlock next:
- Kingdom Monera – bacteria characteristics, shapes (cocci, bacilli, spirilla…), useful & harmful bacteria
- Kingdom Protoctista – Amoeba, Euglena, Paramecium, Plasmodium (malaria), Trypanosoma
- Advantages and disadvantages of each group
- Chapter summary + 10 MCQs + 10 short answers with solutions
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