Bacteria and viruses: structure and life cycles

Bacteria

Bacteria are prokaryotic cells: no nucleus and no membrane-bound organelles.

• Circular DNA in the cytoplasm, often with small rings called plasmids
• 70S ribosomes
• Cell wall made of peptidoglycan (murein)
• Some have a slime capsule, flagella for movement and pili for attachment

Bacteria can be killed or inhibited by antibiotics.

Viruses

Viruses are tiny particles, not cells. Each has nucleic acid (DNA or RNA) inside a protein coat (capsid). Some also have a lipid envelope taken from the host cell membrane, carrying viral glycoproteins. They can reproduce only inside a host cell.

Lytic and latent cycles

• Lytic cycle: the virus attaches to specific receptors on the host cell and its nucleic acid enters. The host's enzymes and ribosomes are used to copy viral nucleic acid and make viral proteins. New viruses are assembled and released, often by bursting (lysis) the cell.
• Latency (lysogeny): the viral DNA is inserted into the host's DNA. In λ phage this is a prophage; in HIV, DNA made by reverse transcriptase forms a provirus. It is copied every time the cell divides but makes no new viruses. Later, often when the host cell is stressed, it becomes active and enters the lytic cycle.

Worked example

Give two structural differences between a bacterium and HIV.

1. A bacterium has a cell wall, cytoplasm and 70S ribosomes; HIV has none of these.
2. A bacterium has circular double-stranded DNA; HIV has single-stranded RNA and reverse transcriptase inside a capsid and envelope.

Answer: Bacteria have ribosomes and a peptidoglycan cell wall; HIV is an RNA virus with a capsid and lipid envelope and no ribosomes.

Check yourself

1. What is the bacterial cell wall made of?
   • Cellulose
   • Peptidoglycan
   • Chitin
   • Phospholipid only
   Show answer

   Peptidoglycan. Bacterial walls are peptidoglycan (murein).

2. Which describes tobacco mosaic virus?
   • Double-stranded DNA with a head and tail
   • Enveloped RNA virus that infects T helper cells
   • A bacterium that infects tobacco plants
   • Single-stranded RNA in a rod-shaped helical capsid, with no envelope
   Show answer

   Single-stranded RNA in a rod-shaped helical capsid, with no envelope. TMV is a helical rod of capsid proteins around RNA.

3. What is the nucleic acid of lambda phage?
   • Single-stranded RNA
   • Double-stranded DNA
   • Two copies of RNA
   • Protein
   Show answer

   Double-stranded DNA. λ phage is a DNA virus of bacteria.

4. Which two viruses in the specification have a lipid envelope?
   • TMV and lambda phage
   • Lambda phage and HIV
   • Ebola virus and HIV
   • TMV and Ebola virus
   Show answer

   Ebola virus and HIV. Ebola and HIV bud out of host cells, taking membrane with them.

5. What happens at the end of the lytic cycle?
   • New viruses are released, often by bursting the host cell
   • Viral DNA is inserted into the host chromosome and stays inactive
   • The host cell divides normally
   • The virus makes its own ribosomes
   Show answer

   New viruses are released, often by bursting the host cell. Lysis releases new virus particles.

6. During latency, what happens to viral DNA?
   • It is destroyed by the host
   • It leaves the cell immediately
   • It is inserted into the host DNA and copied when the host cell divides
   • It makes thousands of new viruses at once
   Show answer

   It is inserted into the host DNA and copied when the host cell divides. Prophage or provirus sits in the host genome.

7. Why are viruses not considered to be cells?
   • They have no cytoplasm or ribosomes and can reproduce only inside host cells
   • They contain DNA
   • They are smaller than eukaryotic cells only
   • They have a cell wall
   Show answer

   They have no cytoplasm or ribosomes and can reproduce only inside host cells. Viruses lack the machinery of a cell.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)
• Open Genetics Lectures (Rotel Pressbooks): One transcription factor can be both an activator and repressor, the lambda phage life cycle
• RCSB PDB-101 Molecule of the Month: Tobacco Mosaic Virus
• MicrobeWiki (Kenyon College): Filoviridae
• CCBC Microbiology: HIV structure and life cycle

Last updated 7 November 2026. Spotted a mistake? Tell us.