Edexcel International A Level Biology

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6.5 Comparison of Bacterial & Viral Structure

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Comparison of Bacterial & Viral Structure


  • Bacteria are single-celled prokaryotes
  • Prokaryotic cells are much smaller than eukaryotic cells 
  • They also differ from eukaryotic cells in having
    • A cytoplasm that lacks membrane-bound organelles
    • Ribosomes that are smaller (70 S) than those found in eukaryotic cells (80 S)
    • No nucleus, instead having a single circular bacterial chromosome that is free in the cytoplasm and is not associated with proteins
    • A cell wall that contains the glycoprotein murein 
      • Murein is sometimes known as peptidoglycan
  • In addition, many prokaryotic cells also have the following structures
    • Loops of DNA known as plasmids
    • Capsules
      • This is sometimes called the slime capsule
      • It helps to protect bacteria from drying out and from attack by cells of the immune system of the host organism
    • Flagella (singular flagellum)
      •  Long, tail-like structures that rotate, enabling the prokaryote to move 
      • Some prokaryotes have more than one
    • Pili (singular pilus)
      • Thread-like structures on the surface of some bacteria that enable the bacteria to attach to other cells or surfaces
        • Involved in gene transfer during sexual reproduction
    • A cell membrane that contains folds known as mesosomes; these infolded regions can be the site of respiration
  • Some bacteria are disease-causing, or pathogenic, but not all bacteria cause harm to other organisms

Cell Components- Prokaryotic cell

Prokaryotic cells have a peptidoglycan cell wall, no membrane-bound organelles, a circular chromosome, and 70S ribosomes


  • Viruses are non-cellular infectious particles
  • They are relatively simple in structure, and much smaller than prokaryotic cells
  • Structurally they have
    • A nucleic acid core
      • Their genomes are either DNA or RNA, and can be single or double-stranded
    • A protein coat called a ‘capsid’ made of repeating units known as capsomeres
  • They do not possess a plasma membrane, cytoplasm, or ribosomes
  • Some viruses have an outer layer called an envelope formed from the membrane-phospholipids of the cell they were made in
    • The fact that lipid envelopes are formed from the membrane of a viral host cell means that very few plant viruses have lipid envelopes
  • Some contain proteins inside the capsid which perform a variety of functions
    • E.g. HIV contains the enzyme reverse transcriptase which converts its RNA into DNA once it has infected a cell
  • Viruses also contain attachment proteins, also known as virus attachment particles, that stick out from the capsid or envelope
    • These enable the virus to attach itself to a host cell
  • Viruses can only reproduce by infecting living cells and using the protein-building machinery of their host cells to produce new viral particles
  • Viruses are classified on the basis of the genetic material they contain and how they replicate
    • They can be classified into the following categories
      • DNA viruses
      • RNA viruses
      • Retroviruses

HIV Structure

HIV contains RNA as its genetic material. It is surrounded by a protein capsid, as well as having an outer lipid envelope and attachment proteins

DNA viruses

  • They contain DNA as genetic material
  • Viral DNA acts as a direct template for producing new viral DNA and mRNA for the synthesis of viral proteins
  • Examples: smallpox, adenoviruses, and bacteriophages
    • Bacteriophages are viruses that infect bacteria, such as the λ (lambda) phage

Bacteriophage viruses, such as the λ phage, are examples of DNA viruses

RNA viruses

  • They contain RNA as genetic material
    • Most have a single strand of RNA
    • They do not produce DNA at all
  • Mutations are more likely to occur in RNA viruses than DNA viruses
  • Examples: tobacco mosaic virus (TMV), ebola virus


  • Special type of RNA virus that does produce DNA
  • They contain a single strand of RNA surrounded by a protein capsid and lipid envelope
  • Viral RNA controls the production of an enzyme called reverse transcriptase
  • This enzyme catalyses production of viral DNA from the single strand of RNA 
  • The new viral DNA is incorporated into the host DNA using integrase enzymes where it acts as a template to produce viral proteins and RNA
  • Example: HIV (Human Immunodeficiency Virus)

Lytic & Latency

  • Viruses can only reproduce within a host cell as they lack the cellular machinery to do so on their own
  • They can enter a host cell in a variety of different ways
    • Bacteriophages inject their genetic material into bacteria
    • Some animal viruses enter the cell via endocytosis by fusing their viral envelope with the host cell surface membrane
    • Plant viruses will often use a vector such as an insect to breach the cell wall
  • Once inside the host cell one of the following pathways can occur
    • Lysogenic
    • Lytic

Lysogenic pathway

  • Some viruses will not immediately cause disease once they infect a host cell
  • Viral DNA known as a provirus is inserted into the host DNA, but a viral gene coding for a repressor protein prevents the viral DNA from being transcribed and translated
    • Every time the host DNA copies itself, the inserted viral DNA will also be copied
  • This is called latency and the time during which it occurs is known as a period of lysogeny
  • Viruses in a lysogenic state may become activated and enter the lytic pathway
    • Activation may occur as a result of, e.g. host cell damage or low nutrient levels inside a cell

Lytic pathway

  • The viral genetic material is transcribed and translated to produce new viral components
  • These components are assembled into mature viruses that accumulates inside the host cell
  • Eventually the host cell bursts which releases large numbers of viruses, each of which can infect a new host cell
    • Cell bursting is known as cell lysis
  • This typically results in disease

The lifecycle of a virus 1The lifecycle of a virus 2

The life cycle of the λ bacteriophage includes a lysogenic and a lytic pathway

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Author: Marlene

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.