6.5 Comparison of Bacterial & Viral Structure

Bacteria

• 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

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

Viruses

• 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 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

Retroviruses

• 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)

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