Comparison of Bacterial & Viral Structure (Edexcel International A Level Biology): Revision Note

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)

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