Viruses

Micro-organisms Study Notes

Term Definitions

  • Termina: A concept in parasitology relating to organisms that depend on a host to survive.

  • Obligate: Refers to organisms that have specific requirements for survival. In this context, it means needing a host.

  • Parasite: An organism that lives off another organism, known as its host, which is harmed by the relationship.

  • Bacteriophage: A virus that specifically attacks bacteria.

Viruses

Classification of Viruses

It is challenging for biologists to classify viruses in terms of whether they are living or non-living entities. They do not fit comfortably into any of the five kingdoms of life due to the following characteristics:

  • Viruses do not directly utilize energy.

  • They cannot maintain their structure actively.

  • Viruses do not partake in essential life processes such as eating, respiration, reproduction, or excretion.

Viruses are termed obligate intracellular parasites because they cannot reproduce independently. Instead, they must hijack the cellular machinery of a host organism to multiply, utilizing the host's genetic material.

Despite these characteristics, viruses contain nucleic acids similar to those found in living organisms, supporting the ongoing debate regarding their classification.

Basic Structure of Viruses

Viruses manifest in various shapes and sizes, illustrating a rich diversity among different viral types, as depicted in Figures 1.1 and 1.2, which illustrate the bacteriophage and the tobacco mosaic virus, respectively. Despite these variations, all viruses share a common basic structure:

  • Inner Core: Comprises either DNA or RNA (but not both). Examples include retroviruses like HIV, which is specifically an RNA virus causing acquired immune deficiency syndrome (AIDS).

  • Protein Coat (Capsid): This outer layer encases the viral genetic material, providing protection. The shape of the capsid can vary among different viruses.

  • Additional Structures: Some viruses possess an additional covering that may include some of the host cell's membrane and various proteins, which can aid in the infection process.

Characteristics of Viruses

  1. Acellular Nature: Viruses are described as acellular because they lack the complete structural characteristics of cells.

  2. Size: They are extremely small, typically ranging from 20 nm to 300 nm in size, making them observable only under an electron microscope.

  3. Diversity in Shape: There exists a significant diversity in the physical form of viruses, further illustrated in Figures 1.1 and 1.2.

Figures

  • Fig. 1.1: Cross section of a bacteriophage, illustrating the structural components such as:
      - Tail fibres
      - Protein capsid
      - Pins
      - Base plate
      - Nucleic acid
      - Collar and sheath

  • Fig. 1.2: The tobacco mosaic virus, demonstrating the differences in structure when compared to the bacteriophage.

Summary of Viral Characteristics

  • Viruses do not have cytoplasm or membrane-bound organelles, reinforcing their classification as acellular entities.

Disease Implications

Understanding the nature of viruses and their structure is critical in medical microbiology, especially in the context of viral pathogenesis and the mechanisms by which viruses interact with host cells, leading to diseases.