Lecture 4 - Viruses, Viroids and Prions

Introduction to Viruses

Viruses are acellular microorganisms and must be considered obligate intracellular parasites. This means that they require a host cell in order to replicate and carry out vital functions. They exhibit a wide range of sizes, morphologies, and types of nucleic acid composition, specifically either RNA or DNA. A complete infectious particle known as a virion is essential for transmitting the genetic material to host cells.

Structure and Size of Viruses

Viruses vary significantly in size, ranging from as small as 24 nm (e.g., Bacteriophage MS2) to larger viruses like the Ebola virus at 970 nm. A typical human red blood cell has a diameter of 10,000 nm for perspective. For instance, bacteriophages, which infect bacteria, can have diameters ranging from 24 nm to 800 nm, while more complex structures like the tobacco mosaic virus and adenovirus arrive at sizes of approximately 250 nm and 90-225 nm, respectively.

Differences Between Bacteria and Viruses

Bacteria and viruses differ in several key aspects:

• Replication Method: Bacteria reproduce via binary fission, whereas viruses must commandeer the host's cellular machinery to replicate.

• Genetic Material: Bacteria contain both DNA and RNA, while viruses only possess either DNA or RNA.

• Size: Bacteria are generally larger, usually 1000 nm or more, while viruses are typically between 30 and 400 nm.

• Antibiotic Sensitivity: Bacteria are sensitive to antibiotics, but viruses are not.

Host Specificity of Viruses

Most viruses are host-specific, targeting a single host organism, yet they can infect a wide array of cell types across different life forms, including eukaryotes (vertebrates and invertebrates, plants, fungi) and prokaryotes (archaea and bacteria). This specificity results from the interaction between viral proteins and host cell receptors.

Viral Structure

Viruses consist of genetic material surrounded by a protective protein coat called a capsid. Some viruses also possess an envelope, which is acquired from the host cell and is composed of lipid membranes with carbohydrate-protein spikes. Virus classification is determined by:

• Genetic Material: Either DNA or RNA, which can be single or double-stranded.

• Capsid Morphology: This can be helical, icosahedral, or complex.

Classification of Viruses

1. Helical Viruses: Spiral-shaped and filamentous, composed of capsomeres. Example: Tobacco Mosaic Virus.

2. Icosahedral Viruses: Characterized by symmetry with 20 faces, composed of capsomeres. Examples include Rabies virus and Adenovirus.

3. Enveloped Viruses: Have an exterior envelope derived from host cell membranes. Examples include HIV and influenza.

4. Complex Viruses: Such as bacteriophages T4 and T7, which have more intricate structures.

Pathogenesis and Viral Life Cycle

The viral life cycle is characterized by several stages:

• Attachment: The virus attaches to specific receptors on the host cell.

• Penetration: The virus enters the host cell via membrane fusion or endocytosis.

• Uncoating and Eclipse: The capsid is removed, revealing the nucleic acid and entering an eclipse phase until replication occurs.

• Synthesis of Viral Components: The host's machinery is utilized to synthesize viral nucleic acids and proteins.

• Assembly: Newly formed viral components assemble into virions.

• Release: Viruses exit the host cell, either through budding (for enveloped viruses) or lysis (for non-enveloped viruses).

RNA Virus Replication

RNA viruses can be classified based on whether they have positive (+) or negative (-) sense RNA. Positive-sense RNA can be immediately translated into proteins, while negative-sense RNA must first be transcribed into a positive-sense strand.

Diseases Caused by Viruses

Viruses can infect various organisms including plants, animals, and bacteria. Examples include:

• Plant Viruses: Tobacco mosaic virus, tomato spotted wilt virus.

• Animal Viruses: Canine distemper virus, parvovirus, human diseases such as measles, smallpox, and influenza. Viruses demonstrate a diverse array of infection strategies, varying from acute infections such as those caused by rhinovirus to chronic infections like HIV.

Oncogenic Viruses

Some viruses are known to cause cancer by integrating oncogenes into the DNA of host cells. Examples of oncogenic viruses include HPV, EBV, and HIV, which can potentially lead to various forms of cancer such as cervical cancer.

Detection and Measurement of Viruses

Viruses can be detected and measured by various methods:

• Plaque assays help visualize viral infectivity by the zones of clearing in cell cultures.

• ELISA methods detect viral proteins or antibodies in clinical samples.

• Advanced techniques like confocal microscopy and electron microscopy are utilized for live cell imaging and morphological studies.

Conclusions

Understanding viruses and their interactions with host cells is crucial for developing treatments, vaccines, and effective measures to prevent viral infections. Their ability to adapt and evolve poses ongoing challenges to public health, necessitating continuous research and surveillance.