Introduction to Virology: Viral Structure

• Viruses are acellular microorganisms, meaning they lack a cellular structure
• Viruses consist of nucleic acid (DNA or RNA) surrounded by a protein coat called a capsid
• The study of viruses is known as virology

Viral Structure

• Virions are individual virus particles
• The main components of a virion are the nucleic acid (genome) and the capsid
• The viral genome contains the genetic information, which can be either DNA or RNA, single-stranded or double-stranded
• The capsid is a protective protein shell that encloses the viral genome
• Capsids are made up of protein subunits called capsomeres
• Some viruses have an envelope, which is a lipid bilayer derived from the host cell membrane
• Viral envelopes contain viral glycoproteins that aid in attachment to host cells
• Viruses are classified based on the type of nucleic acid, the structure of the capsid, and the presence or absence of an envelope
• Common viral shapes include helical (e.g., tobacco mosaic virus), icosahedral (e.g., adenovirus), and complex (e.g., bacteriophages)
• Viral size is much smaller than bacteria, typically ranging from 20 to 300 nanometers

Viral Replication

• Viruses are obligate intracellular parasites, meaning they can only replicate inside a host cell
• The viral replication cycle involves several steps:
  • Attachment
  • Penetration
  • Uncoating
  • Replication
  • Assembly
  • Release
• Attachment is when the virus binds to specific receptors on the host cell surface
• Penetration is the entry of the virus into the host cell, which can occur through:
  • Direct penetration
  • Endocytosis
  • Membrane fusion
• Uncoating involves the release of the viral genome from the capsid
• Replication is the synthesis of viral nucleic acid and proteins using the host cell's machinery
• Assembly is the packaging of the viral genome into new virions
• Release is the exit of new virions from the host cell, which can occur through:
  • Lysis (bursting of the cell)
  • Budding (enveloped viruses)

Viral Classification

• Viruses are classified based on several criteria, including:
  • Type of nucleic acid (DNA or RNA)
  • Strandedness of nucleic acid (single-stranded or double-stranded)
  • Presence or absence of an envelope
  • Capsid shape
  • Host range (types of organisms infected)
• The International Committee on Taxonomy of Viruses (ICTV) is responsible for developing and maintaining the viral classification system

Viral Diseases

• Viruses can cause a wide range of diseases in humans, animals, and plants
• Viral diseases can be acute (short-term) or chronic (long-term)
• Common human viral diseases include:
  • Influenza (flu)
  • Common cold
  • Measles
  • Mumps
  • Rubella
  • Chickenpox
  • Herpes
  • HIV/AIDS
  • Hepatitis
  • COVID-19
• Viral infections can be prevented through vaccination, antiviral medications, and hygiene practices

Bacteriophages

• Bacteriophages (or phages) are viruses that infect bacteria
• Phages can be either lytic or temperate
• Lytic phages replicate inside the host bacterium and cause lysis, resulting in the death of the bacterium
• Temperate phages can integrate their DNA into the host bacterium's chromosome, becoming a prophage
• Prophages can remain dormant for extended periods, and under certain conditions, they can become lytic
• Phage therapy is the use of bacteriophages to treat bacterial infections

Antiviral Drugs

• Antiviral drugs are medications used to treat viral infections
• Many antiviral drugs target specific steps in the viral replication cycle
• Some antiviral drugs are nucleoside analogs, which interfere with viral DNA or RNA synthesis
• Other antiviral drugs inhibit viral enzymes, such as reverse transcriptase or protease
• Antiviral resistance can occur when viruses mutate and become less susceptible to the effects of antiviral drugs

Viral Evolution

• Viruses evolve rapidly due to their high mutation rates and short generation times
• Viral evolution can lead to:
  • Emergence of new viral strains
  • Increased virulence (ability to cause disease)
  • Antiviral resistance
• Antigenic drift is the gradual accumulation of mutations in viral genes, leading to minor changes in viral proteins
• Antigenic shift is the sudden reassortment of genetic material between different viral strains, leading to major changes in viral proteins