Understanding Virology Basics

Virology

• Virology is the study of viruses, complexes of nucleic acids and proteins
• Viruses replicate inside animal, plant, and bacterial cells
• To replicate, viruses utilize host cell functions, acting as parasites
• All viruses are obligate intracellular parasites
• Virus replication inside host cells releases virus particles (virions)
• Virions lack metabolism and organelles
• Virions carry genetic information for new virus particle generation after infection
• Virus sizes range from 20 to 450 nanometers (nm)

Chemical Composition of Viruses

• Viruses are minute nucleoprotein particles
• They consist of a nucleic acid core (DNA or RNA, linear or circular) and a protein coat (capsid) surrounding the nucleic acid
• The protein coat is made of identical structural units called capsomeres
• Virus composition, number, and structure differ

Virus Structure

• Some viruses have an envelope of fat and protein molecules
• A virus particle outside the cell is called a virion
• Each virion contains at least one protein synthesized by specific genes in its nucleic acid

Other Virus-like Particles

• Viroids are disease-causing organisms containing only nucleic acid without structural proteins
• Prions are virus-like particles primarily composed of proteins tightly integrated with a small nucleic acid molecule
• Viruses attach to bacteria using a "tail" and proteinaceous "pins"
• The virus tail contracts, penetrating the bacterial cell wall and membrane to inject viral nucleic acids

Capsid Structure and Function

• The capsid is the protein shell enclosing the nucleic acid, also called the nucleocapsid
• The capsid is composed of protein subunits called capsomeres
• The capsid protects the nucleic acid from enzymes
• The capsid has attachment sites for host cells
• The capsid enables the virion to penetrate the host cell membrane and sometimes inject infectious nucleic acid into the cytoplasm

Virus Classification

• Viruses are classified based on the host cell they infect (animals, plants, fungi, bacteria)
• A more useful classification system is based on the nucleic acid type:
  • DNA viruses are further divided into double-stranded and single-stranded types
  • RNA viruses are positive-sense, negative-sense, and double-stranded
  • Reverse-transcribing viruses (retroviruses) include double-stranded, and single-stranded types
• Baltimore classification groups viruses by their nucleic acid type, strandedness, sense, and replication method into seven categories

Structure, Function, and Uses of Viruses

• A virus is a small parasite that cannot reproduce independently

• Once in a susceptible cell, a virus directs cell machinery to produce more viruses

• An entire infectious virus particle (virion) comprises the nucleic acid and protein outer shell

• Simplest viruses have only enough nucleic acid to encode a few proteins (e.g., 4)

• Complex viruses can encode many proteins (e.g., 100-200)

• DNA viruses typically use cellular enzymes in DNA/mRNA synthesis

• All viruses use normal cellular ribosomes, tRNAs, and translation factors in protein synthesis

• Most viruses utilize the cellular machinery for macromolecular synthesis during the infection phase, directing it to synthesize a limited number of viral mRNAs and proteins instead of the thousands of normal cellular macromolecules

Explore the essential concepts of virology, including virus structure, composition, and replication. This quiz covers the nature of viruses as intracellular parasites and the characteristics that define them. Test your knowledge of these microscopic entities and their impact on living organisms.