Virology Quiz on Arboviruses

Virology: Chapter 1: The Virus

• A virus is a small infectious agent that replicates only inside living cells of other organisms
• Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea
• Viruses are grouped according to their genetic material: DNA or RNA
• Viruses are too small to be seen with a light microscope
• Viruses are obligate intracellular parasites, requiring living host cells to multiply
• Viruses contain a single type of nucleic acid: either DNA or RNA (not both)
• The virion* consists of a nucleic acid core (genome) surrounded by a protein coat (capsid)
• The nucleocapsid is the capsid together with the enclosed nucleic acid
• Some viruses are surrounded by envelopes of lipid, proteins, and carbohydrates
• Viruses replicate by assembling individual components, not by division
• Viruses do not have their own metabolic machinery; they use the host cell's machinery to produce their components
• Viruses that infect bacteria are called bacteriophages or phages

Introduction

• Viruses are too small to be seen with a light microscope.
• Unlike other infectious agents, viruses require living host cells to multiply
• Viruses are filterable agents, passing through filters that retain bacteria
• A single virus particle (virion) contains a single type of nucleic acid (either DNA or RNA) within a protein coat (capsid)
• The capsid, along with the nucleic acid, is called the nucleocapsid
• Some viruses have an envelope surrounding the nucleocapsid
• Viruses replicate by assembling individual components, not through cell division
• Viruses lack their own metabolic machinery, requiring the host cell's systems for replication
• Bacteriophages are viruses that infect bacteria

Bacteria vs viruses

Structure of a virus

• The virion consists of a nucleic-acid core (genome) and a protein coat (capsid)

• The capsid + enclosed nucleic acid = nucleocapsid

• Some viruses have an envelope surrounding the nucleocapsid

• Enveloped Viruses: Have an outer lipid membrane derived from the host cell

• Non-enveloped Viruses: Lack an outer lipid membrane

Viral Structure

• Viral structure is characterized by symmetry(icosahedral symmetry, helical symmetry, or complex symmetry), which the arrangement of capsomeres (protein subunits) determines.
• The nucleocapsid=capsid + genome
• The presence of an envelope, derived from the host cell's membrane, is a characteristic feature of some viruses
• Attachment proteins project from the capsid and bind to host receptors

Viral Classification

• Viruses are classified by their morphological type (helical, polyhedral, enveloped, complex)
• Helical viruses have rod-like shapes, either rigid or flexible
• Polyhedral viruses are multi-sided, often icosahedral (with 20 triangular faces)
• Enveloped viruses have an outer lipid envelope
• Complex viruses have complicated structures, such as bacteriophages

1 - The Capsid

• The capsid is a protein coat surrounding the nucleic acid core
• It is made up of repeating protein subunits called capsomeres
• Capsid functions:
  • Impenetrable protective shell for the nucleic acid core
  • Facilitates viral entry into host cells by adsorption to cell surfaces
  • Protects the nucleic acid from nucleases in biological fluids

Basic Virus Structure

• DNA or RNA + Capsid protein → Nucleocapsid = Naked capsid virus
• Nucleocapsid + Lipid membrane, glycoproteins → Enveloped virus

2 - The Envelope

• Enveloped viruses have an outer envelope derived from the host cell membrane, often with viral glycoproteins
• Viruses lacking an envelope are termed "naked" or "non-enveloped" viruses
• The lipid bilayer component of the envelope is usually of host cell origin

3 - Viral Symmetry

• Icosahedral symmetry: Spherical or polyhedral shape. Typical for many viruses.
• Helical symmetry: Rod-like or filamentous structure.
• Complex symmetry: Irregular shape, often found in bacteriophages

Viral Multiplication

• Lytic cycle: Virus replicates and lyses the host cell
• Lysogenic cycle : The viral DNA is integrated into the host DNA, and the virus replicates along with the host cell.

4 - Viral Nucleic Acid, Proteins, and Lipids

• Viruses contain nucleic acids (DNA or RNA); the type and number of nucleic acids vary among viruses
• Viral proteins are involved in forming the capsid
• Viral proteins have antigenic specificity that varies between viruses
• Enveloped viruses contain lipids derived from the host cell membrane

1- The Capsid

• The nucleic acid in a virus is surrounded by a protein coat called the capsid,
• The capsid is composed of protein subunits (polypeptides) called capsomeres,
• The arrangement of capsomeres is characteristic of a particular type of virus
• The capsid protects the nucleic acid and helps in the attachment of the virus to the host cell.

Stages in a viral replication cycle

• Each viral replication cycle has distinct phases :
  • Attachment : Virus binds to specific receptors on host cell surface
  • Penetration : Virus enters cell by endocytosis or fusion.
  • Uncoating: Release of viral nucleic acid from its protein coat
  • Biosynthesis: Synthesis of viral components (DNA/RNA and proteins)
  • Assembly : Assembly of newly made viral components into complete virions
  • Release : Release of nascent virions (lysis or budding)

Lytic Cycle

• Phage-encoded lysozyme weakens the cell wall, and leads to lysis of the cell
• Phage-encoded lysozyme → the host cell releases numerous viral progeny (infectious virions
• The phage genes cause host cell's protein and nucleic acid synthesis to shut off

Adsorption (attachment)

• HIV attaches via specific tail fiber proteins
• The virus particle gains entry to the cell through interaction with complementary receptors on its surface.

Replication (DNA Viruses)

• The DNA of animal cells, unlike that of bacteria, replicates in the nucleus
• Messenger RNA passes to ribosomes in the cytoplasm for translation
• The translation products are returned to the nucleus for assembly into new virus particles
• Release of naked (non-enveloped) viruses occurs by host cell lysis, while enveloped viruses bud out from the host cell membrane.

DNA Viruses (Deoxyriboviruses)

• Family name, genus name, example virus, size, molecular mass, DNA structure.

RNA Viruses (Riboviruses)

• Family name, genus name, example virus, size, RNA structure.

Viral Symmetry

• The arrangement of the viral capsid around the nucleic acid core determines the symmetry of the capsid.
• Icosahedral symmetry (spherical viruses). Many viruses are icosahedral
• Helical symmetry (rod-like or filamentous viruses).

Viral Nucleic Acid, Proteins, and Lipids

• All viruses contain either DNA or RNA, NOT both.
• The genome can be single-stranded or double-stranded, linear or circular.
• Viral proteins constitute the capsid, protecting the nucleic acid, and possessing antigenic specificity that varies among viruses.
• Enveloped viruses have an outer lipid membrane derived from the host cell membrane.

2- The Envelope

• The virion envelope, usually made of lipids, proteins, and glycoproteins, is derived from the host cell membrane
• Glycoproteins (projections) play a role in attachment.

• Note:* This is a summary of the provided text. Some details, particularly specific examples, were omitted for brevity and conciseness. Please be sure to consult the original source material or use separate study notes from videos or articles for a complete understanding.