2. Learning Objective
Differentiate a virus from a bacterium.
Identify the viral components
Briefly discuss the viral morphology
3. 1.1 Definition of viruses
• Are infectious agents that are too small to be seen with a light
microscope .
• Acellular (absence of nucleus, organelles, cytoplasm,plasma
membrane).
• No ATP generating metabolism
• Do not undergo binary fission
• Sensitive to interferon
4. 1.2 General Characteristics of Viruses
• Obligatory intracellular parasites
• Contain DNA or RNA
• Contain a protein coat
• Some are enclosed by an envelope
• Some viruses have spikes
• Most viruses infect only specific types of cells
in one host
• Host range is determined by specific host attachment sites and
cellular factors
5. 1.2 General characteristics
• Viruses replicate through replication of their nucleic acid and
synthesis of the viral protein.
• Viruses do not multiply in chemically defined media
• Some viruses have enzymes inside the virion. All ss- RNA viruses with
negative polarity have the enzyme transcriptase ( RNA dependent
RNA polymerase) inside virions.
• Retroviruses and hepatitis B virus contain the enzyme reverse
transcriptase.
6. 1.3 Components of viruses
• Nucleic Acid Core (DNA or RNA)
• Capsid: Surrounding protein
coat
• Envelope: Some viruses have
this additional surrounding lipid
bilayer membrane
• Virion: A complete virus
particle
7. Function
Viruses use their nucleic acids (genome) to replicate themselves in host cells
Capsids also play a key role in the attachment of some viruses. Each capsid is
composed of protein subunits called capsomeres.
Enveloped viruses have a typical bilayer membrane outside their capsids and
acquire their envelope after they are assembled in a host cell and “bud”
through host’s membrane. Helps to protect from drying (enhances
transmission),makes virus more susceptible to chemical agents that
dissolve lipids and helps to attach to host cell membrane.
Nucleocapsid comprises the viral genome together with the capsid
8. Naked: viruses with a nucleocapsid and no envelope
Spikes: projections that extend from the viral envelope that may aid in
attachment to the host cell
Glycoprotein: these surface projections serve to attach virions to specific
receptor sites on susceptible host cell surfaces
19. Viral shape
• Helical capsid: consists of a ribbonlike protein that
forms a spiral around the nucleic acid
• Polyhedral capsid: many-sided, and one of the most
common polyhedral capsid shapes is the icosahedron
• Some viruses have a bullet-shaped capsid and some are
spherical
21. TERMINOLOGIES
• VIRION – a complete viral particle
- in naked viruses virion is identical to the nucleocapsid
- in enveloped viruses must acquire envelope before it is considered a virion
• NUCLEOPCAPSID – a protein-nucleic acid complex
• VIROIDS – consist solely of a single molecule of circular RNA without a protein
coat or envelope
• PRIONS – infectious protein particles composed solely of proteins
22. ATYPICAL VIRUS-LIKE AGENTS
• DEFECTIVE VIRUSES – composed of nucleic acid & proteins but cannot
replicate without a helper virus
• PSEUDOVIRION – contain host cell DNA instead of viral DNA within the capsid
23. 1.5 Host range and specificity of viruses
• Most viruses infect specific host cells ie are host specific. Host specificity is
due to:
• specific attachment sites on the host cells called receptors
• Receptor sites for bacteriophage are found in bacterial cell walls or fimbrae
or flagella
• Animal cell membranes contain receptors for animal viruses availability of
cellular factors required for viral multiplication in the host cells.
