Learning Objective Differentiate a virus from a bacterium. Identify the viral components Briefly discuss the viral morphology
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
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
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.
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
FunctionViruses use their nucleic acids (genome) to replicate themselves in host cellsCapsids 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
Naked: viruses with a nucleocapsid and no envelopeSpikes: projections that extend from the viral envelope that may aid in attachment to the host cellGlycoprotein: these surface projections serve to attach virions to specific receptor sites on susceptible host cell surfaces
Viral shape• Helical capsid: consists of a ribbonlike protein thatforms a spiral around the nucleic acid• Polyhedral capsid: many-sided, and one of the mostcommon polyhedral capsid shapes is the icosahedron• Some viruses have a bullet-shaped capsid and some arespherical
Viral genome structure
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
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
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.