3 Viruses Viruses: are noncellular or Acellular infectious agents Virology:study of virusesVirologists:scientists who study viruses
4 General features of Viruses Viruses are infectious agents with both living and nonliving characteristics.1. Living characteristics of virusesa. They reproduce at a fantastic rate, but only in living host cells.b. They can mutate.
5 …General features of Viruses 2. Nonliving characteristics of virusesThey are acellular, that is, they contain no cytoplasm or cellular organelles.They carry out no metabolism on their own and must replicate using the host cell's metabolic machinery.
6 …General features of Viruses Virus particles contains either DNA or RNA (not both)Nucleic Acid is surrounded or coated by a protein shell (capsid)Some viruses possess a membrane-like envelope surrounding the particle
7 …General Properties of viruses Consists of 1 molecule of DNA or RNA enclosed in coat of proteinMay have additional layersCannot reproduce independent of living cells nor carry out cell division as procaryotes and eucaryotes doAn intact viral particle is called a virion.
11 The Structure of Viruses Virion size range is ~ nmAll virions contain a nucleocapsid which is composed of nucleic acid (DNA or RNA) and a protein coat (capsid)Some viruses consist only of a nucleocapsid, others have additional componentsEnvelopesvirions having envelopes = enveloped virusesvirions lacking envelopes = naked viruses
12 VIRAL ENVELOPESMany viruses that infect humans and other animals are enveloped.Envelopes form when viral glycoproteins and oligosaccharides associate with the plasma membrane of the host cell.All envelopes have a phospholipid bilayer.
14 ENVELOPE GLYCOPROTEINS They are firmly embedded in the envelope bilayer.This is facilitated by domains of host membrane proteins called spanners.They can form spikes or other structures on the outside of the virion.These can be used to attach to a host cell.
16 GENOMIC PACKAGINGGenome packaging has an important role in the infection.Viral genomes are packaged in one of three ways:Directly in the capsid-inner side of the protein coatEnclosed in special proteins-nucleic acid binding proteinEnclosed in proteins from the host cell
23 THE INFECTION CYCLEThe infection cycle was first worked out in bacteriophages (bacterial viruses).Animal virus infections can be either lytic or lysogenic.
24 LYTIC VERSUS LYSOGENIC INFECTION In a lytic infection, the host cells fills with virions and bursts.The result is cell death.Lysogenic infections are also known as latent infections.The viral genome becomes incorporated into the host cell’s DNA.It can remain this way for an extended period.The host cell lives.