Viruses are genetic elements that contain either RNA or DNA that replicates intracellularly
but has an extracellular state
Viruses originate for 3 theories
o Regressive theory - degenerated from cellular form of intracellular parasites
o Progressive theory - evolve from cellular nucleic acids that gained the ability to
replicate autonomously
o Co-evolution theory - viruses co evolved with cellular life forms
Virus Morphology
Viruses consist of nucleic acid and a capsid with or without an envelope
The capsid
o The protein coat surrounding the nucleic acid core that protects nucleic acid from
inactivation
o It helps to introduce genome to host cell
Capsomeres
o The repeating subunits that make up the capsid
o Symmetrically arranged to form an impenetrable shell
Protomers
o The polypeptide chain which makes up the capsomeres
The viral envelope
o Derived from the host cell membrane
o Protein subunits seen as projecting spikes on the surface envelope
o Confers chemical, antigenic and biological properties
o More susceptible to lipid solvents
Viruses has two types of symmetry which form the capsid
o Icosahedral
o Helical
The naked capsid virus is made from
o DNA or RNA + capsid protein = nucleocapsid
, The enveloped virus is made from
o Nucleocapsid + lipid membrane glycoproteins
Virus Classification
Viruses are classified by
o The type of viral nucleic acid
o The symmetry and shape of capsid
o The presence or absence of an envelope
o The size of the virus particle
o Serology
Viruses can be classified into 7 arbitrary groups
o Double stranded DNA
o Single stranded (+) sense
o Double stranded RNA segmented genomes
o Single stranded (+) sense RNA
o Single stranded (-) sense RNA
o Single stranded (+) sense RNA with DNA intermediate
o Double stranded DNA with RNA intermediate
Viral Life Cycle
Viruses have two stages
o Metabolically inactive
o Active
Viruses enter the host cell by
o Surface fusion
o Fusion in endosome
o Lysis of endosome
The life cycle of a virus consists of
o Attachment and adsorption
o Penetration
o Uncoating
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