Viruses - Microbiology - Lecture Slides, Slides of Microbiology

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Viruses

Viruses

General Structure of Viruses

-> Size range –

most <0.

μ

m; requires electron microscope

-> Virion

• fully formed virus able to establish an infection

Viruses

General Structure of Viruses

Capsids

• All viruses have capsids - protein coats that enclose and protect their nucleic acid. - Each capsid is constructed from identical subunits called

capsomers

made of protein.

• The capsid together with the nucleic acid are

nucleoscapsid.

• Some viruses have an external covering called

envelope

those lacking an envelope are

naked

Viruses

General Structure of Viruses

Capsids - Envelope

Viruses

General Structure of Viruses

Icosahedral Capsid

Adenovirus

Herpes simplex virus

-> 20-sided with 12 corners

-vary in the number ofcapsomers-Each capsomer may bemade of 1 or severalproteins.- Some are enveloped.

Viruses

General Structure of Viruses

Viral envelope

-> mostly animal viruses-> acquired when the virus leaves the host cell-> exposed proteins on the outside of the envelope, called

spikes

, essential for attachment of the virus to the host cell

Viruses

Viral Morphology

Viruses

General Structure of Viruses

Complex viruses: atypical viruses

-> Poxviruses lack a typical capsid and are covered by a

dense layer of lipoproteins. -> Some

bacteriophages

have a polyhedral nucleocapsid

along with a helical tail and attachment fibers.

Viruses

General Structure of Viruses

DNA viruses

-> usually double stranded (ds) but may be single

stranded (ss) -> circular or linear

RNA viruses

-> usually single stranded, may be double stranded,

may be segmented into separate RNA pieces -> ssRNA genomes ready for immediate translation are

positive-sense RNA. -> ssRNA genomes that must be converted into proper

form are

negative-sense RNA.

Viruses

General Structure of Viruses

Pre-formed enzymes may be present.

-> polymerases – DNA or RNA-> replicases – copy DNA-> reverse transcriptase –synthesis of DNA from RNA

(AIDS virus)

Viruses

How Viruses are classified

Viruses

How Viruses are classified

Viruses

Adsorption and Host Range

Virus coincidentally collides with a susceptible host celland adsorbs specifically to receptor sites on the cellmembrane -> Spectrum of cells a virus can infect –

host range

-> hepatitis B – human liver cells-> poliovirus – primate intestinal and nerve cells-> rabies – various cells of many mammals

Viruses

Penetration

Flexible cell membrane is penetrated by the whole virus or its nucleic acid by: -> endocytosis

• entire virus is engulfed and enclosed in a vacuole or vesicle

-> fusion

• envelope merges directly with membrane resulting in nucleocapsid’s

entry into cytoplasm Endocytosis

Fusion