TRANSCRIBED GEN CAMATO  

 INTRODUCTION  TO  VIROLOGY    MYCOLOGY  AND  VIROLOGY  |  LECTURE  

 

1  

Virus particle

Covering

Capsid  

Envelope  (not  found  in  all  

viruses)  

Central  core  

Nucleic  acid  molecule    

(DNA  or  RNA)  

Various  proteins  (enzymes)  

Viral sizes compared to RBC and Ecoli

VIRAL STRUCTURE HELICAL ICOSAHEDRAL

Schematic diagram illustrating the components of the complete virus particle (the virion). A: Enveloped virus with icosahedral symmetry. B: Virus with helical symmetry

Viruses • Non-cellular form of life • Smallest infectious agents (ranging from • About 20 nm to about 300 nm in diameter)

• Contain only one kind of nucleic acid (RNA or DNA) as their genome. • Obligate intracellular parasites. • Exist as inert particles (virions) outside the cell

¶ Since they have no nuclei (viruses are biologically inert when outside the host cell) they are usually called the boundary between biotic and abiotic elements.

¶ Viruses only become a “living thing” once inside a host cell; viruses are entirely dependent on the host cell for all metabolical functions

• Virus particles (virions) themselves do not 'grow' or undergo division. • Viruses lack the genetic information which encodes apparatus

necessary for the generation of metabolic energy or for protein synthesis (ribosomes)

VIRAL MORPHOLOGY ! Virion

• Is a complete, fully developed, infectious viral particle • Nucleic acid + protein coat

VIRAL STRUCTURE ! Nucleic Acid

• Virus can have either DNA or RNA but never both • Can be single-stranded or double-stranded

r Double-stranded DNA r Single-stranded DNA r Double-stranded RNA r Single-stranded RNA

! Capsid

• This is the protective protein shell surrounding the viral genome • Capsids are typically formed from a small number of protein subunits

(capsomers), which are assembled into repeating, symmetrical structures

• Capsid symmetry r One of the basis of classification. It could be Helical or

Icosahedral

! Helical

• Nucleic acid (RNA) is attached to helix • It is also pleomorphic; it can have different shapes • In terms of flexibility, one with a helical symmetry Is

more flexible while one with an icosahedral symmetry is fixed and more rigid

• Most of RNA with helical symmetry are enveloped

! Icosahedral  

• With hollow space inside • Contains 20 equilateral triangles/facets  

! Envelope

• It is an additional outer membrane • Contains proteins determined by the viral nucleic acid and materials

derived from normal host cell components • Spike – carbohydrate-protein complexes that may project from the

surface of the envelope Functions of Capsid/Envelope

• Protect the nucleic acid from the effects of various enzymes and chemicals outside the host cell

• Help introduce the viral DNA or RNA into a suitable host cell • Stimulate the immune system to produce antibodies

Non-enveloped/Naked virus I Capsid + Nucleic acid Enveloped virus I Envelope + capsid + Nucleic acid

TRANSCRIBED GEN CAMATO  

 INTRODUCTION  TO  VIROLOGY    MYCOLOGY  AND  VIROLOGY  |  LECTURE  

 

2  

NOTE: Ê Which is more susceptible to the adverse effects of the environment?

An enveloped or naked virus? [ An Enveloped Virus *ANSWER [ An envelope is made up of a double lipid layer which is more easily

destroyed unlike the capsid which is made up of proteins

[ A Naked virus such as that found in the GIT can withstand the adverse effects of the environment. It can also thrive longer in the GIT because the capsid with protein components is not easily destroyed

Naked virus/Non-enveloped virus Enveloped virus

� Easily spread (fomites, hand-hand, dust, small droplets)

� Can dry out and still retain infectivity

� Survives adheres conditions of the GIT

� Resistant to detergents

� Requires a wet environment for survival

� Can’t survive the GIT � It initiates a cell-mediated immune

response

HOW VIRUS CLASSIFIED & NAMES

! Earlier virus classification was based on the virus pathogenic property, organ tropism and transmission characteristics.

! The main criteria of virus classification are the following: 1. The type of nucleic acid which is found in the virion (RNA or

DNA) 2. The symmetry and shape of the capsid 3. The presence or absence of an envelope 4. The size of the virus particle

TAXONOMY

• ORDER “virales” Mononegavirales

• FAMILIES

“viridae” Rhabdoviridae Subfamilies – suffix “virinae”

• GENUS

Suffix “lyssavirus”

• SPECIES Has no clear cut pattern May be according to place of discovery, organ which it

affects, commonality or its discoverer CLASSIFICATION ! DNA viruses

• Almost all DNA viruses which infect animals contain double-stranded DNA. Exception is Parvoviridae

• REMEMBER: HHAPPy

Herpesviridae Papoviridae � Hepadnaviridae � Adenoviridae

� Parvoviridae � Poxviridae

! RNA viruses • Broadly subdivided as follows:

¶ Viruses with positive strand (+) RNA genomes: [ Genomes of the same polarity as mRNA and

thus can be immediately translated by the host cell

¶ Viruses with negative strand (-) RNA genomes: [ Genomes of opposite polarity to mRNA and must

be converted positive-sense RNA by an RNA polymerase before translation

¶ Ambisense genomes: [ Contain both (+) and (-) strand RNAs

• Most are ¶ Single-stranded ¶ Enveloped ¶ Show helical capsid symmetry ¶ Replicate in the cytoplasm

• Exceptions: 1) Reoviridae are double-stranded. 2) 3 are nonenveloped:

[ Picorna [ Calici [ Reoviridae.

3) 5 have icosahedral symmetry: [ Reo [ Picorn [ Toga [ Flavi [ Calici * (Rhabdo has helical symmetry but shaped like a bullet)

4) 2 undergo replication in the nucleus: [ Retro [ Orthomyxo

TRANSCRIBED GEN CAMATO  

 INTRODUCTION  TO  VIROLOGY    MYCOLOGY  AND  VIROLOGY  |  LECTURE  

 

3  

• Adsorp>on  

• Uncoa>ng  

• Transcrip>on,  Transla>on,  Replica>on  

• Assembly  and  Release  

VIRAL REPLICATION

! ADSORPTION ! UNCOATING

• The nucleic acid is released from the capsid into the nucleus or cytoplasm.

! TRANSCRIPTION AND TRANSLATION

! ASSEMBLY AND RELEASE

• The structural proteins and genome (RNA or DNA) assemble into the intact helical or icosahedral virion.

• The virion is then released. • Naked Virions:

¶ The cell may lyse and release the virions, or the virions may be released by reverse phagocytosis (exocytosis)

• Enveloped Virions: ¶ The newly formed naked virion acquires its new “clothing”

by budding through the Golgi apparatus, nuclear membrane, or cytoplasmic membrane, tearing off a piece of host cell lipid bilayer as it exits

HOST CELL OUTCOME

• Death ¶ With the viral infection, the host cell’s own function shuts

down as the cell is commandeered for virion replication. This can result in cell death.

• Transformation

¶ Infection can activate or introduce oncogenes. This results in uncontrolled and uninhibited cell growth.

• Latent infection

¶ The virus can survive in a sleeping state, surviving but not producing clinically overt infection. Various factors can result in viral reactivation.

• Chronic slow infection

¶ Some viruses will cause disease only after many years, often decades, of indolent infection.

“Whoever restrains his words has knowledge, and he who has a cool spirit is a man of

understanding.”

- Proverbs 17:27