Virology

Morphology, ultrastructure of viruses. Classification

Different non-cellular forms of infectious agents Viruses are obligate intracellular parasites that consist of

own genome and proteins Viroids are agents that consist solely of a single molecule of

circular RNA within the proteins Virusoid are viruslike agent that consisted of own nucleic

acid (DNA or RNA) and capsid of virus-”helper”. Prions are infectious protein particles that are composed

solely of protein, they contain no nucleic acid. They cause “slow” diseases such as Crautzfeldt-Jakob disease in human and scrapie in sheep.

Defective viruses are composed of viral nucleic acid and proteins but cannot replicate without a “helper” virus, which provides the missing function. They usually have a mutation or a deletion of part of their genetic material.

Similar features: • non-cellular structure, • obligate intracellular parasitism, • absence own metabolism, • dependency on metabolic processes of cell-host, • possibility to cause infectious process.

Dmitri Ivanovsky(1864-1920)

The first virus was unclosed by Dmitri Ivanovsky, Russian scientist.

He studied cause of disease of tobacco. Ivanovsky showed that a disease of tobacco plant was caused by a virus (tobacco mosaic virus).

Definition

Viruses are non-cellular life forms that have their own genome and can multiply only in host cell, using its metabolic processes

Comparison of viruses and cells

Property Viruses CellsType of nucleic acid DNA or RNA but

not bothAlways contain DNA and RNA

Proteins Few Many

Lipoprotein membrane

Envelope present in some viruses

Present in all cells

Ribosome Absent Present

Enzymes None or few Many

Multiplication by binary fission or mitosis

No Yes

Growth Only within cells Most are freeliving

The history of discovered in virology

1892 – Dmitri Ivanovsky discovered first virus - tobacco mosaic virus that caused disease in plant

1898 – Friedrich Loeffler and Paul Frosch unclosed first virus of animals

1900 – Walter Reed showed that viruses may cause disease in human (yellow fever). Their work demonstrated that viruses could infect more than one animal species and that viral disease could be transmitted to human with arthropods.

1911 – Peyton Rous demonstrated that viruses could cause malignant growths in animals, showing that some viruses cause cancer (chicken Rous’ sarcoma virus).

1915 – Frederick Twort and Felix d’Herelle discovered viruses that infect bacteria (bacteriophages).

Properties of viruses Ultramicroscopic size, ranging from 20 nm up to 450 nm. Can pass through bacterial filters. They are not cells, structure is very compact and economical. Do not independently fulfill the characteristics of life. Are inactive outside of the host cell and active only inside

host cell. Are geometric; can form crystal-like masses. Basic structure consists of protein capsid and nucleic acid. Nucleic acid can be either DNA or RNA but not both. Molecules on virus surface impart high specificity for

attachment to host cell. Lack metabolic processes. Lack machinery for synthesizing proteins. Disjunctive method of reproduction. Like alive organisms: capability for reproduce heredity variability, capacity for evolution

Relative sizes of bacteria and viruses

The electron microscope on microbiology, virology and immunology department of NMU.

The crystalline nature of viruses

Light microscope magnification (l,200x) of purified poliovirus crystals

Highly magnified (150,000x) electron micrograph of the capsids of this same virus, demonstrating their highly geometric nature

Scheme of viral structure

Capsid

Covering Envelope

(not found in all viruses)

Central coreVarious proteins (enzymes)

Virus particle

Nucleic acid molecule (DNA or RNA)

Generalized structure of viruses

a) An simple virus is a naked virus (nucleocapsid) consist of a geometrical capsid assembled around a nucleic acid molecule

b) An enveloped virus is composed of a nucleocapsid surrounded by a flexible membrane called an envelope or supercapsid.

Morphology of viruses

- - -

a)b)

c)

d)

i)f)

g)

h)

An array of virus types

Types symmetry of viral nucleocapsid

Isometric (cubical) – poliovirus Helical – tobacco mosaic virus Combined – T-even bacteriophage

Isometric (cubical) symmetry

Polioviruses

Isometric (cubical) symmetry

Adenoviruses

Helical symmetry

Rhabdovirus Tobacco mosaic virus -

Combine type of symmetry

Structure of viruses

A. Model tobacco mosaic virus that has a helical symmetry with capsid surrounding an RNS genome.

B. Many viruses that infect bacteria, such as the T-even bacteriophages, have complex capsid with DNA contained with a head structure.

C. Model adenovirus that has isometric (cubical) symmetry and is naked virus.

D. Model coronavirus that complex capsid and envelope

Viral nucleic acid. Unique properties

Viruses contain either RNA of DNA, never both

Viral genome may consist of linear or circular double-stranded DNA, single-stranded DNA, single-stranded linear RNA, or double-stranded linear RNA

Some RNA viruses have segmented genome that consist of several molecules of RNA.

Viruses with single-stranded RNA can have positive ore negative genome.

Size and genome content of viruses and bacteria

Size Genome content

Smallest virus 20 nm in diameter 4-5 genes

Largest virus 300 nm in diameter

150-200 genes

E.coli cell 1000 nm in diameter

4000 genes

Relationship of viral size to level of dependency on host enzymes for DNA replication

Virus Increa-sing size

Dependence on host cell DNA synthetic enzymes

Parvovirus Depend totally in host cell enzymes

Polyoma virus

Virus codes for a protein that is involved in start of DNA synthesis, rest depends on host enzymes

Adenovirus Virus codes for proteins involved in initiation of DNA synthesis and also DNA polymerase

Poxvirus Totally independent of host cell enzymes

Polarity of viral RNA

1. Viruses with negative genome has single-stranded RNA with negative polarity. They use molecule RNA just as its genetic material. An mRNA must be transctibed by using the negative strand as a template.

For instance, influenza virus

2. Viruses with positive genome has single-stranded RNA with positive polarity. They use molecule RNA as its genetic material and as mRNA that can connect with ribosomes.

Nucleocapsid of Adenovirus

Naked viruses consist of nucleic acid and proteins.

The viral coat structure surrounded the nucleic acid genome of a virus is called the capsid.

Virions of herpesviruses with supercapsid

The envelope is lipoprotein membrane composed of lipid derived from the host cell membrane, protein that is virus-specific, and glycoprotein in the form of spikes on the surface.

The matrix protein, mediates the interaction between the capsid proteins and the envelope

Enveloped viruses that have supercapsid consist of nucleic acid, proteins, lipids and glycoproteins.

Functions of the viral capsid / envelope

1. Protection of the nucleic acid from the effects of various enzymes and chemicals when virus is outside the host cell.

2. Capsid and supercapsid are responsible for helping to introduce the viral DNA or RNA into a suitable host cell, first by binding to the cell surface and then by assisting in penetration of the viral nucleic acid.

3. Parts of the viral capsid and envelope stimulate the immune system to produce antibodies that can neutralize viruses and protect the host organism against future infections.

Functions of viral proteins

Defence of viral genome Receptors that define spectrum of host cell,

which can be affected by the viruses Attachment on host cell Stabilization of nucleic acid molecules (like

histone proteins in eukaryotic cells) Contraction of bacteriophages tails Enzymes Antigens

Enzymes for viral replication

1. Virion enzymes that are constituents of viral particle. There are DNA and RNA polymerases, nuclease, neuraminidase, RNA-dependent DNA polymerase (reverse transcriptase)

2. Viral-induced enzymes that are coded in viral nucleic acid but are not constituents of virion. There are DNA polymerase of poxviruses and herpesviruses, RNA polymerase of polioviruses.

3. Host cell enzymes. There are DNA polymerase of parvoviruses

Morphology of bacteriophages

Shape Description and examples

Tailed BF DNA genome, double-stranded. Virion complex shaped, binary symmetry, variable number of capsomers .T-even coliphages

Grope A. The tails of the phage are long and contractile

Grope B. Tail is long and noncontrative

Grope C. Tail is very short and noncontrative

Cubic BF DNA of RNA genome, icosahedral virion, cubic symmetry, enveloped or naked (X174).

Filamentous BF

DNA genome, single-stranded. Rod-shaped virion, helical symmetry (M13).

Bacteriophages T2

Features that used for classification of viruses

Type and characteristic of viral nucleic acid Size and shape of virion Presence or absence of envelope Type of nucleocapsid symmetry Strategy of virus genome Antigenic characteristic of virion Tissue that affected by virus Disease that virus causes Geographic areas where virus was obtained

firstly

Animal DNA viruses families

Family Strand type

Capsid type Envelope Size

Common Name of Important Members

Poxviridae Double None + 130-300 Smallpox virus; complex virus; brick-shaped

Herpes-viridae

Double Icosahedral + 150-200 Herpes simplex virus, Varicella zoster virus, Epstein-Barr virus

Adeno-viridae

Double Icosahedral - 70-90 Human adenoviruses

Papova-viridae

Double Icosahedral - 45-55 Human papillomavirus

Hepadna-viridae

double Icosahedral + 42 Hepatitis B virus

Parvo-viridae

Single Icosahedral - 18-26 Parvovirus B19

Animal RNA viruses families (1)

Family Strand type

Capsid Type Envelope Size (nm)

Common Name of Important Members

Picorna-viridae

Single Icosahedral - 20-30 Hepatitis A virus, poliovirus, coxsackieviruses, rhinoviruses

Calciviridae Single Icosahedral - 35-40 Norwalk virus

Togaviridae Single Icosahedral + 45-70 Rubella virus, western equine encephalitis

Flaviviridae Single Icosahedral + 40-70 Yellow fever virus, Japanese encephalitis virus

Paramyxo-viridae

Single Helical + 125-250 Parainfluenza virus, mumps virus, measles virus

Animal RNA viruses families (2)Family Strand

typeCapsid Type

Envelope Size (nm)

Common Name of Important Members

Filoviridae Single Helical + 790-970 Ebola and Marburg viruses

Bunyaviridae Single Helical + 90-100 Bunyamwera virus, Hanta virus

Reoviridae Double Icosahedral - 60-80 Human rotavirus, Colorado tick fever virus

Orthomyxo-viridae

Single Helical + 80-120 Influenza viruses

Animal RNA viruses families (3)

Family Strand type

Capsid Type

Envelope Size (nm)

Common Name of Important Members

Rhabdo-viridae

Single Helical + 60-75 Rabies virus

Retroviridae Single Icosahedral + 100 Human immunodeficiency virus (AIDS), oncoviruses

Arenaviridae Single ? + 50-300 Lassa virus; lymphocytic choriomeningitis virus

Corona-viridae

Single Helical + 80-130 Human infectious bronchitis and corona viruses

General taxonomic categories (for herpesviruses)

Kingdom Vira

Family Herpesviridae

Underfamily Alphaherpesvirinae

Genus Alphaherpesvirus

Species simplex

Type 1, 2