By:

Mohd Fahmi Mastuki

Department of Medical Laboratory Technology

Faculty of Health Sciences, UiTM

MLT503 LECTURE 2

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1. Describe the general properties of viruses

2. Describe the structure of viruses

3. Describe the virus genomes

4. State the effects of physical and chemical

agents on viruses

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Learning Outcomes

At the end of the session, you should be able to:

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Virology – study of viral nature, and the relationship between viruses and hosts.

Viruses often cause serious diseases, relate to some cancers and congenital deformities, also can be used as tool for genetic engineering.

Virology

Recognition of viruses

F How long viruses have been within our midst?

1500 BC: Leg deformities indicative of poliomyelitis, pock marks indicative of smallpox

and.

During the 1800's, all infectious agents were considered to be viruses until Koch

developed pure culture techniques which allowed the separation and growth of bacteria. In the

late 1800's: Bacteria were purified and established as disease causing agents. It then became

possible to distinguish them from the "filterable agents", those able to pass through special

filters designed to prevent the passage of bacteria. The first viruses described were foot and

mouth disease (a picornavirus), 1898, Yellow fever (a flavivirus), 1900, Rous sarcoma virus (an

oncogenic retrovirus), 1906.

"Virus" is from the Greek

meaning for "poison" and

was initially described by

Edward Jenner in 1798.

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Viruses may be defined as acellular organisms

whose genomes consist of nucleic acid, and which obligately replicate inside host cells using host metabolic machinery and ribosomes to form a pool of components which assemble into particles called VIRIONS, which serve to protect the genome and to transfer it to other cells.

Definition of Virus VIRUS (latin: poison)

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The Nature of Viruses

http://www.sciencekids.co.nz/pictures/health/smallpoxvi

rus.html

http://www.knowledgerush.com/kr/encyclopedia/Polio_virus/

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The comparative sizes of several viruses and bacteria:

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General properties of viruses

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Structure of Viruses

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Virion

- The complete infectious unit of virus particle

- Structurally mature, extracellular virus particles.

Structure of virions (extracellular state of viruses)

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Viral Genome

Only one type of nucleic acid is found in the virion of any particular type of virus.

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A virus can have either DNA or RNA but never both !!

http://www.daviddarling.info/encyclopedia/V/virus.html

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Genome in virion

DNA RNA

double-stranded single-strandeddouble-

strandedsingle-stranded

linear circular linear circular linear linear (circular)

single single multiple single single multiple single multiple (+)sense (-)sense

single multiple single multiple

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Viral Capsid

The protein shell, or coat, that

encloses the nucleic acid

genome - protection.

Functions:

1. Protect the viral nucleic acid.

2. Participate in the viral

infection. 3. Share the antigenicity

– Core + capsid = nucleocapsid– In many viruses important in attachment of viruses to specific receptors on host cells– Composed of a large number of subunits – capsomeres

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One or more lipoprotein layers surround the

nucleocapsid in enveloped viruses.

The nucleocapsid is symmetrical, with a

precise number and arrangement of

structural subunits surrounding the virus

nucleic acid

SYMMETRY OF THE NUCLEOCAPSID

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Viral Envelope

Surrounds the capsid in several virus families, is

always dependent on cellular membranes

(nuclear or cell membrane, less frequently

endoplasmic reticulum).

Both cell-coded and viral proteins are integrated

in the membrane when these elements are

transformed into the envelope.

Enveloped viruses do not adsorb to the host cell

with the capsid, but rather with their envelope.

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VIRUS CLASSIFICATION

RSV

History - Naming of VirusesUsually based on data available when a virus is discovered:

Diseases viruses are associated with, e.g.:

Poxvirus, Hepatitis virus, HIV, Measles virus.

Cytopathology occuring during infection, e.g.:

Respiratory Syncytial virus, Cytomegalovirus.

Site of infection, e.g.:

Adenovirus, Enterovirus, Rhinovirus, Enterovirus.

Places where viruses were found or people who

discovered them, e.g.:

Epstein-Barr virus, Rous Sarcoma,

Biochemical features, e.g.:

Retrovirus, Picornavirus.

Such names are not useful for orderly classification!!!!

These naming conventions can lead to confusion later e.g.: viral hepatitis is caused by at least 6 different viruses

D

“Infectious”

“Serum”

Viral hepatitis

A

NANB

B

Enterically

transmitted

Parenterally

transmitted

E

C

F, G,

? Other *

* 10-20% of cases of presumed viral hepatitis are still not accounted for.

Related Herpesviruses Cause Many Different Diseases

• HSV Herpes Simplex Virus Cold sores (type 1),

Genital lesions (type 2)

• VZV Varicella Zoster Virus Chicken pox

• CMV Cytomegalovirus Mononucleosis

• EBV Epstein-Barr Virus Mononucleosis,

Burkitt’s lymphoma,

• Nasopharyngeal carcinoma

• and HHV-6, HHV-7, HHV-8…..

(Human HerpesVirus-#)

Therefore if these viruses were classified based on

their symptoms their relationships would be missed.

Different viruses can cause (nearly) the same

symptoms. e.g., the hepatitis viruses

However, different members of the same group

can cause different symptoms. e.g., the herpes

viruses

Thus,

So virologists had to devise more

orderly schemes for classification

Meeting Classification Needs• A universal system of viral classification and a unified

taxonomy was established by the International Committee on Taxonomy of Viruses (ICTV) in 1966. The system makes use of a series of ranked taxons, with the:

• - Order (-virales) being the highest currently recognized.

• - then Family (-viridae)

• - Subfamily (-virinae)

• - Genus (-virus)

• - Species ( eg: tobacco mosaic virus)

The ICTV seeks input from a wide range of virologists and meets every three years to revise

the taxon.

By the year 2000, over 4000 viruses of plants, animals and bacteria had been included in 71 families, 9 subfamilies and 164 genera.

Virus Classification is Based Primarily on Properties of the Virus Particle.

Genetic material Is Most Important!!!form of nucleic acid

ssDNA (+ or - strand)dsDNA ssRNA (+ or - strand) dsRNAsegmented RNA

genetic organizationsequence homology

DNA sequence Hybridization

Morphology:by electron microscopy

Serology:antigenic cross-reactivity

Biological Properties have a low priority

Rotavirus

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BALTIMORE SCHEME – LATEST CLASSIFICATION

Proposed by nobel laureate winner (David Baltimore)

-Nature of genome-Mode of replication-Gene expression

Revised Baltimore scheme-Based on mRNA in replication cycle-7 clases

The Baltimore classification systemBased on genetic contents and replication strategies of viruses. According to

the Baltimore classification, viruses are divided into the following seven

classes:

1. dsDNA viruses

2. ssDNA viruses

3. dsRNA viruses

4. (+) sense ssRNA viruses (codes

directly for protein)

5. (-) sense ssRNA viruses

6. RNA reverse transcribing viruses

7. DNA reverse transcribing viruses

where "ds" represents "double strand" and "ss" denotes

"single strand".

(+) RNA is the one which can function as mRNA for the

synthesis of proteins

(-) RNAcannot function as mRNA

Mammalian RNA Virus Families

Several general features are evident from the classification:

None of the dsRNA viruses are enveloped.

The minus-strand viruses are enveloped with helical nucleocapsids.

Most of the plus-strand strand viruses have icosahedral nucleocapsids.

Plus strand viruses vary in having envelopes.

Most of the plus strand viruses have a single genomic RNA.

Animal DNA Viruses

DNA Viruses differ in many features from RNA Viruses:

Only three families are enveloped.

All families except for the poxviruses replicate in nuclei.

Many families have very complex nucleocapsids.

Comparisons of Vertebrate and Plant Viruses

Vertebrate Infecting Viruses Plant Infecting Viruses

RN

A

Comparison of Vertebrate and Invertebrate Viruses

Invertebrates have DNA Viruses that are not found in Vertebrates

and fewer families of minus strand viruses.

Comparison of Vertebrate and Bacterial Viruses

Major differences between vertebrate and bacterial viruses

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SUBVIRAL PARTICLES

Viroids and Prions

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EFFECTS OF PHYSICAL AND CHEMICAL AGENTS ON VIRUSES

49Fig. 11.1 Microbial control methods

An overview of the microbial control methods.

Several factors influence the effectiveness of antimicrobial treatment.

1. Number of Microbes: The more microbes present, the more time it takes to eliminate population.

2. Type of Microbes: Endospores are very difficult to destroy. Vegetative pathogens vary widely in susceptibility to different methods of microbial control.

3. Environmental influences: Presence of organic material (blood, feces, saliva) tends to inhibit antimicrobials, pH etc.

4. Time of Exposure: Chemical antimicrobials and radiation treatments are more effective at longer times. In heat treatments, longer exposure compensates for lower temperatures.

Control of Microbial Growth:Rate of Microbial Death

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