VIRUSES CLASSIFICATION , LIFE CYCLE OF VIRUSES. CHARACTERISTICS OF VIRUSES

VIRUSES
LIFE CYCLE OF BACTERIOPHAGES

The word virus is derived from Latin word venom which means poisonous fluid that causes infection.
The branch of science that deals with the study of viruses is called Virology. It is the branch of Microbiology.
They show living characters inside the host and non living characters outside the host.
They contain either DNA or RNA as genetic material.
They have different size and shape. They cause diseases in plants, animals and micro-organisms .
Not cellular
Cannot carry on metabolic activities independently.
Contain either DNA or RNA, not both ( true cells contain both ).
Lack ribosomes and enzymes necessary for protein synthesis.
Reproduce only within cells they infect.

CLASSIFICATION OF VIRUSES
Holmes, in 1948, proposed a simple system of classifying viruses based on the type of cell (host) they infect:
Phytophagineae: They infect plants and they RNA as their genetic material. Eg: TMV,CaMV.

Zoophagineae: They infect animals and they have mostly DNA as their genetic material. Eg: Polio virus.

Pagineae: They infect bacterial cells, called bacteriophages they usually have DNA as genetic material.

Based on the viral envelope
Named after David Baltimore, a noble prize winning biologist n 1971.

1. dsDNA viruses Eg: Adenoviruses, Herpiviruses.

2. ssDNA viruses Eg: Paravoviruses.

3. dsRNA viruses Eg: Reoviruses.

4. (+)ssRNA viruses Eg: Picornaviruses.

5. (-)ssRNA viruses Eg: Orthomyxoviruses.

6. ssRNA-RT viruses Eg: Retroviruses.

7. dsDNA-RT viruses Eg: Hepadnaviruses.




Tobacco mosaic:
Causative agent: Tobacco mosaic virus (TMV)
Symptoms: The leaves of infected plants develop mosaic patches ,it is due to destruction of chlorophyll or due to production of abnormal chlorophyll .blisters appear in the region of dark green spots these may be regular or irregular in advanced stages leaves curl and get distorted.



Adsorption of the virion to the bacterial cell.
Penetration and decoating of the nucleic acid .
Protein synthesis.
Breakdown of bacterial DNA.
Arrest of host cell development.
Replication of phage DNA.
Maturation of infective progeny.
Lysis and release of newly formed phages.
Holmes, in 1948, proposed a simple system of classifying viruses based on the type of cell (host) they infect:
Phytophagineae: They infect plants and they RNA as their genetic material. Eg: TMV,CaMV.

Zoophagineae: They infect animals and they have mostly DNA as their genetic material. Eg: Polio virus.

Pagineae: They infect bacterial cells, called bacteriophages they usually have DNA as genetic material.

1. Shylesh Murthy I A Follow for More Biological Slides ( LIFE-Sciences) Viruses

2. INTRODUCTION: The word virus is derived from L atin word venom which means poisonous fluid that causes infection. The branch of science that deals with the study of viruses is called V irology . I t is the branch of M icrobiology. They show living characters inside the host and non living characters outside the host. They contain either DNA or RNA as genetic material. They have different size and shape. T hey cause diseases in plants, animals and micro-organisms .

3. DEFINITION OF VIRUSES A virus….”a piece of bad news wrapped in protein”. Sub microscopic entity consisting of a single nucleic acid surrounded by a protein coat and capable of replication only within the living cells of bacteria, , animals or plants.

4. CHARACTERISTICS OF VIRUSES Not cellular Cannot carry on metabolic activities independently. Contain either DNA or RNA, not both ( true cells contain both ). Lack ribosomes and enzymes necessary for protein synthesis. Reproduce only within cells they infect.

5. Are viruses Living or Non-living? A. Living properties: Obligate intracellular parasites. Multiplication. Proteins and N ucleic acids. Exhibit heredity and undergo Mutation. B . Non living properties: Can be crystallized. No cellular structure. Do not respire or show M etabolic activities.

6. CLASSIFICATION OF VIRUSES Holmes , in 1948, proposed a simple system of classifying viruses based on the type of cell (host) they infect: Phytophagineae : They infect plants and they RNA as their genetic material. Eg : TMV,CaMV . Zoophagineae : They infect animals and they have mostly DNA as their genetic material. Eg : Polio virus. Pagineae : They infect bacterial cells, called bacteriophages they usually have DNA as genetic material.

7. Based on the viral envelope Enveloped viruses: outermost covering is made up of glycoproteins , lipids and proteins Eg : Influenza virus. Less virulent, rarely causes cell lysis and sensitive to environmental factors, more stable. Non-enveloped viruses: outermost covering is made up of capsid. Eg : Adenovirus. More virulent, causes cell lysis and resistance to environmental factors, induce antibodies and can survive in GIT and less stable.

8. BASED ON NUCLEIC ACID: Named after David Baltimore, a noble prize winning biologist n 1971. 1 . dsDNA viruses Eg : Adenoviruses, Herpiviruses . 2 . ssDNA viruses Eg : Paravoviruses . 3 . dsRNA viruses Eg : Reoviruses . 4 . (+) ssRNA viruses Eg : Picornaviruses . 5 . (-) ssRNA viruses Eg : Orthomyxoviruses . 6. ssRNA -RT viruses Eg : Retroviruses. 7 . dsDNA -RT viruses Eg : Hepadnaviruses .

9. BASED ON MORPHOLOGY: Helical viruses Icosahedral viruses Complex viruses

10. BASED ON STRUCTURE:

11. VIRAL DISEASES PLANTS . Tobacco mosaic : Causative agent : Tobacco mosaic virus (TMV) Symptoms : The leaves of infected plants develop mosaic patches ,it is due to destruction of chlorophyll or due to production of abnormal chlorophyll .blisters appear in the region of dark green spots these may be regular or irregular in advanced stages leaves curl and get distorted.

12. MULTIPLICATION OF ANIMAL V IRUSES. Attachment to the surface of the host cells. Penetration into host cells. Uncoating and release of nucleic acid into the cells. Replication of nucleic acid and formation of protein coat . Assembly of nucleic acid and protein coats. Release of virus particles by rupture of cells.

13. VIRAL DISEASES IN ANIMALS The envelop is outside the capsid and helps the virus enter the host cell. Generally a lipid bilayer with glycoprotein spikes The envelop fuses with the cell membrane The ER of the host cell makes the membrane proteins which are transported to the membrane New viruses exits the host in a process similar to exocytosis.

14. LIFE CYCLE OF BACTERIOPHAGE Adsorption of the virion to the bacterial cell. Penetration and decoating of the nucleic acid . Protein synthesis. Breakdown of bacterial DNA. Arrest of host cell development. Replication of phage DNA. Maturation of infective progeny. Lysis and release of newly formed phages .

15. LIFE CYCLE OF BACTERIOPHAGES

16. REFERENCE : A textbook of biology ,by M. SUDHAKAR RAO, CELL BIOLOGY AND MOLECULAR BIOLOGY By Dr. Arumugam , SARAS PUBLICATION. Seventh edition . www.quantamagazine.org/broadband-networks-of-viruses

17. Thank you … ALL THE BEST ,, @ Shy