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. Viruses

2.  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 .

3.  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.  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. A. Living properties:
 Obligate intracellular parasites.
 Multiplication.
 Proteins and Nucleic acids.
 Exhibit heredity and undergo Mutation.
B. Non living properties:
 Can be crystallized.
 No cellular structure.
 Do not respire or show Metabolic activities.

6. 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.  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.

9.  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.

11.  Helical viruses
 Icosahedral viruses
 Complex viruses

13. 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.

15.  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.

16. – 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.

17.  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.

19.  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

20. Thank you…
ALL THE
BEST ,, @ Shy