Questions - Answers
Very short answer type questions
1. Mention the living and non-living characters of viruses.
A: Viruses show living characters like presence of DNA/RNA, genetic continuity through multiplication and undergo mutations.
They also show non-living characters like they do not exhibit life processes, they remain inert lifeless molecules outside the host cell and they cannot multiply outside the host cells.
2. What is the shape of T4 phage? What is its genetic material?
A: Tad pole shape with polyhedral head and helical tail. Genetic material – RNA.
3. What are virulent phages? Give an example.
A: The bacteriophages that attack bacterial cells and cause lysis of the cells are called virulent phages. e.g.: T-even phages (T4 phage)
4. What is lysozyme and what is its function?
A: Lysozyme is a viral enzyme that is produced by bacterial cell and causes dissolution of plasma membrane of the host cell. It helps in breaking of bacterial cell wall to liberate newly synthesized bacteriophages.
5. Define ‘lysis’ and ‘burst size’ with reference to viruses and their effects on host cells.
A: Lysis: The breakage of bacterial cell wall to release newly synthesized phage particles is called lysis.
Burst size: The number of newly synthesized phage particles released from a single host cell is called burst size. (50 to 200).
6. What is a prophage?
A: The phage DNA inserted into bacterial DNA during lysogenic cycle is called prophage. It undergoes replication along with bacterial DNA.
7. What are temperate phages? Give one example.
A: The bacteriophages like λ phages which do not cause lysis of host cell but inject their DNA into the host cell to become a part of it are called temperate phages. e.g.: λ phages.
8. Mention the differences between virulent phages and temperate phages.
A:
Short Answer Type Questions
1. What is ICTV? How are viruses named?
A: * ICTV stands for International Committee on Taxonomy of Viruses.
* It regulates the classification and nomenclature of viruses.
* According to ICTV the viruses are classified into three levels, family, genus and species.
* Viruses are usually named after the disease they cause. e.g.: Polio virus and Influenza virus.
* Using ICTV system AIDS virus is placed under the family Retroviridae, genus Lentivirus and Species Human Immune deficiency Virus (HIV).
2. Explain the chemical structure of viruses.
A: * Virus particles are chemically made up of a protein coat called capsid and a nucleic acid either DNA or RNA but never both together.
* The capsid is made up of protein sub units called capsomeres.
* The nucleic acid forms the genome.
* The nucleic acid in a virus may be single stranded RNA or double stranded RNA, double stranded DNA or single stranded DNA.
* ssRNA is seen in viruses that infect plants.
* dsDNA is found in animal viruses and bacteriophages.
* Some viruses like HIV have two identical molecules of RNA.
3. Write briefly about the symmetry of viruses.
A: Viruses show different kinds of symmetry according to the arrangement of capsomeres in the capsid. Some of the types are:
* Helical viruses which are rod shaped e.g.: Rabies virus and Tobacco mosaic virus.
* Polyhedral viruses with many sides e.g.: Herpes simplex and Polio virus.
* Enveloped viruses which are mostly spherical e.g.: Influenza virus. Measles virus shows envelop with glycoprotein spikes that help in attachment to receptors on host cells.
* Complex viruses – polyhedral in head and helical in tail e.g.: Bacteriophages.
4. Explain the structure of TMV.
A: Structure of TMV (Tobacco Mosaic Virus):
* Tobacco mosaic virus causes mosaic disease in Tobacco.
* It is rod shaped with helical symmetry.
* The size is approximately 300 nm long and 18 nm in diameter with a molecular weight of 39 × 106 Daltons.
* The capsid is made up of 2,130 capsomeres which are arranged in a helical manner around the hollow core of 4nm.
* Each capsomere is made up of a polypeptide chain with 158 amino acids.
* The spirally coiled RNA contains 6500 nucleotides.
5. Explain the structure of T-even bacteriophage.
A: Structure of T-even Bacteriophage:
* T4 bacteriophage resembles a tad pole in its shape.
* It shows a head and a tail joined by a collar.
* The head shows polyhedral symmetry and tail shows helical symmetry.
* The head region encloses DNA.
* The tail is covered by a tail sheath and ends with a base plate with tail pins and tail fibres.
* The tail pins and fibres help in attachment to the host cell.
* The tail sheath helps in injecting the viral DNA into the host cell.
6. Explain the lytic cycle with reference to certain viruses.
A: Viruses like bacteriophages multiply by any of the two methods lytic cycle and lysogenic cycle.
Lytic cycle: Lytic cycle caused lysis or death of host cell. T-even phages which cause lysis are called virulent phages. Lytic cycle involves 5 steps:
1. Attachment or Adsorption
2. Penetration
3. Bio synthesis
4. Maturation
5. Release
Adsorption: It is the attachment of phage particle to the bacterial host cell on complementary receptor sites.
Penetration: The tail sheath of the phage contracts and injects the DNA into the bacterial cell. The capsid of the phage remains outside the host cell as a ghost.
Bio synthesis: As the phage DNA enters the host cells it uses the cell machinery and synthesises phage DNA and capsid proteins. But the complete virus particles do not appear hence called eclipse period.
Maturation: During this phase bacteriophage DNA and capsids assemble into complete virions.
Release: At this stage the viral enzyme lysozyme produced in the host cell dissolves the plasma membrane and breaks the cell wall to release the newly synthesized phage particles or virions.
his phase is hence called lysis phase. The number of newly synthesized phage particles released from a single host cell is called “burst size”. (50 to 200).
7. Explain how temperate phages play a role in transduction.
A: Some bacteriophages like λ phages infect bacterial cells and inject their DNA into the host cell. The phage DNA gets integrated with circular bacterial DNA and becomes a part of it. Such phages which do not cause lysis of the bacterial cell but multiply their DNA along with the host DNA are called temperate phages. The inserted phage DNA is called prophage. The prophage remains latent or inactive inside the host cell but replicates along with host DNA. When the host cell is subjected to UV light or certain chemicals, the phage DNA separates from the bacterial genome is released which carries a part of bacterial DNA. When such viral particle with phage DNA and bacterial DNA infects other host cell it injects the phage DNA and also bacterial DNA into the new host cell thus facilitating transduction. (Transduction is a kind of genetic recombination or reproduction in bacteria like Salmonella typhimurium facilitated by virus).
8. Mention the differences between lytic and lysogenic cycles.
A:
Long Answer Type Questions
1. Write about the discovery and structural organization of viruses.
A: Discovery of viruses: Dmitri Ivanovsky observed that filterable agents smaller than bacteria caused Tobacco Mosaic Disease. Martinus Beijerinck concluded that a contagious living fluid (Contagium vivum fluidum) is responsible for Tobacco mosaic disease. W.M. Stanley - purified the sap of diseased tobacco leaves and crystallized virus that causes Tobacco Mosaic Disease. Later Fraenkel Conrat identified that the genetic material of TMV is RNA.
Structural organization of viruses: Virus particles are chemically made up of a protein coat called capsid and a nucleic acid either DNA or RNA but never both together. The capsid is made up of protein sub units called capsomeres. The nucleic acid forms the genome. The nucleic acid in a virus may be single stranded RNA or double stranded RNA, double stranded DNA or single stranded DNA. ssRNA is seen in viruses that infect plants. dsDNA is found in animal viruses and bacteriophages. Some viruses like HIV have two identical molecules of RNA. The size of viruses may vary from 20 nm to 300 nm. Viruses are classified according to symmetry into the following types:
* Helical viruses – rod shaped – e.g.: Rabies virus and Tobacco mosaic virus.
* Polyhedral viruses – many sided – e.g.: Herpes simplex and Polio virus.
* Enveloped viruses – spherical – e.g.: Influenza virus.
* Complex viruses – polyhedral in head and helical in tail – e.g.: Bacteriophages.
2. Describe the process of multiplication of viruses.
A: Multiplication of viruses is also called replication or reproduction. Viruses like bacteriophages multiply by any of the two methods lytic cycle and lysogenic cycle.
Lytic cycle:
Lytic cycle caused lysis or death of host cell. T – even phages which cause lysis are called virulent phages. Lytic cycle involves 5 steps:
1. Attachment or Adsorption
2. Penetration
3. Bio synthesis
4. Maturation
5. Release
Adsorption: It is the attachment of phage particle to the bacterial host cell on complementary receptor sites.
Penetration: The tail sheath of the phage contracts and injects the DNA into the bacterial cell. The capsid of the phage remains outside the host cell as a ghost.
Bio synthesis: As the phage DNA enters the host cells it uses the cell machinery and synthesises phage DNA and capsid proteins. But the complete virus particles do not appear hence called eclipse period.
Maturation: During this phase bacteriophage DNA and capsids assemble into complete virions.
Release: At this stage the viral enzyme lysozyme produced in the host cell dissolves the plasma membrane and breaks the cell wall to release the newly synthesized phage particles or virions. This phase is hence called lysis phase. The number of newly synthesized phage particles released from a single host cell is called “burst size”. (50 to 200).
Lysogenic cycle:
* Lysogenic cycle does not cause lysis of death of host cell.
* The phage DNA (of λ phage) on entering into host cell gets integrated with the host DNA and remains inactive. Such phages are called ‘temperate phages’.
* The inserted phage DNA is called ‘prophage’.
* The prophage replicates along with the host DNA but remains latent.
* Rarely the phage DNA gets separated from the bacterial DNA and initiates lytic cycle.
