Class 11 Biology – Viruses Notes
Complete notes on viruses, their characteristics, classification, bacteriophage structure, transmission and economic importance.
Viruses
The word virus is derived from a Latin word associated with the meaning of poison or poisonous fluid.
Viruses are described in the handwritten notes as obligatory parasites because they are fully dependent on living organisms for their multiplication.
The branch of biology concerned with the study of viruses is called virology.
Characteristics of Viruses
- Viruses are minute and very simple forms that can be observed with the help of an electron microscope.
- They are much smaller than bacteria.
- They are metabolically inactive outside the host cell because they lack their own complete enzyme systems and protein-synthesis machinery.
- They are therefore described in the source notes as obligatory intracellular pathogens.
- They contain nucleic acid as genetic material.
- Viruses do not possess a true cellular organization.
- They lack a true nucleus, cytoplasm and cytoplasmic organelles such as mitochondria and Golgi complex.
- They respond to factors such as chemicals, heat and temperature.
- Outside the host cell, they may be crystallized and can behave like chemical substances.
- They cannot show an independent mode of life.
- Viruses are highly specific to their hosts.
- They do not reproduce by ordinary cellular methods such as fission or conjugation.
Classification of Viruses on the Basis of Nucleic Acid
The handwritten notes classify viruses into the following three types on the basis of their genetic material:
1. DNA Virus
A virus possessing DNA as its genetic material.
2. RNA Virus
A virus possessing RNA as its genetic material.
3. DNA-RNA Virus
The source notes describe this category as a virus possessing both DNA and RNA as genetic material.
Structure of Bacteriophage
The shape and size of bacteriophages may vary from species to species.
Some phages may be spherical or comma-shaped, while the majority described in the notes possess a tadpole-like structure.
A typical tadpole-shaped bacteriophage consists of:
- Head
- Neck
- Collar
- Tail
- End plate or basal plate
- Tail fibres
Head
The head is polyhedral and is described as bipyramidal or hexagonal in appearance.
Its contents are enclosed by a protein covering called the capsid.
The handwritten notes state that the capsid consists of numerous capsomeres.
The head contains highly folded, double-stranded DNA.
Tail
A short neck and collar connect the head with the tail.
The tail terminates in a hexagonal end plate or basal plate.
According to the source diagram, the basal region bears six tail fibres.
Modes of Transmission of Viruses
The source notes describe several ways by which plant viruses may be transmitted.
Some viruses may remain in soil along with infected plant debris and later become transmitted to crops grown in the same field.
Examples mentioned include tobacco mosaic virus, potato mosaic virus and wheat mosaic virus.
A large number of viruses may be transmitted mechanically.
The notes mention contact between infected and healthy leaves, movement caused by wind, grafting of infected buds and contaminated agricultural tools such as knives used during cutting.
Some viruses may be transmitted through infected seeds.
Examples mentioned in the notes include tobacco mosaic virus and mosaic viruses of legumes.
Some soil-borne viruses can be transmitted by nematodes and fungi occurring in the soil.
The notes mention wheat mosaic virus in this context.
Viruses may be transmitted through slight rubbing or contact between infected and healthy plant organs.
Some viruses are transmitted through specific vectors.
The handwritten notes specifically mention insects and aphids.
When infected plant parts are used in grafting or vegetative propagation, newly propagated plants may also become infected.
The notes state that tobacco mosaic virus may remain infectious in dry tobacco leaves for a long period.
It may be spread through contaminated fingers of smokers, smoke-related contact or pieces of cigarettes after smoking.
Economic Importance of Viruses
1. Positive Effects
Transduction
The handwritten notes state that temperate bacteriophages help in the transduction of genetic material from one bacterium to another.
Variegation in Plants
Some virus-infected plant parts may show attractive or beautiful variegation.
Destruction of Bacteria
Viruses that infect bacteria may destroy bacterial cells.
Use of Bacteriophages
The source notes mention the use of bacteriophages in relation to the treatment of bacterial diseases such as cholera and dysentery.
2. Negative Effects
Diseases in Humans
Viruses cause numerous diseases in human beings.
Examples mentioned in the handwritten notes include:
- Polio
- Rabies
- AIDS
Plant Diseases
The notes list several common viral diseases of plants, including:
- Tobacco mosaic
- Tomato mosaic
- Potato mosaic
Quick Revision
- The study of viruses is called virology.
- A mature virus particle outside the host is called a virion.
- Viruses depend on living host cells for multiplication.
- They are much smaller than bacteria.
- Viruses lack true cellular organization.
- A true nucleus, cytoplasm and membrane-bound cytoplasmic organelles are absent.
- Viruses are metabolically inactive outside a host cell.
- They are highly host-specific.
- They cannot reproduce by ordinary methods such as binary fission or conjugation.
- The handwritten notes classify viruses as DNA viruses, RNA viruses and DNA-RNA viruses.
- A bacteriophage infects bacterial cells.
- A typical bacteriophage has a head, neck, collar, tail, basal plate and tail fibres.
- The bacteriophage head contains genetic material enclosed by a capsid.
- The bacteriophage shown in the source contains double-stranded DNA.
- Viruses may be transmitted through soil.
- Mechanical transmission may occur through infected plant material and contaminated tools.
- Viruses may also spread through seeds, nematodes, fungi, contact and vectors.
- Grafting and vegetative propagation can transmit plant viruses.
- Tobacco mosaic virus may also be spread through contaminated material associated with smokers according to the notes.
- Temperate bacteriophages participate in transduction.
- Bacteriophages destroy bacterial cells.
- Viruses cause diseases such as polio, rabies and AIDS.
- Common viral plant diseases include tobacco mosaic, tomato mosaic and potato mosaic.
Important Questions
- What are viruses?
- What is a virion?
- What is virology?
- Why are viruses called obligatory parasites?
- Write the general characteristics of viruses.
- Why are viruses metabolically inactive outside host cells?
- Why are viruses said to lack cellular organization?
- Classify viruses on the basis of nucleic acid according to the notes.
- What is a bacteriophage?
- Describe the structure of a bacteriophage.
- Describe the structure of the head of a bacteriophage.
- Write the functions of the tail fibres of a bacteriophage.
- List the different modes of transmission of plant viruses.
- Explain transmission of viruses through soil.
- Explain mechanical transmission of plant viruses.
- How are viruses transmitted through seeds?
- Explain transmission of viruses by nematodes and fungi.
- How do vectors transmit viruses?
- Explain transmission through grafting and vegetative propagation.
- How can tobacco mosaic virus be transmitted by smokers according to the notes?
- Write the positive economic importance of viruses.
- Write the negative economic importance of viruses.
- What is the role of bacteriophages in transduction?
- Mention some human viral diseases.
- Mention some common viral diseases of plants.
Discussion
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