VARIETY OF LIFE

 Biology XI Notes

Variety of Life - Short Questions Answers
Biology - XI
Section III - Biodiversity

VARIETY OF LIFE

 Q.1: What is classification? Describe the different characters of classification?

Ans: There are different kinds of living organisms in the world. These organisms differ from each other in size, shape, and color. They are classified into different groups and sub-groups on the basis of their characters. This system of classification is known as Taxonomy.

The living organisms are classified on the bases of following characters:

• Homology
• Comparative biochemistry
• Cytology
• Genetics

Homology:
The living organisms of a particular group have some similar characters; this characteristic is called homology. The organs of the body which have the same fundamental structure but different in their function are called homologous organs. For example, legs of horses, flippers of turtles, wings of birds are homologous organs. They have the same fundamental plan, show different structure but perform common function i.e. locomotion of the body. The homologous organs in different animals show relationship and prove that they have evolved from a common ancestor. In this way, homology is a very important character for classification of living organisms.

Biochemistry (Chemical Constituents):
When morphological characters are not helping in the classification of living organisms, then their chemical constituents are used, for example in bacteria study of chemical structure is useful to identify and classify them, because they have similar cellular structure.

In biochemistry certain techniques are used, such as chromatography and electrophoresis, so their chemical constituents can be studied properly. Amino acid sequence in the proteins or arrangement of nitrogen bases in DNA help to classify the organisms and to determine their evolutionary relationship.

Cytology:
Cytology also helps in the classification of living organisms. By the help of electron microscope it is proved that in bacteria and cyanobacteria (blue green algae), incomplete nucleus is present, so they are placed in the same Kingdom - Monera. Prokaryotic and eukaryotic organisms have been identified on the bases of microscopic study.

Cell study has also helped to know about the number of chromosomes in different organisms. All plants and animals contain a particular fixed number of chromosomes.

Genetics:
It is the final technique, which is applied in taxonomy, because all characters are inherited from parents to new generation. All morphological, biochemical and cytological characters are based upon genetic combination. DNA study is very useful in the classification of organisms.

Q.2: What are the units of classification?

Ans: Units of Classification: (Taxonomic Categories)
In classification of plants, different units are used. In the 18th century, scientists presented the system of classification and formed its units. The lowest and basic unit is called species.

According to the classification system, the similar individuals in a group that breed with one another are called species. They resemble with one another very closely. The different groups of plants are called Taxa. Each plant belongs to a species. Species with many similar characters are placed in a genus, many genera form family. Many families with certain similar characters are grouped in order. A group of similar orders belongs to a class. Many closely related classes constitute a division (or phyla) and many phyla are included in a kingdom.

Q.3: What are Viruses? Describe the properties (characteristics) of Viruses?

Ans: Viruses are very minute non-cellular bodies, considered between living and non-living. The word virus is derived from a Latin word vios, means poison. They are obligate parasites and reproduce only in the living cells. They are visible under electron microscope. In 1892 a Russian virologist Ivanovasky indicated the existence of virus in Tobacco mosaic disease.

Properties of Viruses:

• Viruses are non-cellular parasitic organisms, minute in size from 25nm to 250nm.
• They are considered on the border of living and non-living because they are alive in the body of living organisms and dead outside the living body.
• Viruses are obligate parasites. They reproduce only inside living cells, when they enter a host cell, they control the biochemical activities of the cell.
• They are composed of nucleic acid and proteins.
• They infect only specific hosts, specific tissues, and specific cells. Tobacco mosaic virus can infect only tobacco, potato, and a few other plants of family Solanaceae. Polio virus causes the disease in men and monkey and not in other animals.
• Virus has no cell wall, cytoplasm, and proper nucleus.
• The simplest viruses use the enzymes of the host cell for their protein synthesis and gene replication. The more complex viruses contain about 200 genes. They manufacture their body from the proteins and enzymes of host body.

Q.4: Describe the Structure of Viruses?

Ans: Structurally there are different shapes of viruses. They are rounded, rod-shaped, tadpole like or polyhedral, i.e., consist of many sides. Many viruses have helical or isometric structure.

Helical viruses are rod-shaped or thread-like with numerous helix-like (screw-like) subunits, e.g., Tobacco mosaic virus (TMV). Isometric viruses are spherical in shape.

Virus consists of different parts:

• Viral genome
• Capsid
• Envelopes or covering
• Tail fibres

Viral Genome:
It is the inner part, the nucleic acid. It consists of a single or many molecules of DNA or RNA. In small virus four genes and in the largest virus several hundred molecules are present.

In many viruses, nucleic acid is different. In animal viruses and bacteriophage usually DNA is present and in plant viruses mostly RNA is present. In viruses, cytoplasm, nucleus, and chromosomes are not found.

Capsid:
The central part genome or nucleic acid of virus is covered by an outer protein coat, called Capsid. The capsid is made up of numerous protein units, called capsomeres. The capsid with nucleic acid is called nucleo-capsid.

In simplest viruses, capsid is made up of one or a few different protein molecules. In complex viruses, many different kinds of proteins are present in capsid.

Viral Envelopes:
These are membranous covering around the capsid. It is found in some viruses. This covering helps them to infect their hosts.

Tail Fibres:
In bacteriophage virus, lower part is tail like. At the posterior end of tail some fibre like structures are present, called tail fibres. These fibres take part in the attachment of virus with host cell.

Q.5: What are the different shapes of viruses?

Ans: Shape of Viruses:
Viruses are classified on the basis of their shape and structure. These types are:

• Rod-shaped e.g. Tobacco mosaic virus
• Spherical (Rounded) e.g. Polio virus
• Tadpole e.g. Bacteriophage
• Bullet shaped e.g. Rhabdo virus
• Cuboid or Polyhedral e.g. Adeno virus (Tumor virus)

Q.6: What is the recent classification of Virus?

Ans: The recent classification of viruses is related to their mode of origin. According to this classification viruses are of the following types:

• Unenveloped Plus-Strand RNA Viruses:
  These viruses are without envelope. They act as m-RNA so they are plus-standard.

• Enveloped Plus-Strand RNA Viruses:
  They also act as m-RNA. They have outer envelope, called capsid.

• Minus-Strand RNA Viruses:
  These viruses do not act as m-RNA. They have envelope around the capsid.

• Retrovirus:
  This virus is single stranded RNA virus in HIV or double stranded DNA in Hepatitis-B.

• Double Stranded RNA Viruses:
  They have double strand of RNA with capsid.

• Small-Genome DNA Viruses:
  These viruses contain small DNA genome. They contain envelope and capsid.

• Medium-Genome And Large-Genome DNA Viruses:
  These viruses contain medium sized or large sized genome with capsid, and envelope.

• Bacteriophage:
  These viruses attack bacteria. They have hexagonal head and tail. In the head DNA is present, covered by capsid.

Q.7: Describe the Lytic Cycle of Bacteriophage?

Ans: Lytic Cycle of Bacteriophage:
The reproductive cycle of virus which causes death of the bacterial cell is called Lytic cycle. Due to the infection bacterial cell ruptures and bacteriophage viruses are released. Each phage can infect another cell.

The phage is called T₄-phage. The steps of the lytic cycle are as follows:

• The virus T₄-phage first attaches to the bacterial cell by its protein tail. An enzyme Lysozyme is secreted by the tail, which helps to dissolve the bacterial cell wall.
• The D.N.A. of the virus is transferred into the bacterial cell (Escherichia Coli) from the head region, and the protein coat of the head and tail remains outside.
• Inside the bacterial cell, the viral D.N.A. then controls the bacterial cell activity. The duplication of viral D.N.A. takes place, and the new phage particles start to produce inside the cell.
• It also allows the bacterial cell to produce only the viral type of protein. In this way, D.N.A. and protein of the virus are developed in the bacterial cell.
• The D.N.A. of the virus is migrated into the head portion.
• After a particular period, the bacterial cell bursts, and new bacteriophages (viruses) are set free. This process is called Lysis. This type of cycle is known as the Lytic cycle.

Q.8: Describe the Lysogenic Cycle of Bacteriophage?

Ans: Lysogenic Cycle of Bacteriophage:
Sometimes the viral D.N.A. in the bacterial cell does not take control of biochemical activity. The D.N.A. of the virus and bacteria make an association. In this way, many new generations of bacteria can be produced without any harmful results.

A virus that has both types of reproduction in a bacterial cell is called a temperate virus. When D.N.A. of the virus enters the bacterial cell, it forms a circle and forms an association with the bacterial chromosome. The virus in this state is called a prophage. It remains inactive without causing any harm in the bacterial cell.

When bacteria reproduces, each time viral DNA also replicates and transfers into new cell. The bacteria which are not affected by the viral DNA are called Lysogenic bacteria and the cycle is known as Lysogenic cycle.

Sometimes due to certain reasons such as radiation or chemicals the viral DNA becomes active again and then it starts lytic cycle again. Then it destroys the bacterium as in the lytic cycle.

Q.9: What are the Viroids and Prions?

Ans: Viroids and Prions:

Viroids:

• Viroids are the tiny molecules of Circular RNA, made up 250 to 400 nucleotides.
• They do not have capsid i.e., naked molecules.
• They are smaller and simple as viruses.
• They attack plant cells. They replicate in host plant cells by using cellular enzymes.
• These viroids disrupt the metabolism of a plant cell. They stunt the growth of the whole plant.

Prions:

• Prions are the infectious proteins, cause many degenerative brain diseases, such as scrapie in sheep and mad cow disease.
• Protein cannot replicate itself, but according to one hypothesis prion is a misfolded protein normally present in brain cells. When prion enters a cell which contains normal protein, then prion converts this normal protein into prion, they increase their number by chain reaction, then it becomes abnormal infectious protein, which causes trouble in the cells.

Q.10: Describe Poliomyelitis disease caused by polio virus?

Ans: Poliomyelitis: (Disease by Polio Virus)
This disease is caused by polio-virus. It enters the body through mouth and from alimentary canal it is migrated to the blood vessels. By the circulation of blood is reaches the nervous system and affects the nerve cells. It causes paralysis of certain organs of the body.

Symptoms:
Fever, headache, indigestion, vomiting, stiffness in neck and back.

Control:

• Vaccination of Babies
• Contaminated food and water should not be used.

Q.11: Describe the diseases caused by Rhinovirus (Myxovirus)?

Ans: Diseases Caused By Rhinoviruses: (Viruses of Nose and Throat)
Rhinoviruses are the viruses of nose and throat e.g., Myxovirus, Paramyxovirus.

Colds:
It is the infection of the upper respiratory tract. Many kinds of colds are due to rhinoviruses. These are unenveloped plus-strand-RNA viruses. These are of different kinds. Their more than 200 different types have been discovered.

Control:
Immunization is the method of control, but due to various strains it is difficult.

Influenza:
This disease is commonly called “flu.” It is caused by a virus myxovirus-Influenzae. It takes 2 or 3 days to appear after infection.

Symptoms:
Its symptoms are headache, pain in the muscles, arms, back, throat, and fever. Sometimes influenza may result in pneumonia. Influenza causes severe weakness in the body.

Precautions and Control:

• Avoid crowd and sudden change in temperature.
• Use of antibiotics.

Q.12: Describe the diseases caused by Paramyxovirus?

Ans: Measles and Mumps: (Diseases Caused By Paramyxovirus)
These diseases are caused by Paramyxoviruses. These are large enveloped RNA viruses. Mumps is the inflammation of salivary glands i.e., they become swollen. Mumps is a disease of children, adults are immune to it. It is highly contagious disease. Measles is very common disease of children and adult persons.

Q.13: Describe the diseases caused by Arboviruses?

Ans: Diseases Caused By Arboviruses: (Arthropode Born Viruses)

Yellow Fever, Encephalitis And Dengue:
These are the viral diseases, caused by arboviruses. These viruses live in the body of insects and other arthropodes, from their body these viruses are transferred into man’s body and cause diseases, for example the viruses of yellow fever are transferred from the body of mosquitoes. These viruses attack liver, kidneys, blood capillaries. Fever and jaundice are main symptoms, so the disease is called yellow fever.

Control:

• Use of antibiotics.
• Eradication of mosquitoes and other arthropodes which are involved in the disease.

Q.14: Write a note on Retrovirus?

Ans: Diseases by Retroviruses: (Tumor Virus)
The retroviruses are RNA tumor viruses, cause tumor formation in many animals. The most common virus of this group is HIV, which causes AIDS (Acquired immune deficiency syndrome) in human beings. It was discovered in 1985.

Q.15: Describe the Rabies disease caused by Rhabdovirus?

Ans: Diseases by Rhabdoviruses:
Rhabdoviruses are found in the saliva of animals.

Rabies:
Rhabdovirus causes Rabies disease. It was discovered in the 19th century by Louis Pasteur. This disease is also called hydrophobia. Usually, it is found in dogs and foxes. The viruses are transferred into the body of man by biting the mad dog. The viruses are present in the saliva of the animal.

Control:
Vaccination is the control of disease.

Q.16: Describe the Small Pox disease?

Ans: Small Pox: (Disease by Pox Virus)
It is caused by Pox virus, which is the DNA enveloped virus. It is a contagious disease and common in children. Due to the infection, pustules or packs are developed in the skin, which leave permanent spots.

Symptoms:
Fever, pain in the back, headache, and pustule formation on skin.

Control:
Vaccination is the best control. WHO has played an important role to control this disease in Pakistan.

Q.17: Write a note on plant diseases caused by virus?

Ans: Plant Diseases Caused by Virus:
Viruses attack many important plants and cause diseases in them, so the crop is badly damaged and yield is greatly reduced. Some plants are tobacco, tomato, sugarcane, wheat, corn, rice, beet, etc.

Tobacco mosaic virus disease is very common. The virus is called TMV (Tobacco mosaic virus). In this disease white, yellow, green spots are produced on leaves. It looks like a mosaic. So the disease is called Tobacco mosaic virus disease.

The viruses cause infection in plants in two ways:
Horizontal Transmission:
In this infection, the plant is infected from an external source through insects or injured parts.

Vertical Transmission:
In this infection, the plant gets it from the parent plant by inheritance.

Control of Viral Diseases:

• There is no proper control of viral diseases of plants.
• The best control is the use of resistant varieties.
• Clean and disease-free seeds should be used for the cultivation of crops.

Q.18: Describe the AIDS disease cause, ways of spread, symptoms, and treatment?

Ans: Human Immune Deficiency Virus (HIV):
HIV, the human immune deficiency virus, causes a disease in human beings called AIDS, Acquired Immune Deficiency Syndrome. The virus was named in 1986. HIV infects the Lymphocytic cells T₄ (Helper cells), which is a very important part of the immune system. Due to this virus, the lymphocytic cell T₄ immune system of the body becomes very weak. The virus replicates in T₄ cells or helper cells. These affected cells do not motivate other T-cells to fight against the virus. When the body of the host is affected by the virus continuously, helper T-lymphocytes is decreased. T-lymphocytes (T₄) are actually the killer of microorganisms.

By their decreased number, the immune system is damaged. As a result of which, the body can be attacked by other bacterial and viral diseases.

Transmission:
The HIV-virus can be transmitted to other persons by blood or semen because it is active only in body fluid. In this way, there are two methods of its transmission.

• Entrance of infected blood
• Sexual contact

Entrance of Infected Blood:
There are different ways of entering the infected blood into the body of a healthy person. These are as follows:

• Blood Transfusion:
  When already infected blood by HIV is transfused into the body of a person, then this virus enters the body.

• By Intravenous Methods:
  When unsterilized syringes and needles are used, HIV may enter the body. The intravenous drug users are actively involved in spreading the disease.

• Through Cuts and Wounds:
  Through cuts and open wounds, the virus can be transferred from one person to another.

• • From Mother To Baby:
    Virus can be transferred from mother to baby through placenta at birth or through milk and food from mother.

  • Sexual Contact:
    Sexual contact is the main cause of transmitting the virus from infected person to non-infected person.

  Signs & Symptoms:

  • In the beginning, flu-like illness and for months or years no prominent symptoms of disease.
  • Immune system is damaged i.e., immune deficiency is the main symptom.
  • Due to immune deficiency, other diseases also attack the body. Pneumonia, fever, dementia, diarrhea are also the symptoms.
  • Skin cancer, called Kaposi’s sarcoma, may occur. It causes death.
  • Blood poisoning (septicaemia) and other forms of cancer may occur.
  • Lymphocytes (cells of immune system) are affected, so brain cells are damaged, brain shrinks, memory loss, mental disorder take place. Behavior of patient is also changed.

  Control, Treatment And Prevention:

  • Great efforts are being made to develop effective drugs and vaccines to control the disease. The best drug is azidothymidine or Zidovudine, commonly known as AZT. This drug was prepared in 1987.
  • Another drug Ribavarrin is also used to control the viral infection. It suppresses the AIDS virus.
  • Sumarin drug is also effective to control the viral reproduction in host body.

  Methods Of Prevention:

  • Clean needles and sterile syringes should be used for injection.
  • Before transfusion, blood should be properly checked and examined. It should be healthy and without infection.
  • Close contact with infected person should be avoided.
  • Proper guidelines by education should be provided to people. They should be aware of the disease.

  Q.19: Describe the Hepatitis disease?

  Ans: Hepatitis:
  Hepatitis is a liver disease, caused by virus. It is the inflammation i.e., swelling and burning of liver. In this disease, there is enlargement of liver, pain in the abdomen, fatigue, fever and jaundice. Hepatitis is sometimes mild or sometimes very strong, it may cause liver cancer.

  Types of Hepatitis:
  There are different types of Hepatitis:

• Hepatitis A:
  It is the infectious hepatitis. It can be transmitted from infected person through unhygienic condition and by the contact with excreted undigested food. The virus which causes this disease is non-enveloped RNA virus.

  Hepatitis B:
  It is caused by unusual DNA virus. This hepatitis is serum hepatitis. It is transmitted by the exchange of body fluids, such as blood stream, mother’s milk, saliva, from mother to baby during birth and sexual contacts.

  Symptoms:
  The main symptoms of disease is jaundice and loss of appetite and fatigue. Acute infection causes liver damage and death may occur.

  Hepatitis-B is a serious dangerous, highly infectious disease of Asia, Africa, Middle East, Philippines and other countries.

  Control:

  • Hygienic conditions should be maintained.
  • Screening of blood, tissues, or organs of donors is necessary.
  • Effective vaccination and drugs should be used to control the infection.

  Hepatitis C:
  It passes through blood from mother to the body of new baby or by sexual contact.

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