Variety of Life


Overview to Variety of Life

Variety of life is determined by the looking into the classification or taxonomies in which living organisms are grouped or categorized.

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:

  1. Homology
    The living organisms of a particular groups have some similar characters, this characteristic is called Homology. The organs of the body which have 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.
  2. Comparative biochemistry
    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.
  3. Cytology
    Cytology also helps in the classification of living organisms. By the help of electron microscope it is proved that in bacteria and cyonobacteria (blue green algae), incomplete nucleus is present, so they are placed in the same Kingdom-Monera. Prokeryotic and eukaryotic organisms have been identified on the bases of microscopic study.Cell study has also helps to know about the number of chromosomes in different organisms. All plants and animals contain particular fixed number of chromosomes.
  4. 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 organ.

Units of Classification OR Taxonomic Categories

In classification of plants different units are used. In 18th century scientists presented the system of classification and formed it’s 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 from 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.


Viruses are very minute non-cellular bodies, considered between living and non living. The word virus is derived from a Latin world “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:

  1. Virus are non-cellular parasitic organisms, minute in size from 25nm to 250nm.
  2. 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.
  3. Viruses are obligate parasites. They reproduce only inside living cells. Whenthey enter a host cell, they control the biochemical activities of the cell.
  4. They are composed of nucleic acid and proteins.
  5. Virus has no cell wall, cytoplasm and proper nucleus.

Structure of Viruses:

Structurally there are different shapes of virus. 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:

  1. 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.
  2. 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.
  3. Envelopes or covering
    These are membranous covering around the Capsid. It is found in some viruses. This covering helps them to infect their hosts.
  4. 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.

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. Bacteriophage often consist of a head and a hollow tail region. From the tail fibres are produced. Around the tail a protein sheath is also present. The steps of lytic cycle are as follows:

  1. The virus 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.
  2. The D.N.A. of virus is transferred into the bacterial cell from the head region and the protein coat of the head and tail remains outside.
  3. 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.
  4. It also allows the bacterial cell to produce only the viral type of protein. In this way D.N.A. and protein of virus are developed in bacterial cell.
  5. The D.N.A. of the virus is migrated into the head portion.
  6. 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 lytic cycle.

Lysogenic Cycle of Bacteriophage

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

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

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