Acellular life

• Viruses are abundant in nature and can infect and parasitize a wide variety of organisms from bacteria to mammals. They are considered to be very small biological entities composed of a small number of macromolecules produced by, and thus derived from, the organism they infect. Viruses cause many diseases of international importance.
• Amongst the human viruses, smallpox, polio, influenza, hepatitis, human immunodeficiency virus (HIV-AIDS), measles and severe acute respiratory syndrome(SARS), coronavirus are particularly well known.
• Antibiotics can be very effective against diseases caused by bacteria but compounds that treat microbial infections are ineffective against viruses and most control measures have to rely on vaccines or relief of the symptoms to encourage the body's own defense mechanisms.
• Viruses also cause many important plant diseases and are responsible for major losses in crop production and quality in all parts of the world.
• In just ten minutes, a virus may take over a cell, copy itself hundreds of times, and kill the cell. Some viruses have a calculated replication time of about 20 seconds. By comparison, the fastest bacterial replicators only double their biomass every 20 minutes or so.
• Viruses lie on the border line between living and non living things. They are considered as living because viruses possess DNA or RNA. They have the ability of reproduction. They can undergo mutation and genetic recombination.
• Apart from these living characters viruses possess some non-living characters. They are sub cellular or non cellular structure. They do not respire, excrete, can be crystallized and stored in much the same way as chemicals.
• By the late 1800's pioneer bacteriologists like Louis Pasteur, Robert Koch had demonstrated that bacteria cause many diseases of man and other organisms.
• In 1892 a Russian biologist named Iwanowsky demonstrated that the agent of tobacco mosaic disease could be transmitted through a filter from which bacteria could not pass.
• By 1930 most people believed that the viruses are small particles not visible under the compound microscope, and they were regarded as dead particles composed of many rod-shaped structures.
• This started a debate whether viruses are living or dead? W.M. Stanley took purified TMV, dissolved it in water and on the leaves of healthy tobacco plants. The leaves soon showed the mottle condition, characteristics of TMV disease. It was found that viruses had reproduced itself in the living cells of the host. This proved that viruses had some living characteristics.
• Spherical Virus

  e.g. Polio virus
• Tadpole shaped virus


  e.g. Bacteriophage
• Rod shaped virus

  e.g. tobacco mosaic virus
• Plant viruses are parasites on plants and cause diseases in them.
• Bacteriophage (phage) is a virus that infects only on bacteria.
• Bacteriophage


  • They are obligate intracellular parasites that multiply within a living cell
  • They have a head structure which can vary in size and shape
  • Some have a tail attached to the phage head, which is a hollow tube through which the nucleic acid passes during infection
• HIV


  • It is an RNA virus which may exist in different shapes from round to long, spaghetti-like
  • The genome of this virus is associated with different viral proteins and is surrounded by a lipid membrane, which means that it belongs to the "enveloped" group of viruses
  • It has eight separate pieces of nucleic acid (RNA) that make up the viral genome and each piece of RNA is a gene
• Influenza virus

  • It is an RNA virus which may exist in different shapes from round to long, spaghetti-like
  • The genome of this virus is associated with different viral proteins and is surrounded by a lipid membrane, which means that it belongs to the "enveloped" group of viruses
• Viruses have evolved many mechanisms by which they can tackle the host's immune responses. Some of the prominent ways are that their capsules, proteins, and glycerin coats do not bind the adhesion molecules used by macrophages and neutrophils so they are safe from being phagocytosed.
• Many viruses, unlike bacteria, cover their cell walls with host proteins. So in this way the body's immune system is able to recognize them as foreign bodies.
• Many viruses produce mutants (antigenic variations) at regular basis. And a vaccine developed to control the spread of one mutant virus becomes ineffective against the new mutant, so controlling them is a continuous challenge. For example, influenza virus and HIV.
• Viruses do not possess any life sustaining characteristics, and do not require any energy. In fact, without proper host viruses lie dormant indefinitely. However, viruses are specific to a certain kind of cell. They also have preferred ways of entrance into their hosts. A virus' method of entry is very specialized, and it is central to the main ways a virus is able to locate its victims.
• The virus that infects and becomes parasite on the bacterium is called Bacteriophage. There are many strains of the phage but only one kind of phage can attack only one strain or one species of bacteria. There are two types of bacteriophage namely lytic cycle and lysogenic cycle.
• Lytic cycle

  1. Phage first attaches itself by its tail to the cell wall of bacterium at a point called receptor site

  2. The phage contains an enzyme called lysozyme, which digest the cell wall of bacterium

  3. An opening is made in the bacterial cell wall

  4. The phage contracts and injects its DNA inside the host while the protein coat and the tail remain outside

  5. Inside the bacterial cell the phage DNA takes over the biosynthetic machinery of the host to synthesize its own DNA and protein molecule

  6. The phage multiplies and increases in number

  7. The daughter phages exert pressure on the cell wall of bacterium

  8. The bacterial cell ruptures (lysis occurs) and release the new phages, which are now ready to attack new bacteria and start their cycle again
• Bacteriophage


  Virus that infects bacteria
• Types of bacteriophage life cycle

  • Lytic cycle
  • Lysogenic cycle
• Lytic cycle

  1. Phage attaches to bacterial cell wall

  2. Phage injects DNA into bacterial cell

  3. Phage DNA takes over host cell machinery

  4. Phage multiplies

  5. Bacterial cell ruptures and releases new phages
• Lysogenic cycle

  1. Phage DNA integrates with bacterial chromosome

  2. Bacterium lives and reproduces normally

  3. Phage DNA can later become activated and enter lytic cycle