Nov 232012
 

Centrioles are non-membranous organelles which are present in the cytoplasm near the nucleus of the cell. They are present in all animal cells and in flagellated motile cells of lower plants. They have ability to repcentrioleslicate and occur in pairs which are called diplosomes. In a diplosome, two centrioles lie at a right angle to each other. Diplosome is covered by cytoplasmic sheath which is known as centrosphere. The complex structure which is formed by centrospheres and centrioles is called centrosome.

Each centriole is cylindrical and about 500 nm long and 200 nm in diameter. Each centrole is composed of nine groups of microtubules arranged in a circular manner, i.e. 9 triplets+0 arrangement of microtubules. Each of nine groups of microtubules is a triplet which is composed of three microtubules at the periphery end and no microtubules at the centre. The adjacent triplets appear to be connected to each other by fibrils giving a cart-wheel structure of centrioles in transverse section (T.S.).


Centrioles perform different functions inside the cell. They help to form spindle fibres. During cell division, at the beginning of nuclear division, centrioles replicate, separate and migrate to opposite poles from where they give spindle fibres. Similarly, they also help to form basal body of cilia and flagella.

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Nov 202012
 

We already know that viruses are one of the amazing organisms which have different interesting, unbelievable and unique traits or characteristics.

Characteristics of Viruses

  • Viruses are minute and simplest of all known organisms. They can only be seen with an electron microscope.
  • Viruses range in diameter from twenty to three hundred mµ.
  • They are able to pass through filters which do not allow even the smallest bacteria.
  • They are all obligate intracellular parasites and are able to carry out the typical life functions until they are inside a host cell.
  • Before 1935 A.D., viruses were regarded variously as toxic substances, enzymes, form of bacteria, unusual metabolic products etc.
  • They are composed of protein and nucleic acid. The nucleic acid form a central core, covered by a nongenetic protein coat, called the capsid.
  • Viruses infect every kind of living organism on the earth i.e. plants, fungi, protozoa, insects, birds, mammals and humans. They attack and kill even bacteria. The bacterial viruses are called bacteriophages or simply phages and those that attack the fungi are called mycophages.
  • They cause many highly infectious diseases of animals including man, angiosperms, bacteria and fungi.
  • They can multiply only in living cell.
  • Viruses are highly specific to their host. Each type of virus will recognize and infect only certain types of cell.

Nature of Viruses

Virus is an interesting scientific puzzle as it shows both living and non-living characteristics. The exact nature of virus is still an unsolved problem.

Non-living characteristics of Viruses

  • Viruses lack true cell structure and represent a different category of organization from cells.
  • They are incapable of performing life functions without a proper host. Thus, they depend upon hosts for their survival.
  • They lack enzyme system and do not have metabolic activity of their own.
  • Some viruses can be crystallized like a chemical substance, e.g. tobacco mosaic virus.
  • They can be precipitated by a number of chemical substances.
  • Viruses do not respire.

Living characteristics of Viruses

  • They can grow and multiply within the host cell.
  • They can infect healthy organisms and are capable of spreading diseases.
  • They respond to chemicals, light, temperature.
  • They are transmitted from the diseased host to the healthy ones.
  • Viruses show mutation.
  • They contain nucleic acid as their genetic material.

At present most of the virologists have agreed that viruses are nucleoproteins of high molecular weight and have the power of multiplication.