MODERN CELL DOCTRINE

The postulation of the cell theory  by schleiden and schwann generated  more interest in biologist  to investigate  all aspects of cell  and consequently , the knowledge about cells  became rich ;more facts emerged  will respect to cells .I n  light of the  available  facts,Rudolph  Virchow [1858] made another  biological generalization  that  new cells can arise  only from  pre-existing cells,i.e.,  ominis cellula  e cellula , leading  to the establishment  of the law  of lineage .This  made it clear  that the succession  of cells is continuity  of life and the vital  function of the body  such as metabolism  , growth , inheritance , diesease , aging , death , etc ., are varied  aspects  of the funxctioning and the behavour of the cells .These  facts modified  the  cell theory into modern cell  doctrine .The  main postulates   of the doctrine include   :                                                                                                                                                      1.The  cell is the structural  and functional  unit of  all living  things.                                                                       2.It is the  unit of development  of all living things .                                                                                              3. All  cells come from  pre-existing cells.                                                                                                       4. They are hereditary units     5. The cell  is the unit of pathology.                                                                                                       Two types  of Cells                                                                                                                               The  cells of organisms belong  to two  basic categories : the  prokaryotic  cells and the  eukaryotic  cells .The prokaryotic  cells lack the  membrane bound nucleus.Nor they have mitochondria ,golgi  body  and  lysosomes .Their   so called nucleus has just a single  chromosome , consisting  of a DNA molecule , without histone  and lying  exposed  in the  cytoplasm  Their  enzymes  involved  in the protein  synthesis  and respiration  , are associated  with the cell  membrane .Cell of bacteria  and  cyano bacteria  [blue green algae ]  are  prokaryotic  cells .                                                                                                                          The  eukaryotic  cells are true cells  of animals  from  from  protozoa  to mammalia , and plants  from algae to angiousperms .These  cells have a distinct  nucleus  ,bounded  by a nuclear  membrane .The nucleus has  nucleolus  and varying number of chromosomes.                                                                                                                                                          

PHYSICAL NATURE OR STRUCTURE

 It is  often coluorless or greyish ,translucent and somewhat viscous  semiliquid , that is ,  gelatinous  in texture .Its  viscosity varies from  cell to cell  in the same organism  ,and  from  organism   to organism .It is heavier than water  .Some substances  of the protoplasm    such as sugars  ,salts and simple acids and bases ,etc ., form crystalline  or true solution  but others  exist  in colloidal  solution .Thus  ,the protoplasm  may be called a complex  crystallo-colloid mixture .In colloidal system ,samll  finely divided  particales of matters [0.001u-0.1u]  are  suspended in a continuos medium  .The colloids   frequently posses   glue-like  consistency and are  not crystalline  in structure  .A very  viscous colloid  is called  a gel  and more  fluid  colloid a sol . Sometimes  sol  and gel are  converted into one another  .Thus  protoplasm  is a reversible colloid , that is , it changes into  sol or gwel when necessary  .However it  exists  normally in a state between  a sol and gel .       View  about  physical structure  .Under  light microscope  the protoplasm  of the different cell shows  different appearance    or different structure  , even the same cell shows  different structures at different times  .This has led the workers  to think  differently  about the structure  or appearance  of the protoplasm.                                                                                                                                                            The  view   of the workers are in the form  of the following theories :                                                                   1. Fibrillar theory .This theory , proposed by flemming  , states  that  the suspended  colloidal particles are in the form of fine fibres .                                                                                                                       2. Alveolar theory .This theory  ,proposed by Butschli [1892]  ,holds that the protoplasm  is an alveolar strucute  .It appears  to be  a foaming mass of globules  embedded in a  viscid medium  .                             3. Granular  theory .According to  this theory  , the protoplasm  presents  , a  granular  appearance ; many fine  granules are evenly  distributed  in a  liquid   medium  .This theory was proposed by  altman [1893] .        4 Reticular theory . This  theory states that the  structure  of the protoplasm  is reticular ,  the suspended particles  are in the form of  a network of fibres.                                                                                                                                            

LIFE SUBSTANCE: THE PROTOPLASM

What  is life ? What is to be alive? These are the questions which have not  been satisfactorily  answered even to this date .Many attempts  have already been made  and are being made  to disclose the mystery of life but we   have still not succeeded  .We know  only this  that  the life is the quality of the living substance,that is , the protoplasm , and it is always associated  with the protoplasm . Thus  protoplasm  is the physical basis of the life.                                                                                                                                                                             Discovery of Protoplasm                                                                                                                                                  The  protoplasm in the cell  was observed by corti  as  early as 1772 .In 1835 ,Dujardin  noted the constant  occurence  of a jelly -like  substance ,unlike ordinary  gelatin  or albumin in  cells of certain   simple animals ,for which he proposed  the name  sarcode .He  recognised  it as  the living material  of these animals  .Four years later ,Purkinje [1839] coined  the name  protoplasm  for the sarcode .In 1846 Von Mohl  also  proposed the name  protoplasm  for a substance which he found  in the cell  of plants in 1861 Max Schultze observed  the cell  contents of protzoa  and of plants and animal cells .He concluded  that the  contents of all cells were essentially    similar, and  he proposed  a theory which was later named  protoplasm  theory  by  Hertwig [1892] .This  theory postulates that the cells are accumulation of the protoplasm  with the  nucleus and a cell membrane .This theory  established the close  similarity of the living  substance of animals and plants  .Huxley [1968] defined protoplasm  as the physical basis  of life.                                                                              STRUCTURE OF PROTOPLASM                                                                           The protoplasm  is a complex mixture of  many chemical substances  .It keeps living thing  alive .Rather , it houses the life .Therefore , it is known as  life substance .The body  of all living  organisms  is composed  of the protoplasm  .The body  and cells  of different  organisms  differ  greatly  from one another  but there  is a basic  similarity in physical and chemical  structures  of their protoplasm  .The living state  of the protoplasm  is due to  its  composition   and physico-chemical  properties.                                                                       

SIGNIFICANCE OF VIRUSES IN ORIGIN OF LIFE

Since viruses have  characteristics  of both living and non-living objects with very simple organization as very minute particles  of nucleo- protein  , it may be concluded that the primitive life was like viruses .Their  posession  of the characteristics of both  the living  and non-living  objects suggests that  non-living inorganic  substances evolved into organic  substances  before  the origin  of the primitive  life.Further , since  viruses  may  synthesize DNA and protein  in the host cell , they  may be taken as  the  first  forms  of life  as per Oparin's  explanation  of the origin of life .However , the modern  view  about the  viruses  is that they  have  originated  from  the early  existing  cellular  form of life  and therefore , they can not be taken  as the early primitive  forms of life .Moreover,C.F.Luria [1971]  ,the noble prize winner , has called them bits  of heredity , looking  for a chromosome.                                                                                                                                                                                         ECONOMIC IMPORTANCE OF VIRUSES                                                                        Viruses  are responsible  for several diseases  of plants  and animals. In plants , they cause  leaf roll of potato , rugose  mosaic of potato, leaf curl  of tomato , leaf curl of papaya ,mosaic of papaya ,yellow  ringspot of lady finger ,mosaic of tobacco ,etc.                                                                                                                                  In man ,the viruses  cause small pox ,influenza,common cold ,mumps ,measles ,rabies,poliomyeletis  [polio],etc. The HIV [Human immuno deficiency virus]  cause AIDS [acquired immune deficiency  syndrome ] which  drastically cut the life of men and women .Besides , the viruses are supposed  to cause  cancer and tumor formation.                                                                                       

HOST'S DEFENCE AGAINST VIRAL INFECTION

In order   to combat the viral infection, the host -cells synthesize special coded proteins called interferons .These by intact animal or cultured cells in response to the viral infection ,and believed to be body's first  line of defense against viruses .In fact,these  inhibit replication of viruses.Various interferons, known so far ,fall under three main categories [1]IFNa,[2]IFNb,[3]IFNy .All  vertebrates species  are able to  synthesize  interferons . However ,normal cells  do not generally synthesize  interferons  until these are induced to do so .Their  synthesis  is induced  by the viral  infection .The different kinds of  the host-cells .The IFN -a , is  produced predominantly  by leucocytes ,IFN-B  mostly by fibroblasts and IFN-y only by  lymphocytes .                                                                    Antiviral Activity of  Interferons                                                                                                                                               Interferons  were first recognized  to interfere  with the viral infwection in cultured cells .In intact animals these are produced soon  after the virus infection , leading  to decrease in the virus -production .Apparently  ,these act as antibodies  against viruses .In fact, these  induce  synthesis of other proteins  which  inhibit the viral replication .                                                                                                             NATURE OF VIRUSES                                                                                      Since  the discovery of viruses , the controversy  has been going  on whether they  are living or non- living objects  because they have some of the characteristics  of  the both .Many biologists consider them  non-living because of their following  characteristics :                                                                                     1. Viruses  are not in the form of cells.They lack cell membrane and cytoplasm .                                          2. They   are composed  of aggregations of the units of heredity  enclosed in a protein sheath  or coat .             3. They do not  ingest food .                                                                                                                          4 .They do not have  energy  producing structure  nor they  respire  like cellular life .                                       5.  They can be  crystallised  like an ordinary substance  and stored  for  long periods  in bottle .                                                                                                                                                                                                                                                                    

SHAPE AND SIZE OF VIRUSES

Viruses are such smaller than the smallest bacterium .Their  size ranges  from 30to 300 nm  .That  is why their structure  was not known before discovery of the electron microscope .                                                                                             Viruses have assumed  different shapes .Some are helical or rod-shaped,e.g., tobacco  mosaic virus [TMV]  and influenza virus .Many  viruses are spherical   but on close examination ,their  shape is found to be many sided  or polyhedron.e.g., adeno virus ,herpes virus ,turnip yellow mosaic virus [TYMV] etc .Some viruses have complex shape .For example ,T4 bacteriophage  ,T4 bacteriophage ,or virus, vaccinia virus,etc.                                                                                                                                                                        
                       Structure of Viruses                                                                                                                              The electron  microscope  has shown  that all viruses exist as discrete particles ,each of which is called a virion .In other words ,a virion  is an individual of a virus .All  viruses have  a similar organization;avirion has acentral core of coiled strand of a nucleic acid,DNA or RNA  and a protein  coat called capsid  which surrounds the core .Some viruses  ,for example , herpes virus,have an outer  envelope ,composed  of polusacchearide or lipoprotein.                                                                                                                      The capsid consist of a number of  protein subunits  called capsomeres .The arrangement  of capsomers arround  the core  imparts  a particular  shape to virion of a  virus.The animal viruses , bacteriophages  and cyanophages  have DNA whereas plant viruses  other than  bacteriophages have RNA .                                             

HOST AND TUPES OF VIRUSES

The  available records shows that  the viruses attack bacteria ,cyanobacteria[Blue-green
algae],flowering plants and animals,Depending on the kind of the living organism attacked by them ,the viruses belong to the following four types;                                                                                                                     1.Bacterial viruses.usually called  bacteriophages,these viruses attack bacteria .If a small amount of  bacteriophage suspension  is added  to a liquid  medium  with rich bacterial growth , then the turbid liquid medium becomes  clean and full  of bacteriaophages.                                                                                         2.Blue-green algae viruses.These  viruses usually  called cyanophages,attack blue-green algae ,now called cyanobacteria.                                                                                                                                                   3.Plant viruses .These viruses attacks flowering plants ,e.g.,tobacco-mosaic virus,tomato-wilt virus ,yellow and vein mosaic virus,etc.                                                                                                                                         4.Animal viruses .These  viruses attack  animals  and include  influenza virus, rabies  virus,pox virus,mumps virus ,polio virus ,measle virus ,vacinnia virus ,adeno virus ,etc.                                                         DNA and RNA Viruses .Depending   on the presence  of DNA or RNA,the viruses  are either DNA virus or RNA virus .The DNA virus contains DNA   in its core ,whereas the RNA virus  has RNA in its core .The bacteriophages  are DNA  virus and plant viruses  are either  DNA or  RNA  virus .                                        Reverse transaction  viruses .These  are animal viruses ,strongly  linked  to cancer or tumor  formation ,and have  RNA  in thier core .Usually  the DNA makes copy  of sequence of its  nitrogenous bases in the RNA [transcription], but in these viruses ,the RNA makes  copy of sequence  of its nitrogenous  bases  on the DNA  [reverse transcription] hence these are called reverse transcription viruses or retroviruses in the abbreviated form .The hepatitis B virus , Epstein-barr virus ,herpes virus and papilloma virus are examples of retroviruses.