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.                                                                                                                

LIFE STYLE OF VIRUSES

The viruses become active and   reproduce within the cells  of other organisms  but they remain inert outsides the cells ,showing  no activity .Evidently ,they are obligate parasites .While  infecting  or attacking  other organisms  they show host  specificity .In general ,viruses are  specific in  attacking hosts, a particular  virus attacks a particular host ,choosing one and the same  organ  or closely  related organs .However  ,a few viruses ,for example ,rabbies virus and vaccinia virus  can infect man as well as cows  ,sheep,rabbits,monkeys ,rat ,mice ,guinea pigs,hamsters ,chick embryos ,etc.                                                                                                             In certain  instances  ,there is much  precision in host specificity ;there are some viruses  which attack only a particular  kind of  tissue .For example ,viruses of pox  and measles attack  and tomato-wilt virus attacks only leaves of tomato .Certain viruses ,attack bacteria infect  only male cell  but not the female cell.                                         Many viruses remain concealed within the host cell for a very  short to a long  period without  showing  any adverse effect on the host .Sch viruses are  known as latent virus .Some viruses that infect  two hosts ;remain  latent  in one host  but become virulent  in another .For  example  a virus known to  exist and  reproduce  in potato  without  showing  any pathogenic  symtoms .become  effective on transfer to tomatoes .                                                                                                                                                             Nomenclature of Viruses                                                                                                                           It  appears that the virus  nomenclature  is more arbitrary  and in confused  state.Instead of  adopting  the binomial  nomenclature ,viruses are named after the disease they cause ,for  example ,pox virus,polio  virus , tobacco -mosaic virus ,tomato wilt virus ,cherry-ringspot virus ,etc ., or  after the tissue or organ  they attack ,for example ,the ,the adeno-virus  that attacks adenoids .In certain  cases ,the naming is  more arbitrary ,for example ,the polyoma virus is  named so because it is  able to cause  numerous kind of tumors .The code number are often  used to name  the bacterial viruses .For example ,viruses that attack the  intestinal bacteria,Escherichia coli have  been  named  by using  T with  numeral  as suffix from 1to 7 [T,toT1]  .                                                                                                                                        

VIRUSES

Viruses are the smallest objects  that can  pass through  the filter  capable  of trapping even the smallest bacteria .These  are very minute particles  of  nucleoproteins   which can be usually viewed  only under  the electron microscope ;however, some are so minute that they can  not be viewed even under  the electron  microscope .Inspite  of this  ,scientists , better  say ,virologists  [specialists of viruses] ,have evolved special  techniques  to study  their structure ,behaviour and reproduction .These deserve special  consideration  in our survey of the  distinction between  the living and non living objects because they show  of the characteristics of both  and thus  ,they form a  connecting link between  the two types of objects .In order to evacuate  them more clearly as alink  between the two types of objects ,information about their structure  and lifestyle is essential .Such information is also important  due to this fact  that the viruses  are pathogenic ,causing harmful diseases  in man ,animal and plants .                                                                                                                                       Discovery of virus  .People  were not familiar with viruses till the end  of the 18th  century  but they definitely knew them to cause diseases .First of all ,Edward Jenner [1796] tried to  know about small pox and then Louis Pasteur [1882] tried to know  about the virus , causing  rabies .However  ,in 1892 Ivanovsky discovered that  the bacteria -free  extract of the tobacco plant ,already  affected by the  tobacco-mosaic disease,was successful in causing these diseases in healthy  tobacco plants  .this  fact was confirmed  by Beijernick [1898] and he named this extract  or filtrate  as contagium  vivum  fluidum  which  ,later , was named  filterable virus  ,and  finally ,virus literally meaning poison .The first animal  virus  responsible  for the foot-mouth  disease of cattle  was discovered by Reeds  in 1902 .The bacteria attacking viruses were discovered  for the first time by twort [1916]  in england and then  by d'Herelle [1917] in France .Later on ,W.M.Stanley [1935]  was able to isolate  Tobacco Mosaic  Virus [TMV] from the diseased  tobacco leaves .Since then many more viruses  have been discovered  and the diseases  caused by the m  have been recognized                                                                                                                                                                          CHARACTERISTICS OF VIRUSES                                                                                                Viruses are  curious and interesting .These exhibit  the following  characteristics :                                         1.Virus is a very minute particle  of nucleoproteins which can be seen  only with the  help of the electron microscope .                                                                                                                                                        2.  These  are very simple in organization  and lack cellular structure .                                                            3. These are very simple in organization  and lack cellular structure .                                                                 4. These   are composed of  DNA or RNA ,single or double stranded  and proteins as a sheath ,called  capsid .                                                                                                                                                              5. These donot have cytoplasm and plasmalemma [plasma membrane ].                                                                                                                                                                                 

EVIDENCES IN SUPPORT OF BIOCHEMICALORIGIN OF LIFE

Evolution of complex  organic molecules from simple molecules  ,as held by Oparin,has been proved by many scientists all over the world .This is well supported  by experimental  or  laboratory  studies.in this context  ,Stanley miller [1953] has taken alead to provide  experimental proof .                                                                                              Stanley Miller's Experiment                                                                                                                             IN 1953 ,Stanley Miller performed a simulation experiment  under  guidance of his professor ,Harold Urey  to test hypothesis of biochwmical origin of life .In this  experiment ,he  tried to create thse conditions  that might  have existed  in the primitive  atmosphere .For this  he built an apparatus as shown in Fig .2.1  to circulate CH4,NH3,H2and H20 and pass to circulate CH4,NH3,H2andH2O and pass electric  discharges.The assemblyu  of the  apparatus consisted  of two  flasks- one  large and another small , a condenser  and glass tubes .Two tungsten electrode  ,connected to an  electric source  were fitted  in the large flask  to provide  energy  of electric discharge ,simulating effect of lightning  of  the primitive  atmosphere and for this reason ,Stanley Miller's  apparautus  is called  Spark .                                                                         Discharge Apparatus .The  large flask contained  a  mixture of CH4,NH3 and H2[2:1:2]  and the small flask   had water  .The experiment  started  with switching on the electric source  and boiling the water  and it continued  for  a week .Then , on observation ,Miller  found  a dark coloured condensed  liquid .He  collect it and analysed  chromatographically .The analysis showed that  the fluid  was a mixture  of sugars,amino acids [e.g., glycine ,alanine ,aspartic acid,etc.]and  fatty acids .Miller's  this finding supports Oparin's theory  of origin of life  that during  course of origin of life , simple  molecules  evolved  into complex organic  molecules .                                                                                                                                                                                        Other Evidences                                                                                                                                           Oparin's  view has also been supported  experimentally  by Melvin and Calvin[1951] , Sydney.F.Fox[1964]  and Abelson ,Oro  and Kamat[1960,1965 ] .The latest support comes  from Dr.H.R.Khorana [1968]  who has synthesized  a DNA molecule ,consisting of 77nuccleotides ,in a test tube  .For this synthesis ,Dr.Khorana's synthesis of DNA molecule suggest that  during the course of origin of life ,nucleic acids  would have  formed  by linking  together of nucleotides  in the primitive  ocean.