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.
Tuesday, September 27, 2011
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.
Saturday, September 24, 2011
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 .
Tuesday, September 20, 2011
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 .
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.
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.
Saturday, September 17, 2011
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 ].
Sunday, September 11, 2011
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.
Subscribe to:
Posts (Atom)