Microorganisms exist as either prokaryotes or eukaryotes. The prokaryotic cells are morphologically much simpler than eukaryotic cells. The prokaryotic cells lack internal membrane bound organelles that are present in the cytoplasm of eukaryotic cells such as mitochondria and chloroplast etc. Though simple, prokaryotic cell contains a variety of structures, however, all the structures are not found in every prokaryotic organism, A prokaryotic cell is bounded by a chemically complex cell wall. A cell membrane lies below the cell wall and surrounds the inner cytoplasmic matrix. This cytoplasmic matrix include unbounded nucleoid, ribosomes and some inclusion bodies. External to the cell wall there occurs extra components possessing different functions. The most typical example for prokaryotic cell organization is the bacterial cell. Structurally a bacterial cell consists of Structures external to the cell wall, cell wall and structures internal to the cell wall.
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Microorganisms exist as either prokaryotes or eukaryotes. The prokaryotic cells are morphologically much simpler than eukaryotic cells. The prokaryotic cells lack internal membrane bound organelles that are present in the cytoplasm of eukaryotic cells such as mitochondria and chloroplast etc. Though simple, prokaryotic cell contains a variety of structures, however, all the structures are not found in every prokaryotic organism, A prokaryotic cell is bounded by a chemically complex cell wall. A cell membrane lies below the cell wall and surrounds the inner cytoplasmic matrix. This cytoplasmic matrix include unbounded nucleoid, ribosomes and some inclusion bodies. External to the cell wall there occurs extra components possessing different functions. The most typical example for prokaryotic cell organization is the bacterial cell. Structurally a bacterial cell consists of Structures external to the cell wall, cell wall and structures internal to the cell wall.
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ULTRA STRUCTURE OF BACTERIAL CELL
Microorganisms exist as either prokaryotes or eukaryotes. The prokaryotic cells are morphologically much simpler than eukaryotic cells. The prokaryotic cells lack internal membrane bound organelles that are present in the cytoplasm of eukaryotic cells such as mitochondria and chloroplast etc. Though simple, prokaryotic cell contains a variety of structures, however, all the structures are not found in every prokaryotic organism, A prokaryotic cell is bounded by a chemically complex cell wall. A cell membrane lies below the cell wall and surrounds the inner cytoplasmic matrix. This cytoplasmic matrix include unbounded nucleoid, ribosomes and some inclusion bodies. External to the cell wall there occurs extra components possessing different functions. The most typical example for prokaryotic cell organization is the bacterial cell. Structurally a bacterial cell consists of Structures external to the cell wall, cell wall and structures internal to the cell wall.
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Virus literally means ‘poison’. They are also called as filterable molecules during olden days as they are able to pass through filter pores which do not permit bacteria to pass. They are the submicroscopic entities. Andre Luoff, a virologist and Nobel Laureate, defined the viruses as “Viruses are Viruses”.
The general characteristics of Viruses are :
1. Viruses are a cellular organisms that are infectious in nature.
2. They are 10 to 100 times smaller than most bacteria and range from 20 to 200nm size.
3. They exist in two states namely extra cellular and intracellular.
4. They are obligate intracellular parasites
5. Viruses are incapable of independent growth in artificial media. They lack metabolic machinery of their own to generate energy or to synthesize proteins. Viruses depend on host cell machinery to carry out vital metabolic functions. However, they consist some genetic information for reproduction.
6. They reproduce only in living host cells such as plants, animals and microorganisms.
7. They structurally complete nature and infectious virus is called viron.
8. They possess either DNA or RNA as genetic material but never both.
9. The nucleic acid can be either single stranded or double stranded.
10. Genetic material is enclosed in a highly specialized protein coat called capsid.
11. The capsid may be naked or enveloped.
12. Some viruses such as bacteriophases possess a banal structure of head and tail attached.
13. They are neither prokaryotic or eukaryotic. They do not consist cytoplasm and cytoplasmic organelles.
14. Several viruses cause different diseases in plants, animals and human beings.
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VIRUS
Virus literally means ‘poison’. They are also called as filterable molecules during olden days as they are able to pass through filter pores which do not permit bacteria to pass. They are the submicroscopic entities. Andre Luoff, a virologist and Nobel Laureate, defined the viruses as “Viruses are Viruses”.The general characteristics of Viruses are :
1. Viruses are a cellular organisms that are infectious in nature.
2. They are 10 to 100 times smaller than most bacteria and range from 20 to 200nm size.
3. They exist in two states namely extra cellular and intracellular.
4. They are obligate intracellular parasites
5. Viruses are incapable of independent growth in artificial media. They lack metabolic machinery of their own to generate energy or to synthesize proteins. Viruses depend on host cell machinery to carry out vital metabolic functions. However, they consist some genetic information for reproduction.
6. They reproduce only in living host cells such as plants, animals and microorganisms.
7. They structurally complete nature and infectious virus is called viron.
8. They possess either DNA or RNA as genetic material but never both.
9. The nucleic acid can be either single stranded or double stranded.
10. Genetic material is enclosed in a highly specialized protein coat called capsid.
11. The capsid may be naked or enveloped.
12. Some viruses such as bacteriophases possess a banal structure of head and tail attached.
13. They are neither prokaryotic or eukaryotic. They do not consist cytoplasm and cytoplasmic organelles.
14. Several viruses cause different diseases in plants, animals and human beings.
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In bacteria sexual reproduction is of three types :
Transformation : The process of uptake of a complete or fragment of a naked DNA molecule by a competent cell from the medium and incorporation of this molecule into recipient chromosome in a heritable form is called as transformation. This type of gene transfer is discovered and demonstrated by Fred Griffith. The process of transformation is random and any portion of a genome may be transferred between bacteria. The process of transformation is an important and natural route of genetic exchange in bacteria. The transformation, as natural phenomenon, is found only in certain gram +ve and gram –ve bacteria such as Streptococcus, Bacillus, Thermoactinomyces Haemophilus, Azotobacter, Pseudomonas etc. In laboratories, the transformation is carried out artificially by treating the cells with calcium chloride. This treatment makes the cell membrane more permeable to DNA. However, the frequency of the transformation increases with high concentration of DNA.
Conjugation : The process of transfer of genetic material from one cell to another cell through direct cell-cell contact is called as conjugation. This process of conjugation was discovered in E.coli by J.Lederberg and E.L. Tatum in 1976. Later, scientists namely Hayes, Jacols and wollman proved that gene transfer in this process occurs only in one direction and it is non- reciprocal. This one-way gene transfer takes place from male or donor cell to female or recipient cell. Normally much longer DNA fragment is transferred in this conjugation process when compared to that of transformation process. Besides the main chromosome, some of the bacterial cells contain one or more. Small DNA molecules called as plasmids. This extra chromosomal genetic material can also be transferred from one cell to another cell through conjugation process.
Transduction : Transduction is defined as a process of genetic transfer from donor bacterial cell to recipient bacterial cell mediated through a bacteriophage. This process is first discovered by Norton Zinder and Joshua Lederberg in Salmonella, by a variety of bacteriophages. Transduction is one of the most common mechanism for gene exchange and recombination in bacteria. There are two type of transduction processes namely generalized transduction and specialized transduction.
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SEXUAL REPORDUCTION IN BACTERIA
In bacteria sexual reproduction is of three types :
Transformation : The process of uptake of a complete or fragment of a naked DNA molecule by a competent cell from the medium and incorporation of this molecule into recipient chromosome in a heritable form is called as transformation. This type of gene transfer is discovered and demonstrated by Fred Griffith. The process of transformation is random and any portion of a genome may be transferred between bacteria. The process of transformation is an important and natural route of genetic exchange in bacteria. The transformation, as natural phenomenon, is found only in certain gram +ve and gram –ve bacteria such as Streptococcus, Bacillus, Thermoactinomyces Haemophilus, Azotobacter, Pseudomonas etc. In laboratories, the transformation is carried out artificially by treating the cells with calcium chloride. This treatment makes the cell membrane more permeable to DNA. However, the frequency of the transformation increases with high concentration of DNA.
Conjugation : The process of transfer of genetic material from one cell to another cell through direct cell-cell contact is called as conjugation. This process of conjugation was discovered in E.coli by J.Lederberg and E.L. Tatum in 1976. Later, scientists namely Hayes, Jacols and wollman proved that gene transfer in this process occurs only in one direction and it is non- reciprocal. This one-way gene transfer takes place from male or donor cell to female or recipient cell. Normally much longer DNA fragment is transferred in this conjugation process when compared to that of transformation process. Besides the main chromosome, some of the bacterial cells contain one or more. Small DNA molecules called as plasmids. This extra chromosomal genetic material can also be transferred from one cell to another cell through conjugation process.
Transduction : Transduction is defined as a process of genetic transfer from donor bacterial cell to recipient bacterial cell mediated through a bacteriophage. This process is first discovered by Norton Zinder and Joshua Lederberg in Salmonella, by a variety of bacteriophages. Transduction is one of the most common mechanism for gene exchange and recombination in bacteria. There are two type of transduction processes namely generalized transduction and specialized transduction.
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Some organisms are very small and can’t be seen with the unaided human eye. Hence these small and invisible organisms is called MICROBIOLOGY.
Organism with a diameter of 1mm or less are can’t be seen with the human eye. Hence these organisms are called after the invention of microscope. Traditionally microorganism had been included in Botany or Zoology. The present day biologists can’t agree with the inclusion of these microorganisms in Botany or Zoology because they exhibit greater diversity than plants or animals.
Classification of Microorganisms:
At present five groups of organisms are identified in micro organisms.
1. Bacteria
2. Viruses
3. Algae
4. Fungi
5. Protozoa.
Hence Microbiology has been classified into five branches.
(1) Viruses : These are acellular, infectious agents. They have no living characters except replication, Nuclic acid. They cause several diseases.
(2) Bacteria : these are smallest unicellular organisms. They show prokaryotic cellular organization. Ex : Eubacteria, Cynobacteria, Mycoplasmas, Rickettsiae and Chlamydiae
(3) Algae : these are plgmented, photosynthetic autotrophic micro organisms. Ex : Chylamydomonas, Spirogyra etc.
(4) Fungi : these are heterotrophic, non-green, eukarotic microorganisms. They posses a thick wall usually made up of polysaccharides (chitin). The body is made up of filaments called hyphae with forms into a mycelium. Ex : Pencillium, Puccinia, Mucor, Agaricus etc.
(5) Protozoa : These are commonly defined as unicellular, eukaryotic, non- photosynthetic microorganisms. Ex : Amoeba, Plasmodium, Paramecium.
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IMPORTANCE AND SCOPE OF MICROBIOLOGY AS A MODERN SCIENCE
Some organisms are very small and can’t be seen with the unaided human eye. Hence these small and invisible organisms is called MICROBIOLOGY.Organism with a diameter of 1mm or less are can’t be seen with the human eye. Hence these organisms are called after the invention of microscope. Traditionally microorganism had been included in Botany or Zoology. The present day biologists can’t agree with the inclusion of these microorganisms in Botany or Zoology because they exhibit greater diversity than plants or animals.
Classification of Microorganisms:
At present five groups of organisms are identified in micro organisms.
1. Bacteria
2. Viruses
3. Algae
4. Fungi
5. Protozoa.
Hence Microbiology has been classified into five branches.
(1) Viruses : These are acellular, infectious agents. They have no living characters except replication, Nuclic acid. They cause several diseases.
(2) Bacteria : these are smallest unicellular organisms. They show prokaryotic cellular organization. Ex : Eubacteria, Cynobacteria, Mycoplasmas, Rickettsiae and Chlamydiae
(3) Algae : these are plgmented, photosynthetic autotrophic micro organisms. Ex : Chylamydomonas, Spirogyra etc.
(4) Fungi : these are heterotrophic, non-green, eukarotic microorganisms. They posses a thick wall usually made up of polysaccharides (chitin). The body is made up of filaments called hyphae with forms into a mycelium. Ex : Pencillium, Puccinia, Mucor, Agaricus etc.
(5) Protozoa : These are commonly defined as unicellular, eukaryotic, non- photosynthetic microorganisms. Ex : Amoeba, Plasmodium, Paramecium.
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Microorganisms are widespread and omnipresent. They live in water, air and soil. They also live in side the plants and animals. Microorganisms are widely employed in many industries connected with the manufacture of food, medicines, textiles and chemicals. They improve soil fertility. Microorganisms are also associated with the health and welfare of human beings. Microorganisms like viruses, bacteria and fungi cause several diseases in animals, human beings and plants. The spoilage of food materials, industrial substances, wood, iron pipes is also due to the action of microorganisms.
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MICROBES AND THEIR DISTRIBUTION
Microorganisms are widespread and omnipresent. They live in water, air and soil. They also live in side the plants and animals. Microorganisms are widely employed in many industries connected with the manufacture of food, medicines, textiles and chemicals. They improve soil fertility. Microorganisms are also associated with the health and welfare of human beings. Microorganisms like viruses, bacteria and fungi cause several diseases in animals, human beings and plants. The spoilage of food materials, industrial substances, wood, iron pipes is also due to the action of microorganisms.
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Microorganisms serve as specific agents for large scale chemical transformations, specially variety of geochemical changes. Winogradsky and Beijerink proved that microbes play important role in the Carbon cycle, Nitrogen cycle, Sulphur cycle etc. they also proved the role of microorganism in the fixation of atmospheric nitrogen fixation.
Microbiology contributed significantly to the development of new branches like biochemistry and Genetics. Buchner discovered cell- free alcoholic fermentation which became a foundation to the biochemistry. The studies on microorganisms helped in the discovery of vitamins, enzyme.
Beadle and Tatum isolated a series of biochemical mutants from fungus called Neurospora. Delbruck and luria carried researches on mutation in bacteria. Several mechanisms of genetic transfer were discovered in bacteria (conjugation, transduction, transformation).
Avery Mc Leod and Mc cart basing on the studies of genetic transformation studies in bacteria discovered that genetic matter in living organisms is DNA.
Microbiology made many contributions for the development of molecular biology.
Microorganisms are responsible for the manufacture of various foods of man. The biochemical reactions of microorganism helps in the process of production of foods like Curd, Bread, Alochol, Soya Souce, SCP protein etc. the fruiting bodies of some fungi like Agaricus, Plurotus etc are used as mushrooms, SCP protein are synthesized from culturing of micro organisms like Chlorella, Spirulina, Yeasts etc.
Microorganisms helps in several industries like medicine industry, jute industry. Hormones, Vitamins, Vaccins are also manufactured from microorganisms. They also used in Recombinant technology, cell fusion technology etc.
Hence the branch microbiology developed into a independent modern branch of science.
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IMPORTANCE OF MICROBIOLOGY
Microorganisms serve as specific agents for large scale chemical transformations, specially variety of geochemical changes. Winogradsky and Beijerink proved that microbes play important role in the Carbon cycle, Nitrogen cycle, Sulphur cycle etc. they also proved the role of microorganism in the fixation of atmospheric nitrogen fixation.Microbiology contributed significantly to the development of new branches like biochemistry and Genetics. Buchner discovered cell- free alcoholic fermentation which became a foundation to the biochemistry. The studies on microorganisms helped in the discovery of vitamins, enzyme.
Beadle and Tatum isolated a series of biochemical mutants from fungus called Neurospora. Delbruck and luria carried researches on mutation in bacteria. Several mechanisms of genetic transfer were discovered in bacteria (conjugation, transduction, transformation).
Avery Mc Leod and Mc cart basing on the studies of genetic transformation studies in bacteria discovered that genetic matter in living organisms is DNA.
Microbiology made many contributions for the development of molecular biology.
Microorganisms are responsible for the manufacture of various foods of man. The biochemical reactions of microorganism helps in the process of production of foods like Curd, Bread, Alochol, Soya Souce, SCP protein etc. the fruiting bodies of some fungi like Agaricus, Plurotus etc are used as mushrooms, SCP protein are synthesized from culturing of micro organisms like Chlorella, Spirulina, Yeasts etc.
Microorganisms helps in several industries like medicine industry, jute industry. Hormones, Vitamins, Vaccins are also manufactured from microorganisms. They also used in Recombinant technology, cell fusion technology etc.
Hence the branch microbiology developed into a independent modern branch of science.
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The first person who discovered microorganism was a Dutch merchant Anton Van Leeuwenhoek. He named them as animalcules or little animals. Later scientific experiments helped to disprove the theory of spontaneous generation. Lewis Pasteur stated that microorganisms bring about chemical changes in organic infusions. The disease ‘late blight of potato’ which is responsible for Irish famine was due to fungus. Berkely identified that the disease causing organism of ‘late blight of potato’ is fungus. Brigerinck isolated nitrogen fixing bacteria like Azatobacter and Rhizobium. Winogradsky isolated sulpur and Iron bacteria.
The first antibiotic penicillin was discovered by Alexander Fleming. This antibiotic is produced by a fungus called pernicillium notatum. Waksman discovered the antibiotic called streptomycin from bacteria. Lederberg and Tatum discovered a type of sexual reproduction called conjugation in bacteria. Watson & Crick proposed a model for the structure of DNA which is called as ‘Double helical structure. Jacob and Monod proposed Operon Concept’ for explaining gene concept. Diener discovered viroids. Nerenberg and Koran proposed genetic code triplet code.
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HISTORY OF MICROBIOLOGY
The first person who discovered microorganism was a Dutch merchant Anton Van Leeuwenhoek. He named them as animalcules or little animals. Later scientific experiments helped to disprove the theory of spontaneous generation. Lewis Pasteur stated that microorganisms bring about chemical changes in organic infusions. The disease ‘late blight of potato’ which is responsible for Irish famine was due to fungus. Berkely identified that the disease causing organism of ‘late blight of potato’ is fungus. Brigerinck isolated nitrogen fixing bacteria like Azatobacter and Rhizobium. Winogradsky isolated sulpur and Iron bacteria.
The first antibiotic penicillin was discovered by Alexander Fleming. This antibiotic is produced by a fungus called pernicillium notatum. Waksman discovered the antibiotic called streptomycin from bacteria. Lederberg and Tatum discovered a type of sexual reproduction called conjugation in bacteria. Watson & Crick proposed a model for the structure of DNA which is called as ‘Double helical structure. Jacob and Monod proposed Operon Concept’ for explaining gene concept. Diener discovered viroids. Nerenberg and Koran proposed genetic code triplet code.
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Refers to any heritable change in nucleotide sequence of a gene of the organisms irrespective of altered phenotypic expression of characters of the organisms. A gene codes for a protein, therefore, the physical and chemical properties of proteins are changed due to alteration in genes. Genes are made up of nucleotide sequences. Hence, changes in a gene refer to changes in nucleotide or nucleotide sequence. For example, if the normal sequence was ATT, the change to AAT may lead to entirely different amino acids into a polypeptide chain. Therefore, mutation is the result of stable and heritable changes in nucleotide sequence of DNA. These changes may include alteration of a single base pair of nucleotides or addition/deletion of one or more nucleotides in the coding region of a gene.
When an amino acid substitution has no detectable effect on phenotype, it is known as silent mutation. On the other hand if a bacterium carries such a mutation in the enzyme that synthesizes an essential amino acid, it can grow very slowly unless the medium is supplied with that substance. This type of mutation is called leaky mutation.
For an organism to exist with stability, it is necessary that the nucleotide sequence of its gene must not be altered to such an extent that can promote instability. Changes to some extent brings about alteration in phenotypic expression. Though changes in genetic make up is harmful to an organism, but it is necessary to generate variability in organism and contribute to the process of evolution in nature.
The process of formation of a mutant organism is known as mutagenesis. Mutagenesis occurs in an organism by two mechanisms, (i) spontaneously (ii) through physical or chemical agents. The mutation that arises all of sudden without any effort is called spontaneous mutation. The mutation that arises through induction by addition of chemicals (i.e. mutagens) or radiation is called induced mutation.
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MUTATION AND MUTAGENESIS
Refers to any heritable change in nucleotide sequence of a gene of the organisms irrespective of altered phenotypic expression of characters of the organisms. A gene codes for a protein, therefore, the physical and chemical properties of proteins are changed due to alteration in genes. Genes are made up of nucleotide sequences. Hence, changes in a gene refer to changes in nucleotide or nucleotide sequence. For example, if the normal sequence was ATT, the change to AAT may lead to entirely different amino acids into a polypeptide chain. Therefore, mutation is the result of stable and heritable changes in nucleotide sequence of DNA. These changes may include alteration of a single base pair of nucleotides or addition/deletion of one or more nucleotides in the coding region of a gene.When an amino acid substitution has no detectable effect on phenotype, it is known as silent mutation. On the other hand if a bacterium carries such a mutation in the enzyme that synthesizes an essential amino acid, it can grow very slowly unless the medium is supplied with that substance. This type of mutation is called leaky mutation.
For an organism to exist with stability, it is necessary that the nucleotide sequence of its gene must not be altered to such an extent that can promote instability. Changes to some extent brings about alteration in phenotypic expression. Though changes in genetic make up is harmful to an organism, but it is necessary to generate variability in organism and contribute to the process of evolution in nature.
The process of formation of a mutant organism is known as mutagenesis. Mutagenesis occurs in an organism by two mechanisms, (i) spontaneously (ii) through physical or chemical agents. The mutation that arises all of sudden without any effort is called spontaneous mutation. The mutation that arises through induction by addition of chemicals (i.e. mutagens) or radiation is called induced mutation.
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The genes of transposase and resolvase i.e. tnpR and tnpR are identified by recessive mutations. The above enzymes accomplish the two stages of TnA mediated transposition. Like IS- type elements the transposition stage involves the ends of the elements. A unique feature of TnA family is that a specific internal site is required for resolution.
The mutants of tnpA cannot transpose because the enzyme transposase will not be encoded. However, transposase recognizes the ends of elements and binds to 25 bp long sequence located within 38 bp of the inverted terminal repeat. Transposase also makes the staggered 5 bp long sequence breaks in target DNA where transposon is to be inserted. Resolvase functions in two ways, (i) it acts as repressor of gene expressions, and (ii) provide the resolvase function. The frequency of transposition gets increased in tnpR mutants because tnpR represses the transcription of both tnpA and its own gene. Inactivation of tnpR protein allows the increased synthesis of tnpA resulting in the increased transposition frequency. Therefore, the amount of tnpA transposase is a limiting factor in transposition.
The genes, tnpA and tnpR express divergently from an ATP rich enter-cistronic central region. The effects of tnpR are also medicated by its binding in this region. TnpR resolvase gets involved in recombination between direct repeats of Tn3 in a co integrate structure. But in Tn3, resolution reaction occurs only at a specific site.
The res is the site where the recombination carried by tnpR resolvase occurs. The res site is identified by cis-acting deletions. The deletions block transposition resulting in accumulation of co integrates. The sites bound by tnpR resolvase have been determined by foot printing the DNA- protein complex. It binds independently at each of three sites i.e. I, II and III, each 30-40 bp long. Site I is the region genetically defined as the res site. In the absence of site I, resolution reaction does not proceed. However, resolution also involves binding at sites II and III. In the absence of either of II and III sites reaction proceeds poorly. Site I overlaps with the start point for tnpA transcription and site II with the start point for tnpR transcription.
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GENETICS OF TRANSPOSITION
The genes of transposase and resolvase i.e. tnpR and tnpR are identified by recessive mutations. The above enzymes accomplish the two stages of TnA mediated transposition. Like IS- type elements the transposition stage involves the ends of the elements. A unique feature of TnA family is that a specific internal site is required for resolution.The mutants of tnpA cannot transpose because the enzyme transposase will not be encoded. However, transposase recognizes the ends of elements and binds to 25 bp long sequence located within 38 bp of the inverted terminal repeat. Transposase also makes the staggered 5 bp long sequence breaks in target DNA where transposon is to be inserted. Resolvase functions in two ways, (i) it acts as repressor of gene expressions, and (ii) provide the resolvase function. The frequency of transposition gets increased in tnpR mutants because tnpR represses the transcription of both tnpA and its own gene. Inactivation of tnpR protein allows the increased synthesis of tnpA resulting in the increased transposition frequency. Therefore, the amount of tnpA transposase is a limiting factor in transposition.
The genes, tnpA and tnpR express divergently from an ATP rich enter-cistronic central region. The effects of tnpR are also medicated by its binding in this region. TnpR resolvase gets involved in recombination between direct repeats of Tn3 in a co integrate structure. But in Tn3, resolution reaction occurs only at a specific site.
The res is the site where the recombination carried by tnpR resolvase occurs. The res site is identified by cis-acting deletions. The deletions block transposition resulting in accumulation of co integrates. The sites bound by tnpR resolvase have been determined by foot printing the DNA- protein complex. It binds independently at each of three sites i.e. I, II and III, each 30-40 bp long. Site I is the region genetically defined as the res site. In the absence of site I, resolution reaction does not proceed. However, resolution also involves binding at sites II and III. In the absence of either of II and III sites reaction proceeds poorly. Site I overlaps with the start point for tnpA transcription and site II with the start point for tnpR transcription.
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The transfer of genetic material from one cell to another by a bacteriophage is called transduction. The phenomenon of transduction was first discovered by Zinder and Lederberg while searching for sexual conjugation in Salmonella species.
The infection by a bacteriophage is accomplished in several stages such as adsorption, penetration, replication, assembly, lysis and release. In brief the virus particles first attaches to specific receptor site on bacterial call wall surface. The genetic material penetrates the bacterial call, and replicates independently by using cell machinery of the host. Consequently, the virus DNA is replicated into multiple copies, and synthesizes phage proteins. Complete phage particles are assembled and finally cell is lysed resulting in release of virus particles.
Depending on mode of reproduction that reproduction the bacteriophages are of two types, the virulent phage and the temperate phage, the phage that reproduce by using a lytic cycle are called virulent phages because they destroy the host cell such as T phage, phage lambda, etc. in contrast the temperate phages ordinarily, do not lyse the bacterial cell. The viral genome behaves as episome like F factor and becomes integrated into the bacterial chromosome. The latent form of phage genome that remains within the host without harm and integrates with chromosome is called prophage. Bacteria containing prophage are known as lysogenic bacteria and the relationship between phage and its host is called lysogeny. The lysogenic bacteria can produce phage particles under some conditions, and the phage is able to establish the phenomenon of lysogeny and behaves as temperate phage.
Usually, transduction occurs most readily between the closely related species of same genus of a bacterium i.e. intrageneric. This preference is due requirement for specific cell surface receptor for recognition of the phage. In addition, intergeneric transduction has been shown between the closely related enteric bacteria such as between E. coli salmonella or shigella species. Several genetic traits for example fermentation potential, antigens, chemical resistance are transducible.
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TRANSDUCTION
The transfer of genetic material from one cell to another by a bacteriophage is called transduction. The phenomenon of transduction was first discovered by Zinder and Lederberg while searching for sexual conjugation in Salmonella species.
The infection by a bacteriophage is accomplished in several stages such as adsorption, penetration, replication, assembly, lysis and release. In brief the virus particles first attaches to specific receptor site on bacterial call wall surface. The genetic material penetrates the bacterial call, and replicates independently by using cell machinery of the host. Consequently, the virus DNA is replicated into multiple copies, and synthesizes phage proteins. Complete phage particles are assembled and finally cell is lysed resulting in release of virus particles.
Depending on mode of reproduction that reproduction the bacteriophages are of two types, the virulent phage and the temperate phage, the phage that reproduce by using a lytic cycle are called virulent phages because they destroy the host cell such as T phage, phage lambda, etc. in contrast the temperate phages ordinarily, do not lyse the bacterial cell. The viral genome behaves as episome like F factor and becomes integrated into the bacterial chromosome. The latent form of phage genome that remains within the host without harm and integrates with chromosome is called prophage. Bacteria containing prophage are known as lysogenic bacteria and the relationship between phage and its host is called lysogeny. The lysogenic bacteria can produce phage particles under some conditions, and the phage is able to establish the phenomenon of lysogeny and behaves as temperate phage.
Usually, transduction occurs most readily between the closely related species of same genus of a bacterium i.e. intrageneric. This preference is due requirement for specific cell surface receptor for recognition of the phage. In addition, intergeneric transduction has been shown between the closely related enteric bacteria such as between E. coli salmonella or shigella species. Several genetic traits for example fermentation potential, antigens, chemical resistance are transducible.
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GENETIC NOTATION
If a bacterial cell synthesizes amino acid leucine, it is represented as Leu+, and if it does not it is denoted as Leu+. The symbol has capitalized (not italicized) the letters. It denotes that Leu- has some defective genes which cripple the cell to synthesize leucine. The defective gene is represented as leu- (italicized three letters). However, if more than one gene are needed to synthesize leucine, it is denoted as leuA, leuB, etc. and the functional gene as leuA+ leuB, etc.
It should be kept in mind that the bacteria always have a single set of genes i.e. they are haploid. The eukaryotic organisms are haploid as well as diploid but the dominance of these two phases in the life of an organism differs. The diploid cells of organisms contain two sets of genes. The double sets of functional genes are represented as leu+/leu. They may have normal or defective gones. The functional form of a gene is called wild type.
However, if a bacterium is resistant to certain antibiotics it is represented by giving the symbols, for example Tet (resistant to tetracycline), Amp (resistant to ampicillin), Kan (resistant to Kanamycin), etc. the microorganisms susceptible to the above antibiotics are represented as Tet, Amp, Kan, etc.
Mutations occurring in genotype are also represented by numbers in the order in which they have been isolated. For example, if leucine mutation has occurred on 58 and 79 position it is written as leu58 and leu79 respectively. More specifically to denote the mutation on a particular gene e.g. A and B, it is written as leuA58 and leuB79 if mutation has occurred in leuA and leuB genes, respectively.
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It should be kept in mind that the bacteria always have a single set of genes i.e. they are haploid. The eukaryotic organisms are haploid as well as diploid but the dominance of these two phases in the life of an organism differs. The diploid cells of organisms contain two sets of genes. The double sets of functional genes are represented as leu+/leu. They may have normal or defective gones. The functional form of a gene is called wild type.
However, if a bacterium is resistant to certain antibiotics it is represented by giving the symbols, for example Tet (resistant to tetracycline), Amp (resistant to ampicillin), Kan (resistant to Kanamycin), etc. the microorganisms susceptible to the above antibiotics are represented as Tet, Amp, Kan, etc.
Mutations occurring in genotype are also represented by numbers in the order in which they have been isolated. For example, if leucine mutation has occurred on 58 and 79 position it is written as leu58 and leu79 respectively. More specifically to denote the mutation on a particular gene e.g. A and B, it is written as leuA58 and leuB79 if mutation has occurred in leuA and leuB genes, respectively.
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ORIGIN AND EVOLUTION OF GENETIC CODE
From the discussion of genetic code some important facts came into light as the degeneracy and universality of the genetic code. It may be supposed that present from of genetic code would have evolved from a more primitive code which must have occurred about three billion years ago. Since the evolution of bacteria, the code has remained fixed. Any mutation that altered the sequence would changed the reading frame of mRNA resulting in changes in specifying amino acids. Wong has discussed in detail about the evolution of genetic code
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