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Wednesday, September 30, 2009

DNA (Short Note)

Wednesday, September 30, 2009 - 1 Comment

DNA is Deoxyribose nucleic acid. It occurs in chromosomes (nuclear reticulum). DNA has deoxyribose with one less oxygen atom in its molecule. DNA is double stranded molecule. Biologists are of view that all secrets of life are embodied in DNA or in other words it is the chemical basis of life. DNA is sole genetic (hereditary) material migrating intact from generation to generation through reproductive units or gametes and is responsible for the development of specific characters of a plant. It also controls the biosynthetic processes of the cell including protein synthesis, Watson and Crick gave model of DNA in 1953 which double stranded structure. Each strand (helix) is a chain of several nucleotides and is known as poly nucleotide chain. Each nucleotide consist of a de oxyribose sugar, one nitrogen base and one molecule of phosphoric acid. Nitrogen bases are adenine, guanine, cytosine and thiamine.

Dicyclic nitrogen bases are purines, therefore adnine and guanine are purines. Monocyclic nitrogen bases like cytosine and thiamine are known as Pyramidines.

Linkage and Crossing Over (Short Note)

According to Conservative estimate there are thousands of genes in four pairs of chromosomes of Drosophila. It means that each chromosome contains large number of genes, similar is the situation in all organisms. The chromosomes behave as single units. All the genes in a given chromosome tend to remain together during inheritance. This tendency of genes in a chromosome to remain together is called ‘Linkage’. This linkage is not absolute and the genes do not remain locked up in the same chromosome for ever. Otherwise the inheritance of traits would also have remained constant. During meiosis the homologous chromosomes come together and form pairs, a process called ‘Synapsis’. Soon after they sometimes exchange segments mutually, a process is called ‘Crossing over’. This exchange occurs ranelonly along the length of chromosome. After separation the chromosomes carry some genes that were earlier located in different member of their pair of homologous chromosome. Such exchanges of chromosomal segments or crossing over may occur at more than one point in homologous chromosomes in single meiotic division.

Genetic Engineering (Short Note)

It is the manipulation of genetic material of any organism. Genetic engineering usually utilize bacterial cells and their plasmids, which are small circular DNA molecule. They can replicate feely within bacterial cells. Genetic engineering can produce cells that contain a foreign gene. These results ar capable of producing new and different Protein. As a result of growth of these cells so many identical copies of plasmid with a foreign gene are produced.

Genetic engineering bacteria can be used in the environment, for serving in the field of agriculture etc. These bacteria can also be used to promote health of plants to make them resistant towards insects as bioremediation (pollution cleaner) to synthesize organic chemicals to dect metals, to enhance genetic research to produce pharmaceutical products. In agriculture these bacteria can be used to promote the health of plant in different ways.

Genotype (Short Note)

The sum total of all the hereditary units or genes which an organism contains are together known as genotypes. Briefly it means that phenotypes are not inherited, only the genes which control the development of phenotypes are indeed inherited. Whatever is the phenotype of an animal, it is not the result of genotype only but is accumulative result of the interaction of the genotype and environment. The organism having same genotype might produce different phenotypes in different types of environment. However the ability of an organism to produce a character is largely determined by its genotype, although the environment can modify the action of a gene. In brief neither the genes alone nor environment alone can produce a character. The development of phenotype actually depends upon the factors i.e. genes and surrounding environment.

Phenotype (Short Note)

All the physical appearances by which recognize an individual are together known as phenotypes. For example rabbit has specific shape and structure which includes so many characteristics i.e. length of legs, length of ears, length of tail, mouth, abdomen, thorax etc: and overall body shape. Almost of all the animals, plants, bacteria and viruses are bio-chemically very similar. Therefore we can say that the phenotype may be visual, microscopic, structural, fundamental or biochemical i.e. phenotype of metabolism of the process of photosynthesis, the transmission of nerve impulse along the nerve fibres, muscular contraction, blood circulation the development of secondary sexual characters and the photoperiodism and the fall of leaves in some trees are examples of physiological phenotypes. Phenotype may be of reproductive capacity. Reproduction may be at molecular level, cellular level (gametes) or individual level. Phenotype is combination of many features which together contribute to the characteristic phenotype of an individual.

Monday, September 28, 2009

Law of Segregation (Short Note)

Monday, September 28, 2009 - 0 Comments

The factors for contrasting characters remain associated in pairs in the somatic cells of each plant throughout its whole life. Later in its life history when spores (and subsequently gametes) are formed as a result of reduction division, the factors located in homozygous chromosomes become separated out and each of the four spores (and gametes) will have only one factor (tallness or dwarfness) of the pair but not both i.e. a gamete becomes pure for particular character. This law is also called law of purity of gametes. Mendel also experienced on other pairs of alternative characters and he found that in every case the characters followed the same scheme of inheritance. Thus in the garden pea he discovered that coloured flower was dominant over white flower; yellow seed over green seed and smooth seed over wrinkled seed.

Law states that ‘the genes or factors representing a pair of contrasting characters, when brought together by the union of two gametes coexist in the cells of the offspring and later become separated in the gametes, so that each male or female gamete carries only one gene for a member of the pair but never both.

Test Cross (Short Note)

Tall plants produce in F2 generation may either have TT or Tt genotypes. Mendel devised simple way to distinguish the genotype. It is known as test cross. The organism of dominant phenotype (tall) but unknown genotype is crossed to homozygous recessive individual. For example a tall plant is crossed with a dwarf plant. If the tall plant is homozygous (TT), all F1 plants would be tall. But if the tall plant is heterozygous (Tt), one half one F1 plants would be tall (tt) and one half dwarf (tt) exhibiting ratio of 1:1. This ratio is called test cross ratio.

Back Cross (Short Note)

The term test cross is often used interchangeably with back cross. However two are not necessarily the same. The test cross is a cross of an individual of dominant phenotype to homozygous recessive. On the other hand back cross is mating of an individual of known phenotype to anyone of homozygous parents. For example in above mentioned example the cross between F1 hybrid to homozygous recessive individual is test cross but if we cross the F1 hybrid to a homozygous tall (TT), it is a back cross. Back cross may have special value in some genetic analysis such as Casey incomplete dominance. Because the homozygous necessive individual is also one of the parents, therefore test and back crosses are regarded same.

Multiple Alleles (Short Note)

A gene for a trait having three or more allelic forms are called multiple alleles. A well known example of multiple alleles in human beings is that of blood groups. The type of blood group that a person may have, depends upon the presence or absence of certain specific substances in the red blood cells. These substances are antigens and are of two kinds Antigen A and Antigen B. A person with Antigen A has blood group A and that with antigen B, has blood group B. A person with A and B antigens has blood group AB. Similarly a person lacking both antigens falls in blood group O. It is now known that there are three alleles responsible for this trait Gene 1A and 1B are responsible for the synthesis of antigens A and B respectively. Their alternative genes (i) synthesize no antigens at all. As only two alleles can occur in the individual, therefore, three alleles can have only four possible combinations. Alleles 1A and 1B are co dominant and both are dominant over allele ‘i’ Blood groups are helpful in determining the paternity.

Co-Dominance (Short Note)

It may be described as phenomenon of inheritance in which both alleles of a contrasting character are dominant and express themselves in heterozygous individual neither masking the effect of one another. In a cross between a tree breeding short horn red cattle and a true breeding white short horn cattle, the offspring have roan colour. A close examination of the skin or roan coloured animal shows that the animal does not possess an intermediate shade of skin colour, but is appears so because of mixture of red hair and white hair. It is clear that none of the two genes is dominant over the other. Such pairs of alleles of a gene are said to be ‘Co dominant’ and the phenomenon as ‘Co Dominance’.

Evidences of Evolution

According to theory of evolution organisms living today have arisen from earlier types of organisms by a process of genetic change that has occurred over a period of several billion years. The fact that all organism have arisen from a common ancestor explains why they have the same mechanisms for the storage and utilization of genetic information, many of the same types of cellular organelles and similar types of enzymes and metabolic pathways. Evolution explains how a single species can give rise to numerous other species. The biologists may argue over the mechanisms of evolution; they agree that all life descended with modification from single common ancestor.

(1) EVIDENCES FROM PALEONTOLOGY

It is study of fossils. Fossils are remains of life from the past. Fossils are formed when organisms become buried in sediment, the calcium in bone and other hard tissue is mineralized and the sediment is converted into rock. They may include entire organism, hard skeletal structures and casts petrifications, impressions, imprints and fossilized faecal pellets. Earliest known fossil plants are about 410 million years old. Seed bearing plants with feather like leaves similar to living cycads were abundant in Mesogoic era. Oldest fossils of flowering plants or angiosperms are from early cretaceous period.

(2) EVIDENCES FROM COMPARATIVE ANATOMY

Comparing the structures of the parts of the bodies of different organisms is probably the most commonly used evidence of evolution. In order to gather comparative evidence for evolution, biologists study external characteristics, examine bones and teeth dissect organs systems, study sections of tissue under the light microscope and examine finer details of cells and tissues under the election microscope.

(i) Homologous Structures:

Comparative anatomy reveals that certain structure features are basically similar. For example the basic structure of all flowers consist of sepals, petals, stamens, stigma, style and ovary yet the size colour, number of parts and specific structures and different in different species. Organ having similar basic pattern but modified into different forms to perform characteristics functions are called homologous structures.

(ii) Adaptive radiation:

When a group of organisms share a homologous structure which is differentiated to perform a variety of different functions, it illustrates a principle known as adaptive radiation variations between different species within the class enable them to adapt to different habitat.

Analogous Structure: Similar structures, physiological process or modes of life in organisms that differ in their structural but show adaptations to perform the same functions are called analogous structures. E.g. Presena of thorns o plant stens and spines on animals. The existence of analogous structures suggest convergent evolution. In this type of evolution. The environments act through the agency of natural selection favouring advantageous variations.

(3) EVIDENCE FROM COMPARATIVE EMBRYOLOGY

The evidence of progressive development on the basis of embryology can be seen in plants. the gametophytes of primitive mosses and forms are represented by Protonema produced by germination of the spores has similar structure, physiology and pattern of growth to filamentous green algae from which they thought to be evolved. Alternation of generation in plant life cycles and homologous variation in it are considered to be due to adaptations to various environmental conditions e.g. Gymnosperms are regarded as a group in-between land plants and plants which still requires water for fertilization. In cycas male gametophyte resemble pollen grains of angiosperms in that it is distributed by wind. As it develops pollen tube is formed but instead of carrying gametes it act as hanstorium for absorption of food. The gametes are flagellated and swim to ovule to fertilize the egg present there. Cycas therefore represents intermediate group of plants. it suggests that gymnosperms are adapted to different environment and are phylogenic intermediate forms between non vascular plants and angiosperms. Many of these intermediate forms are extinct and are represented by fossils.

(4) EVIDENCE FROM COMPARATIVE BIOCHEMISTRY

As techniques of biochemical analysis have become more precise, this field of research provided evidences for evolution. The occurence of similar molecules in complete range of organisms suggest the existence of biochemical homology in similar way to the anatomical homology shown by organs and tissues. Most of the research which has been carried out on comparative biochemistry has involved analysis of primary structure of protein molecules such as cytochrome, haemoglobin and most recently nucleic acid molecules particularly r RNA.

Variation and Kinds of Variation

Term variation means the differences in characteristics shown by organisms belonging to the same species. Organisms within the same species vary from one another variations may be

(1) Genetic Variations

(2) Environmental Variations

(1) GENETIC VARIATIONS

Variation between individuals are caused by genes. Most genes have different forms called alleles. In sexually reproducing organisms these alleles are reshuffled each time a new organism is produced. Reshuffling occurs during meiosis as a result of crossing over and independent assortment. The random fusion of gametes from two parent also produces new combinations of alleles in the offspring. Another source of genetic variation is mutation, crossing over, independent assortment and random fusion produce new combinations of alleles while mutations produce completely new alleles. Genetic variations are inheritable therefore when organisms produce they pass on some of their genes to their offspring genetic variations affect DNA of the organisms.

(2) ENVIRONMENTAL VARIATIONS

Some variations among individuals are not caused by their genes. Plants exhibit such variations. They may differ in size and colour of leaves because one may be growing in shade and nutrient deficient soil and other in light and nutrient rich soil. Such differences that arise during organisms life time are called environmental variations. These variations do not affect DNA of organisms, therefore are not inherited.

A study of phenotypic difference in any large population shows that two types of variation occur

(1) Discontinuous variation

(2) Continuous variation

(1) Discontinuous Variations: There are certain characteristics within a population which exhibit a limited form of variation. Variations in this case produce individuals showing differences with no intermediate forms. For example a person belongs to one of the four blood groups in ABO blood system, A, B, AB and O. There are no in-betweens and only a few clearly defined groups of individuals. Such variations are called discontinuous variations.

(2) Continuous Variations: The characteristics show complete gradation from one extreme to the other without any break. For example human skin or eye colour cannot be categorised into clearly defined colours. Continuous variations may be caused by genes or by environment of by both. Variation in leaf length is oxford ragwort plant is caused by environment entirely because all cells in the plant were produced by mitosis from a single zygote and so contain exactly the same genes.

Sources of variations

Genetic variation do not occur in asexually reproducing organisms as replication of DNA is perfect. Any apartment variation in these organisms may be due to influence of environment. There is opportunity for genetic variation to arise in sexually reproducing organisms.

The main causes of variation are:

(1) Gene Recombination

(2) Mutations

(1) Gene Recombination: Gene recombination originate as a result of crossing over during meiosis, random migration of chromosomes at the time of call division and random fusion of male and female gametes during fertilization.

(2) Mutation: The gene reshuffling do not generate major changes in the genotype, which are necessary to give rise to new species. These changes are produced by mutations.

A mutation is a change in the amount or the structure of DNA of an organisms. This produces change in the genotype which may be inherited by cells derived by mitosis or meiosis from the mutant cell. Mutation may result in the change in appearance of characteristic in a population. Mutations occurring in gamete cells are inherited, where as those occurring in somatic cells can only be inherited by daughter cells produced by mitosis. They are called somatic mutations.

A change in the amount of arrangement of DNA is known as chromosomal mutation or chromosomal aberration where as change in structure of DNA at a single locus is called point mutation or gene mutation. The term mutations is usually used to describe gene mutation.

Chemical Mutagens: Certain chemicals such as nitrous acid, hydroxylamine, dimethyl solfonate, methyl ethyl sulphate, acridive etc act as mutagens. In addition a variety of other chemical substances including mustard gas, Caffeine, formaldehyde, Colchinine, certain components of tocacco and increasing number of drugs, food preservatives and pesticides also cause mutations.

Induced Mutation: A mutation that is produced artificially during experimentation by using mutagens such as X-rays, ultra violet rays etc are called induced mutations.

Spontaneous mutation: The mutation which arises for no apparent reason and cause a genetic alteration is called spontaneous mutation. It is random and natural. It may occur at any site on a chromosome. It is found that majority of them produce just a slight effect and are harmful.

Evolution and Theories of Evolution

Evolution means development of life with time. Evolution means that living things change. A species may slowly change into new species or even into two or more new species. It means that the plants and animals that are present on earth now are not the first plants and animals, it also means that many plants and animals that once flourished are no longer alive, as one species evolves into other species, the original species no longer exists. The knowledge about these extinct species is provided by their remains preserved in the rocks called fossils Theory of evolution has special place in the study of history of life. It helps us to understand the unit as well as the diversity of plants and animals. But evolution has not made the organisms completely different DNA always carries the instructions for inheritance. Evolution provides an unifying picture of life. Life has common pattern because all life is interrelated through evolutionary descent. This is evident from classification which has its scientific basis in evolutionary interrelationship. By evolution scientists try to discover how this process of change occurs.

(1) THEORY OF EVOLUTION BY NATURAL SELECTION

Charles Darwin during voyage accumulated geological and fossil evidence that supported the idea that life changes with time. He studied flora and fauna of main land South America. He found evidence development of a variety of forms among finches (a kind of birds) from single ancestral group due to adoption to feeding on different kinds of food. He also studied the effect of light on plant growth that later led to discovery of plant hormones, plants of various habitats. He said there is contest among the members of a population for food to which called ‘Survival of the fittest’. He found that same struggle exists among all living things. He suggested that all species show variations with time. Some variations are of advantage in struggle for existence. Organisms with favourable variations are most likely to breed and to pass on their favourable characteristics. In this way new species arise from existing ones because the natural selects the fittest, evolution by ‘natural selection’.

LAMARCK’S THEORY OF EVOLUTION - LAMARCKISM

French naturalist Lamarck in 1809 gave hypothesis based on two conditions.

(1) The use and disuse of parts

(2) The inheritance of acquired characters.

According to Lamarck changes in the environment may lead to changed patterns of behaviour which can necessitate new or increased or disuse of certain organs or structures. Extensive use would lead to increased size, disuse would lead to degeneracy and atrophy. These traits acquired during life time of the individual were believed to be heritable and thus transmitted to offspring.

Weisman theory of continuity of Germplasm

Weisman proposed theory of continuity of germplasm. He postulated that somatic or body acquired characters resulting in variation did not affect the germ or gamete cells (in other words DNA, the genetic material) which are responsible for transmission of genetic characters. The germ cells are protected and nourished by somatic cells and are passed on intact and unmodied from generation to generation. Weisman on the basis of his theory argued that bodily modifications brought out by the environmental changes and by the use and disuse of organs cannot affect the germ cells and therefore cannot be transmitted to the next generation.

According to Weisman only germ cells of an individual contain all the determinants for all hereditary characters but the somatic cells contain only those determinants which determine the development of particular characters of the tissues or organs which they form. For example the nucleus of nerve cell contains those determinants which control the development of characters of nerve cell. Same nucleus behaves differently in different cells because of differences in their cytoplasm.

Dominance, Incomplete Dominance, Co-Dominance, Test Cross and Back Cross

DOMINANCE

If two different alleles of the same gene (T and t) come together by fusion of gametes, they co exist as such although one of them expresses itself called dominant and the other is unable to express itself and is called recessive. If F1 plants are tall therefore the allele of tallness is dominant and allele of dwarfness is recessive. Therefore Tall character is dominant on recessive.

INCOMPLETE DOMINANCE

In case of flower colour Mendel found that red colour of flower was dominant over white colour of flowers. White coloured flowers reappeared in F2 following Mendel’s principle of segregation. Later a deviation to this rule was observed in a plant species commonly called four o clock (Mirabilis Jalapa). The plant produces flowers with red and white colours. When pure breeding red (R) flowered plants were crossed with pure breeding white (r) flowered plants, F1 plants were Pink (Rr) flowered. Appearance of pink flowers, an intermediate shade between red and white provided relief to those who believe in blending inheritance. But when F1 hybrids were crossed (Rr × Rr), F2 generation showed a phenotypic ratio of 1 red: 2 pink: 1 white instead of typical 3: 1 ratio. Although the ratio contradicted Mendel’s principles but it provides strong support to the concept of particular inheritance.

Appearance of red and white factors (genes) in F2 indicates that these have not been altered or blended white present together in the pink flowered individuals. F2 reds and whites are same as the parental red and white. This was declared a case of ‘incomplete dominance’. In this case neither of the genes is dominant to the other. Each expresses itself in the presence of its allele to produce intermediate effect. Another important aspect of incomplete dominance is that phenotypic ratio (1: 2: 1) is the same as genotypic ratio (1 homozygous dominant: 2 heterozygous dominant: 1 homozygous recessive) resulting from cross when two monohybrids are crossed, when dominance is incomplete, a cross of two monohybrids (Rr × Rr) gives a phenotypic ratio of 1: 2: 1 which is identical to genotypic ratio. The heterozygous individual shows incomplete dominance. Where as when the dominance is complete the phenotypic (3: 1) and genotypic ratio are complete.

CO DOMINANCE

In case of blood types in human, both genes (A & B) produce an effect in a heterozygous individual. This is called co dominance. The genes which govern A and B blood types are alleles. Each control the formation of different red blood cell protein or antigen. Antigen in case of person having blood group A and antigen b in individuals with blood group B. Neither gene is dominant to the other. Heterozygous individuals with blood group AB contains both antigens a and b. Both proteins are detected in equal amounts in the red cells.

TEST CROSS

Tall plant produced in F1 generation may either have TT or Tt genotypes. Mendel devised a simple way to distinguish the genotype. It is known as test cross. The organism of dominant phenotype (tall) but unknown genotype is crossed to homozygous recessive individual. For example a tall plant is crossed with a dwarf plant. If the tall plant is homozygous (TT), all F1 plants would be tall. If the tall plant s heterozygous (Tt) one half of F1 plants would be tall (Tt) and one half dwarf (tt), exhibiting ratio of 1: 1. That ratio is called ‘test cross’.

BACK CROSS

The term test cross is often used interchangeably with back cross. However, two are not necessarily the same. The test cross is a cross of an individual of dominant phenotype to homozygous recessive. On the other hand back cross is mating of an individual of known phenotype to any one of homozygous parents. For example in the above mentioned example the cross between F1 hybrid to homozygous recessive individual is a test cross, but if we cross the F1 hybrid to homozygous tall (TT), it is a back cross. The back cross may have special value in genetic analysis such as in case of incomplete dominance. Because the homozygous recessive individual is one of the parents therefore test cross and back crosses are regarded same.

Plant Breeding Techniques

Plant breeding techniques are as under:

(1) Selection (2) Hybridization (3) Mutation breeding

(4) Polyploid breeding (5) Tissue culture (6) Genetic engineering

(1) SELECTION

It is the process of isolating desirable genotypes from undesirable ones. Selection is oldest method and basis of all plant breeding techniques. The genotypes most suitable for human needs are isolated. It is carried out in a number of ways depending on the mode of reproduction and types of breeding method used. Selection tends to change gene frequencies and is thus responsible for creating new gene pools. A plant breeder selects plants according to his specific needs, for example higher grain yield, sugar or potato content or early maturity. Since artificial selection tends to change gene frequencies in genotypes, changes in the genetic make up of new varieties are expected.

(2) HYBRIDIZATION

If sufficient genetic variability is not available for effective single plant or mass selection, the variability can be created artificially through hybridization. The varieties or species with desirable characteristics are selected and crossed to produce hybrids. The populations are heterozygous and plants with combined desirable features of the parents are selected for testing and further growth. The hybridization result in a single true breeding strain possessing best characteristics of the parent varieties. Large number of plants are used in these crosses because the greater the number of plants used, the greater the chances of obtaining the desired combination of characters and the easier the development of a variety with good agronomic qualities.

(3) MUTATION BREEDING

Mutation is sudden change in hereditary material of a cell. Mutation involves:

(a) A change in gene from one allele to other.

(b) Rearrangement of chromosome material.

(c) Loss or duplication of chromosome segment.

Changes in genes are often called point mutations. The use of mutation in plant breeding is popularly known as ‘mutation breeding’. Mutations are induced through physical mutagens, as x-rays, fast neutrons, thermal neutrons, ultra violet radiation and beta radiation. In addition to physical mutagens large numbers of chemical mutagens are also used to induce mutations in crop plants. Whole plant or any part of plant can be treated by radiation seeds, are most commonly used for irradiation. They offer a number of advantages. They are easily to handle and store and can be maintained for extended periods of time. When dry seeds are almost inert biologically and sever environment cause no significant biological damage.

(4) POLYPLOID BREEDING

Polyploid is a condition in which individuals have more than two chromosome sets or genomes in their somatic cells. In contrast to the normal diploid (2n) they may be triploid (3n), tetraploid (4n), pentaploid (5n), and hexaploid (6n) and so on.

Polyploid plants may arise by duplication of the chromosome sets for single species, auto polyploidy, or by combining chromosome sets from two or more species, alloploidy. Alloploidy in more common method of ploidy in nature. An alloploid in which total chromosome compliment of two other species is combined to form a fertile species hybrid is called amphiploid. Many commonly cultivated crop species have evolved in nature as polyploids for example oats, cotton, tobacco, sorghum, Brassica, common wheat, forage grasses and legumes.

(5) TISSUE CULTURE TECHNIQUE

It is commonly used to describe in vitro and aseptic cultivation of any part on a nutrient medium. The technique was first introduced by Haberlandt in 1902. It has provided an efficient selection tool for modern plant breeding.

In part, if a plant breeder wished to propagate a hybrid plant, a method of vegetative reproduction was used, such as taking stem, leaf or root cuttings. This is a relatively rapid way of reproduction in plant, but not all plants can reproduced like this Tissue culture is a method of vegetative reproduction on mass scale. Within a few years several thousand plants, mostly identical to the original plant can be produced. However some mutations may occur giving occasional variant plants. The propagation of plants by tissue culture is also used after genetic engineering.

(6) GENETIC ENGINEERING

The aim of genetic engineering is to remove a gene from one organism and transfer it into another in such a way that the gene is expressed in its new host. The host is now known as transgenic. Genetic engineering provides a way of overcoming barriers to gene transfer between species. Indeed the genes in question have often been taken by organisms in different kingdoms, such as a bacterial gene but into a plant or a human gene into a bacterium. Unlike selective breeding where whole sets of genes are transferred; genetic engineering results only in the transfer of single gene. Genetic engineering must obtain the wanted gene, clone the gene to produce many copies and insert a copy of the gene into the host DNA.

Sunday, September 27, 2009

Mutation, Gene Mutation, Chemical Mutagen and Recombination

Sunday, September 27, 2009 - 0 Comments

MUTATION

Genes along with their chromosomes are fairly stable and transmit heredity more or less uncharged. They sometimes undergo changes called ‘Mutation’. Mutation is any change in the amount, organization or content of genetic material. It does not include exchange as a result of recombination between homologous chromosomes. Mutations may be obvious and observable by cytological techniques. Some changes in chromosomes are called chromosomal aberrations. They may be however invisible when they are in genes. Both types of mutations when passed from adult to the offspring alter the hereditary instructions.

GENE MUTATION

A change in genetic massage of a cell is called Mutation. Mutational changes that affect the message itself producing alterations in the sequence are called ‘point mutations’; since they usually involve only one or a few nucleotide. In both bacteria and eukaryotic individual genes may move from one place on the chromosome to another by a process called ‘transposition’. When a particular gene moves to different location there is often alternation in its expression or in that of neighbouring genes. In eukaryote large segments of chromosomes may change their relative location or undergo duplication. Such chromosomal rearrangement often has drastic effects on the expression of the genetic message.

Point mutations involving only one or few nucleotide, result either from chemical or physical damage to the DNA or from spontaneous pairing errors that occur during DNA replication. First class of mutation is of particular practical importance because modern industrial societies produce and release into the environment many chemicals capable of damaging DNA. These chemicals are called Mutagens.

CHEMICAL MUTAGENS

Many mutations result from the direct chemical modification of DNA bases. The chemicals that act on DNA fall into three classes (i) Chemicals that look like DNA nucleotides but pair incorrectly when they are incorporated into DNA (ii) chemicals that remove the amino group from adenine or cytosine, causing them to mispair and (iii) chemicals that add hydrocarbon groups to nucleotide bases, also causing them to mispair.

RECOMBINATION

Scientists J. Lederberg and E. L. Talum discovered specific type of sexual reproduction in E. coli bacteria. Two mutant kinds were found in E. coli bacteria. One mutant strain was named as Y10 and other mutant strain was known as Y24. Scientists prepared mixed cultures of Y10 and Y24 by providing all six additional substances which were necessary for their growth. They also discovered various recombinations of six mutations. Because the rate of back mutation was already determined in these bacteria which were very low rate therefore it was explained that the discovery of wild type bacteria and other recombinations of mutations could be the explanation that the exchange of hereditary material might have occurred during a process that was named as conjugation.

Genetic Engineering and Biotechnology and Basic Genetic- Engineering techniques

GENETIC ENGINEERING:

It is the manipulation of genetic material of any organism. Genetic engineering usually utilized bacterial cells and their plasmids, which are small circular DNA molecule. They can replicate freely within bacterial cells. Genetic engineering can produces cells that contain a foreign gene. These cells are capable of producing new and different protein. As a result of growth of these cells so many identical copies of plasmid with a foreign gene are produced. Basic steps in recombination of DNA technology are:

(1) Preparation of recombinant DNA (r DNA) molecule.

(2) Insertion of r DNA into host cell.

(3) Multiplication and production of numerous copies of host with r DNA in it.

(4) Selection of bacteria with required gene.

TRANSGENIC PLANTS:

Free living plants in the environment that have a foreign gene inserted into than are said to be transgenic organisms or genetically engineered organisms. Plants in particular are easy to genetic manipulation because they can grow by tissue culture, where entire plant can be grow from single cell. Some techniques have been developed to produce transgenic plants. r DNA can be introduced into embryo or must be in cell wall-removed cell called protoplast. The only plasmid for transgenic plants cell is Ti Plasmid, transferred by its host Agro bacterium to many but not all plants.

Main aims of developing transgenic plants are:

(i) To cultivate more nutritious plants, seeds of these plants contain all amino acids required by human. Protein enhanced beans, soybeans, corn and what are now developed.

(ii) Plants require less fertilizer and are able to make rise of nitrogen from atmosphere.

(iii) Plants grow under harsh and unfavourable condition.

GENE SEQUENCING:

Gene sequencing is a method of determining mucleotide sequence of a gene (DAA molecules) developed in late 1970s by Fredrick Singer. This gene sequence can provide great deal of information. It is often the fastest way to determine the amino acid sequences of its polypeptide, more over; it provides the location of restriction site within a gene which can be manipulated further later on.

The main phases of gene sequencing are firstly to cut genes into specific small pieces, then each fragment is individually sequenced.

Most sequences that are determined are being collected in computer data bank that are valuable for biotechnology.

Genomic Library: It consist of genetic information of a species in a preferred environment. This library provides easy access to a preferred gene for its further copying or manipulation. In order to establish a genomic library of particular species, its DNA is fragmented by means of restriction enzymes and then the fragments are inserted into plasmids and bacteriophages which are introduced into bacteria. Such bacteria are cultured at controlled conditions so that they can be used later on.

Role of BioteChNology in diagnosis of diseases:

Biotechnology is now playing very important role in the diagnosis of infections as well as genetic diseases. The use of PCR and DNA propes in providing an excellent tool for the diagnosis of such diseases even before the onset of sumptoms. Medical scientists can now diagnose more than 200 genetic diseases using such technologies. Though hybridization alalysis, now it is possibleto detect abnormal alletic forms of genes present in DNA samples.

Genetherapy

One of the potential benefits of genetic engineering is to treat genetic diseases in individuals. Theoretically it could be possible to replace or supplement the defective allele with a functional, normal allele. This could be inserted into somatic cells of the child or adult or into the germ cells or embryonic cells.

First illness likely to be treated by this techniques is called sever combined immuno deficiency disease (5CID) which is characterized by a very poor immune system. In this case the cells of bone marrow can not produce an enzyme called adenosine deaminase (ADA).

Gene therapy of germ live cells is another matter which has raised complex issues of safety and ethics. In this type recombinant DNA would be inserted into human sex cells so not only the treated individual be affected but so would all the individuals descendents.

GENETIC COUNSELLING

In most cases genetic counselling entails predicting the chances of recurrence of condition that has already affected one or more members of a family. Such counselling depends first on precise diagnosis of disorder; the counsellor must be able to explain why the disorder occurred and how it is inherited. Counselling is important for parents of a child with genetic disorder such as cystic fibrosis, hemophilia, Down’s syndrome.

Some uses and applications of biotechnology in agriculture and medicine: Biotechnology brings revolution in the field of agriculture, medicine and other fields of biological sciences. One of the most important advantage of biotechnology in that it allows mass production of proteins which were difficult to obtain in past. Another field of biotechnology is the production of hormone. Human growth hormone produced by biotechnology is used to treat dwarfs. Insulin also produced by biotechnology is being used to treat diabetics. Biotechnology also contributes to diagnose by making DNA probes available. DNA probe is a specific sequence of single stranded DNA often radio active which binds by complimentary base pairing to a gene of interest.

Cotton, corn, potato and Soyabean plants have been engineered to be resistant to either insect predation or herbicides which are also environmentally safe. In 1999 transgenic crops were grown on more than 70 million acres around the world.

Biotechnology made progress in enhancing food quality of crops like soyabeans which produced oleic acid unsaturated fatty acid.

Conjugation and Gene Recombination in E. Coli also Transduction and Transformation

Conjugation and Recombination:

J. Lederberg and E. L. Tatum discovered specific type of sexual reproduction E. coli bacteria which was named as Conjugation. Wild type E. coli usually grow on a minimal medium containgin inorganic salts and glucose. Two mutant kinds were found in E. coli bacteria which could not grow on the minimal medium. One mutant strain was named as Y10 and the other mutant strain was known as Y24.

Y24 strain was unable to synthesize three chemical substances i.e. threonine, Leucine (both amino acids) and a vitamin biotin. Various experiments revealed that both the strains can give rise to wild type back mutants but at very low rate i.e. 1/1000000 (one per million) or even 1/10000000 (one per hundred million). The scientists prepared mixed cultures of Y10 and Y24 by providing all six additional substances which were necessary for their growth. They discovered wild type bacteria from the mixed culture. They also discovered various recombinants of six mutations. Because the rate of back mutation was already determined in these bacteria which was very low rate, therefore it was explained that the discovers of wild type bacteria and other recombination of mutations could not have possibly originated due to back mutation.

This only was to explain the origin of coild type bacteria and recombinants could be the explanation that the exchange of hereditary material might have occurred during a process that was named as Conjugation.

SEXUAL MATING TYPE IN BACTERIA:

In E. coli the bacterial strains between which the exchange of hereditary material occur by Conjugation are known as mating types one mating type which donates its hereditary material to the other is usually known as F+; other bacterial strain which receives the hereditary material is called as F-. F+ cannot mate with F+ type; similarly F cannot mate with F- type. Frequency of recombination of hereditary material is relatively low. Period of 430minutes is normal time of Conjugation.

TRANSDUCTION:

When a phase virus is allowed to cause infection in bacteria, a few bacteria survive and are not destroyed by phage, although the phages chromosome had entered the bacteria. Such resistant bacteria are supposed to carry phage chromosome in inactive form known as prophage of temperate phage; however such passive phage itself is known as temperate phage. Resistant strains of bacteria which escape from Lysis are known as Lysogenic. The presence of phages chromosome in Lysogenic bacteria in an inactive form is useful to bacteria because it enables them to with stand infection and prevents the reproduction of phage particles. In 1952 Zinger and Lederberg discovered that such inactive phage in the bacteria might act as a carrier for the transfer of genes from one bacterium to another bacterium, the process is known as transduction.

Example: In Salmonella (bacteria) there are two mutant strains, one of which is tryptoplan (amino acid) dependent and the other is cysteine dependent. When these two mutant strains were grown together and were allowed to be infected with T4-phage virus, three types of bacteria were discovered from the culture i.e. wild type bacteria, tryplophan dependent bacteria and cysteine dependent bacteria. The rate of occurance of wild type bacteria in this case is much greater than can be expected by back mutation, therefore, it was concluded that the wild type bacteria in this case might have been produced by Transduction i.e. some of the wild type T4-phage virus which were containing some of the genes of the other bacteria would have been responsible to induce the property of synthesizing typtophan and cysteine.

TRANSFORMATION:

If some living strain of bacteria is exposed to purified DNA extract of some of other strain of bacteria, recombinant type bacteria arise which may show recombination for one gene that are known as single gene transformation or they may show recombination for two genes which are known as double transformants or may show recombination for three genes that are known as triple transformants. Various experiments of transformation on all Mutants have shown that many genes are involved to synthesize and enzyme amylomaltase which is actually responsible to metabolize maltose.

Example: There are two strains of bacteria one of which is resistant to sulpha drug and amethopterin (a toxic substance), the other strain is resistant to streptomycin and micrococin. When mixed culture of these two strains were grown in Laboratory or the DNA extract of one strain is added to the living culture of the other strain osme bacteria resistant to all the four substances were produced. Formation of such recombinant type bacteria by adding DNA extract of the other strain was indicative of genetic transformation.

Regulation of Gene Expression

The hereditary apparatus of the body works in much the same way as that of most primitive bacteria; all organisms use the same mechanism. RNA copy of each active gene is made and at a ribosome RNA copy directs the sequential assembly of a chain of amino acids. There are many minor differences in the details of gene expression between bacteria and eukaryote and a single major difference. The basic apparatus used in gene expressions appear to be the same in all organisms. It apparently has persisted virtually unchanged since early in the history of life. The process of gene expression occurs in two phases which are called transcription and translation.

(1) TRANSCRIPTION: First stage of gene expression is the production of RNA copy of the gene called messenger RNA or mRNA. Like all classes or RNA that occur in cells mRNA is formed on DNA template. The production of RNA is called transcription; messenger RNA molecule (as well as other t RNA and r RNA) is said to have been transcribed from DNA.

Transcription is initiated when a special enzyme called RNA Polymerase binds to a particular sequence of nucleotides on one of DNA strands. This sequence is located at the edge of a gene.

(97) FIG 6.10 PAGE 157 BIOLOGY XII SINDH TEXT BOOK BOARD

Starting at that end of the gene, RNA polymerase proceed to assemble a single strand of RNA with a nucleotide sequence Complementary to that of the DNA strand it has bound. Complementarity refers to the way in which two single strands of DNA that form a double helix relate to one another, with A (ademine) pairing with T (thymine) and G (guanine) pairing with C (cytosine). RNA strand complementary to thymine is uracilc.

As the RNA polymerase moves along the strand into gene, encountering each DNA nucleotide in turn. It adds the Corresponding Complementary RNA nucleotide to the growing RNA strand. When the enzyme arrives at a special stop signal at the for edge of the gene, it disengages from the DNA and releases the newly assembled RNA chain. This chain is complementary to DNA strand from which the polymerase assembled it; thus it is RNA transcript (copy) called primary RNA transcript, of the DNA nucleotide sequence of gene.

TRANSLATION:

The second stage of gene expression is the synthesis of a Polypeptide by ribosomes, which use the information contained in and mRNA molecule to direct the choice of amino acids. This process of mRNA directed Polypeptide synthesis by ribosomes is called ‘Translation’ because nucleotide sequence information is translated into amino acid sequence information.

Translation begins when r RNA molecule within the ribosome binds to one end of mRNA transcript. Once it has bound to mRNA molecule, a ribosome proceeds to move along mRNA molecules in increments of three nucleotides. At each step it adds amino acid to a growing polypeptide chain. It continues to do this until it eucounters a ‘stop’ signal that indicates the end of Polypeptide. It then disengages from mRNA end releases newly assembled Polypeptide.

Genetic Code: There is sued the word code several times to refer to the information stored in DNA and ultimately translated into amino acid sequence of Proteins. This genetic code is conceptually similar to Morse Code. One set of symbols (bases in nucleic acids, dots and dashes in Morse Code) can be translated into another set of symbols (amino acids in Proteins, letters of the alphabet). The question is what combinations of bases stand for which amino acids?

Translation in Protein synthesis

Translation:

The Mechanism of Protein synthesis: Translation is the final stage of gene expression during which mRNA molecules are translated into Polypeptides. The process is complex and occurs only in association with ribosomes. Three major types of RNA molecules produced during transcription, messenger, ribosomal and transfer RNA work together to synthesize proteins by the process of translation. However the main fact about translation is that sequence of amino acids in the polypeptides being synthesized in specified by the sequence of nucleotides in mRNA molecule being translated. The genetic code determines the sequence of nucleotides in mRNA molecule that specified the sequence of amino acids in a polypeptide. The process of translation can be subdivided into the following four phases.

(1) t RNA charging (2) Chain initiation

(3) Chain Elongation (4) Chain termination

(1) Charging t RNA:

Role of t RNA: Each cell contains a number of different kinds of t RNAs. Each t RNA is distinguished in functional terms by its specificity for one of 20 amino acids involved in Protein synthesis for example t RNA tyr is specific tyrosine and t RNA 2Ly for glycerine.

A t RNA forms a covalent linkage with its amino acid and can recognize and attach to a Codon specifying that amino acid. There may be more than one type of t RNA molecule for single amino acid as most amino acids are Coded by more than one Codon.

CHARGING OF AMINO ACYLATION:

Before translation starts t RNA molecules must be chemically linked to their respective amino acids. This is called charging or amino acylation. The charging is controlled by a group enzymes called amino acyl synthetases. There is specific amino acyl synthetase enzyme for each amino acid.

(2) Initiation of Polypeptide chain:

Initiation of translation involves ribosomal subunits, mRNA molecule, a specific initiater t RNA, GTP, Mg++ and at least three non ribosomal protein initiation factors which enhance binding between various components. Ribosomes do not exist free in the cytoplasm, rather they are dissociated into their large and small subunits. Small ribosomal subunit binds to mRNA molecule. The attachment takes place at specific point just upstream of the initiation Condon of the gene. The translation process itself starts when a charge t RNA base pair with small subunit. The resultant structure comprising of mRNA, small ribosomal subunit and charged t RNA is called initiation complex.

(3) Elongation of Polypeptide chain:

Once the initiation Complex has been formed, the large subunit of the ribosome attaches to the complex. The energy is provided by hydrolysis of GTP molecule. Large subunit contains two binding sites for charge t RNA molecules, P-site or Peptidyl and A site or amino acyl sites. The initiation t RNA binds to P-site provided AUG triplet is in corresponding position of the small subunit. The sequence of second triplet (anticodon) in m RNA dictates which charge t RNA molecule will become attached to A-site. If correct t RNA molecule is present, Peptidyl transferase catalyze the formation of Peptide bond which links two amino acid together. This enzyme is present in large ribosomal subunit. At the same time the covalent bond between amino acid and t RNA occupying P-site is broken. The product of this reaction is dipeptide which is attached to t RNA at A-site. This phase during which growing polypeptide chain increase in length by one amino acid is called elongation.

Friday, September 25, 2009

Transcription in RNA Synthesis

Friday, September 25, 2009 - 0 Comments

It has been Commonly observed that:

(1) In EuKaryotes, DNA the genetic material is largely restricked to the nucleus and Proteins are synthesized in association with ribosomes formed of RNA in the cytoplasm. Therefore DNA does not participate in protein synthesis directly.

(2) The amount of Protein in a cell is generally proportional to the amount of RNA but not to the amount of DNA.

(3) In EyKaryotes the cellular RNA is synthesized in the nucleus where DNA is found.

It has been suggested that an intermediate RNA molecule is involved in Protein synthesis. The genetic information stored in DNA is transferred to RNA during the initial stage of gene expression. The process by which RNA molecules are synthesized on a DNA tempelate (blue print or guide) is called “Transcription”.

This RNA is called mRNA (messenger RNA) or transcript, because it contains message which directs ribosomes to assemble amino acids in a order which is specific for a given kind or Protein.

EVIDENCE OF EXISTENCE OF mRNA:

Two important evidences suggest the existence of mRNA. These are:

(a) DNA-RNA complementarity: It has been show that the base pairs of RNA produced by various organisms were present in the same ratios as in their DNA from where these are synthesized i.e. from template DNA.

(b) DNA-RNA hybridization: Hall, Spiegelman and other denatured DNA by heating. They found that two strands of double helix separate. When the solution is cooled certain proportion of DNA strands rejoined and rewound. This suggest that complementary strands recognised each other and joined to perform double helix. In another experiment they added RNA to the solution of denatured DNA and solution of DNA. These existence of complementarity between DNA and RNA indicate that DNA act as template on which complementary RNA is made.

The process of transcription occurs on a DNA template in the synthesis of complementary single stranded RNA molecule. Most evidence suggest that only one of the two strands of DNA duplex is transcribed. The strand that is transcribed is called sense strand and the complementary strand is called antisense strand. Transcription may be divided into three stages.

(1) Template binding (2) Elongation (3) Termination

(1) TEMPLATE BINDING AND IMITIATION

It involves RNA polymerase and DNA molecule. The initial binding of an RNA polymerase enzyme to DNA molecule must occur at a specific position just in front of the gene to be transcribed. These attachments points are called Promoters.

Promoter: Promoter is a short nucleotide sequence that is recognized by an RNA polymerase enzyme as a point at which it can bind to DNA in order to begin transcription. Promoters occur just lipstream of genes and no where else.

(2) Elongation:

During this stage RNA polymerase enzyme migrates along the DNA molecule, melting (breaking base pairings) and unwinding double helix as it moves along. A structure called ‘open promoter complex’ is formed in the region of -10 box where breakage of base pairing and unwinding of DNA double helix occurs. After the formation of open promoter complex, the sigma factor dissociates and the holoenzyme is converted into core enzyme. At the same time first two ribonucleotide are base paired to the template polynucleoptide.

One elongation begins the synthesized portion of RNA molecule gradually dissociated from the template (guide) strand allowing the double helix to return to its original state. Thus only a limited region of DNA molecule is melted at any one time. The open region also called transcription bubble contains between 12 and 17 RNA-DNA base pairs. The transcription is longer than the gene.

(3) TERMINATION

The termination of transcription occur only as suitable positions shortly after the ends of genes. The termination does not involve specific sequence analogous to the promoter but it is brought by a more complex signal. The termination signals are complementary palindromes.

TRANSCRIPTION IN EUKARYOTES:

Transcription in Eukaryotes occurs by a process very similar to that in Prokaryotes. Most important differences are thet in eukaryotes initiation of transcription is more complicated and termination does not involve stem loop structures. There are three different kinds of RNA polymerases in eukaryotes each have their own promoters, so each enzyme transcribes only its own set of genes. For example RNA polymerase II cannot transcribe transfer RNA genes as these have promoters for RNA polymerase III.

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