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    The genetic material of bacteria is found in .

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    Question

    The genetic material of bacteria is found in _________.

    A

    Nucleus

    B

    Cytoplasm

    C

    Cell membrane

    D

    Ribosome

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    स्रोत : www.toppr.com

    Genetics

    The genetic material of bacteria and plasmids is DNA. Bacterial viruses (bacteriophages or phages) have DNA or RNA as genetic material. The two essential functions of genetic material are replication and expression. Genetic material must replicate accurately so that progeny inherit all of the specific genetic determinants (the genotype) of the parental organism. Expression of specific genetic material under a particular set of growth conditions determines the observable characteristics (phenotype) of the organism. Bacteria have few structural or developmental features that can be observed easily, but they have a vast array of biochemical capabilities and patterns of susceptibility to antimicrobial agents or bacteriophages. These latter characteristics are often selected as the inherited traits to be analyzed in studies of bacterial genetics.

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    Medical Microbiology. 4th edition.

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    Chapter 5Genetics

    Randall K. Holmes and Michael G. Jobling.

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    General Concepts

    Genetic Information in Microbes

    Genetic information in bacteria and many viruses is encoded in DNA, but some viruses use RNA. Replication of the genome is essential for inheritance of genetically determined traits. Gene expression usually involves transcription of DNA into messenger RNA and translation of mRNA into protein.

    Genome Organization

    The bacterial chromosome is a circular molecule of DNA that functions as a self-replicating genetic element (replicon). Extrachromosomal genetic elements such as plasmids and bacteriophages are nonessential replicons which often determine resistance to antimicrobial agents, production of virulence factors, or other functions. The chromosome replicates semiconservatively; each DNA strand serves as template for synthesis of its complementary strand.

    Mutation and Selection

    The complete set of genetic determinants of an organism constitutes its genotype, and the observable characteristics constitute its phenotype. Mutations are heritable changes in genotype that can occur spontaneously or be induced by chemical or physical treatments. Organisms selected as reference strains are called wild type, and their progeny with mutations are called mutants. Selective media distinguish between wild type and mutant strains based on growth; differential media distinguish between them based on other phenotypic properties.

    Exchange of Genetic Information

    Genetic exchanges among bacteria occur by several mechanisms. In transformation, the recipient bacterium takes up extracellular donor DNA. In transduction, donor DNA packaged in a bacteriophage infects the recipient bacterium. In conjugation, the donor bacterium transfers DNA to the recipient by mating. Recombination is the rearrangement of donor and recipient genomes to form new, hybrid genomes. Transposons are mobile DNA segments that move from place to place within or between genomes.

    Recombinant DNA and Gene Cloning

    Gene cloning is the incorporation of a foreign gene into a vector to produce a recombinant DNA molecule that replicates and expresses the foreign gene in a recipient cell. Cloned genes are detected by the phenotypes they determine or by specific nucleotide sequences that they contain. Recombinant DNA and gene cloning are essential tools for research in molecular microbiology and medicine. They have many medical applications, including development of new vaccines, biologics, diagnostic tests, and therapeutic methods.

    Regulation of Gene Expression

    Expression of genes in microbes is often regulated by intracellular or environmental conditions. Regulation can affect any step in gene expression, including transcription initiation or termination, translation, or activity of gene products. An operon is a set of genes that is transcribed as a single unit and expressed coordinately. Specific regulation induces or represses a particular gene or operon. Global regulation affects a set of operons, which constitute a regulon. All operons in the regulon are coordinately controlled by the same regulatory mechanism.

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    Genetic Information in Microbes

    The genetic material of bacteria and plasmids is DNA. Bacterial viruses (bacteriophages or phages) have DNA or RNA as genetic material. The two essential functions of genetic material are replication and expression. Genetic material must replicate accurately so that progeny inherit all of the specific genetic determinants (the genotype) of the parental organism. Expression of specific genetic material under a particular set of growth conditions determines the observable characteristics (phenotype) of the organism. Bacteria have few structural or developmental features that can be observed easily, but they have a vast array of biochemical capabilities and patterns of susceptibility to antimicrobial agents or bacteriophages. These latter characteristics are often selected as the inherited traits to be analyzed in studies of bacterial genetics.

    Nucleic Acid Structure

    Nucleic acids are large polymers consisting of repeating nucleotide units (Fig. 5-1). Each nucleotide contains one phosphate group, one pentose or deoxypentose sugar, and one purine or pyrimidine base. In DNA the sugar is D-2-deoxyribose; in RNA the sugar is D-ribose. In DNA the purine bases are adenine (A) and guanine (G), and the pyrimidine bases are thymine (T) and cytosine (C). In RNA, uracil (U) replaces thymine. Chemically modified purine and pyrimidine bases are found in some bacteria and bacteriophages. The repeating structure of polynucleotides involves alternating sugar and phosphate residues, with phosphodiester bonds linking the 3′-hydroxyl group of one nucleotide sugar to the 5′-hydroxyl group of the adjacent nucleotide sugar. These asymmetric phosphodiester linkages define the polarity of the polynucleotide chain. A purine or pyrimidine base is linked at the 1′-carbon atom of each sugar residue and projects from the repeating sugar-phosphate backbone. Double-stranded DNA is helical, and the two strands in the helix are antiparallel. The double helix is stabilized by hydrogen bonds between purine and pyrimidine bases on the opposite strands. At each position, A on one strand pairs by two hydrogen bonds with T on the opposite strand, or G pairs by three hydrogen bonds with C. The two strands of double-helical DNA are, therefore, complementary. Because of complementarity, double-stranded DNA contains equimolar amounts of purines (A + G) and pyrimidines (T + C), with A equal to T and G equal to C, but the mole fraction of G + C in DNA varies widely among different bacteria. Information in nucleic acids is encoded by the ordered sequence of nucleotides along the polynucleotide chain, and in double-stranded DNA the sequence of each strand determines what the sequence of the complementary strand must be. The extent of sequence homology between DNAs from different microorganisms is the most stringent criterion for determining how closely they are related.

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    Bacterial DNA – the role of plasmids — Science Learning Hub

    Like other organisms, bacteria use double-stranded DNA as their genetic material. However, bacteria organise their DNA differently to more complex organisms.

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    Bacterial DNA – the role of plasmids

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    Like other organisms, bacteria use double-stranded DNA as their genetic material. However, bacteria organise their DNA differently to more complex organisms.

    Rights: The University of Waikato Te Whare Wānanga o Waikato

    Bacterial DNA

    Bacteria have a single circular chromosome that is located in the cytoplasm in a structure called the nucleoid. Bacteria also contain smaller circular DNA molecules called plasmids.

    Bacterial DNA – a circular chromosome plus plasmids

    The DNA of most bacteria is contained in a single circular molecule, called the bacterial chromosome. The chromosome, along with several proteins and RNA molecules, forms an irregularly shaped structure called the nucleoid. This sits in the cytoplasm of the bacterial cell.

    In addition to the chromosome, bacteria often contain plasmids – small circular DNA molecules. Bacteria can pick up new plasmids from other bacterial cells (during conjugation) or from the environment. They can also readily lose them – for instance, when a bacterium divides in two, one of the daughter cells might miss out on getting a plasmid.

    Every plasmid has its own ‘origin of replication’ – a stretch of DNA that ensures it gets replicated (copied) by the host bacterium. For this reason, plasmids can copy themselves independently of the bacterial chromosome, so there can be many copies of a plasmid – even hundreds – within one bacterial cell.

    Plasmids help bacteria to survive stress

    Rights: Dr Stephanie Dellis, Molecular Biology Lab at the College of Charleston

    Antibiotic resistance

    Some plasmids can make their host bacterium resistant to an antibiotic. In this image, two halves of an agar plate containing the antibiotic kanamycin have been spread with the same strain of. The bacteria spread on the right-hand side of the plate contain a plasmid that confers resistance to kanamycin, so they can grow colonies even when it is present. On the left-hand side of the plate, the bacteria lack the kanamycin resistance plasmid and have been unable to grow.

    Plasmids contain just a few genes, but they make a big difference to their host bacterium. The genes are usually not essential for the bacterium’s day-to-day survival – instead, they help the bacterium to overcome occasional stressful situations. For instance, many plasmids contain genes that, when expressed, make the host bacterium resistant to an antibiotic (so it won’t die when treated with that antibiotic). Other plasmids contain genes that help the host to digest unusual substances or to kill other types of bacteria.

    Plasmids make themselves indispensable

    Keeping a plasmid is hard work for a bacterial cell, because replicating DNA (including plasmid DNA) uses up energy. However, by protecting its bacterial host from stress-related death, a plasmid maximises its chances of being kept around. Under stressful conditions, bacteria with the plasmid will live longer – and have more opportunity to pass on the plasmid to daughter cells or to other bacteria. Bacteria without the plasmid are less likely to survive and reproduce.

    Some plasmids take extreme measures to ensure that they are retained within bacteria. For example, some carry a gene that makes a long-lived poison and a second gene that makes a short-lived antidote. These plasmids are effectively holding their host bacterial cell hostage – if they are ever lost from the cell, they won’t be able to provide the antidote and the cell will die.

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    Plasmid vector

    Key features of a typical plasmid vector are an origin of replication (to ensure the vector is copied within bacteria), a gene for antibiotic resistance (to ensure the vector is not lost by bacteria) and a set of recognition sites for restriction enzymes (to make it straightforward to insert foreign DNA into the vector).

    Plasmids in biotech – delivering DNA

    Plasmids have been key to the development of molecular biotechnology. They act as delivery vehicles, or vectors, to introduce foreign DNA into bacteria. Using plasmids for DNA delivery began in the 1970s when DNA from other organisms was first ‘cut and pasted’ into specific sites within the plasmid DNA. The modified plasmids were then reintroduced into bacteria.

    Why plasmids are excellent DNA delivery vectors

    Decades after their first use, plasmids are still crucial laboratory tools in biotechnology:

    Scientists can force bacteria to keep them. Virtually all plasmids that are used to deliver DNA contain genes for antibiotic resistance. Once bacteria have been treated with a plasmid, scientists grow them in the presence of antibiotic. Only those cells that contain the plasmid will survive, grow and reproduce. The others will be killed by the antibiotic.

    स्रोत : www.sciencelearn.org.nz

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