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Genetics of Bacteria -

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  • Genetics of Bacteria -

    Genetics of Bacteria
    - The use of bacteria and bacteriophages has been essential to the accumulation of knowledge in many areas of genetic study.
    - For example, much of what is known about molecular genetics, recombinational phenomena, and gene structure was initially derived from experimental work with these organisms.
    - Both bacteria and their viruses have extremely short reproductive cycles.
    - Hundreds of generations, giving rise to billions of genetically identical or¬ganisms, can be produced in short periods of time.

    Bacterial Mutation and Growth
    - It has long been known that pure cultures of bacteria give rise to cells that exhibit heritable variation, particu¬larly with respect to growth under unique environmental conditions.
    - Mutant cells that arise spontaneously in an otherwise pure culture can be isolated and established indepen¬dently from the parent strain by using established se¬lection techniques.
    - As a result, mutations for almost any desired characteristic can now be induced and iso¬lated.
    - Because bacteria and viruses usually contain only a single chromosome and are therefore haploid, all mu¬tations are expressed directly in the descendants of mu¬tant cells, adding to the ease with which these microorganisms can be studied.
    - Bacteria are grown in a liquid culture medium or in a Petri dish on a semisolid agar surface.
    - If the nutrient components of the growth medium are very simple and consist only of an organic carbon source (such as a glu¬cose or lactose) and a variety of ions, including Na+, K+, Mg2+, Ca2+, and NH4+ present as inorganic salts, it is called minimal medium.
    - In order to grow on such a medium, a bacterium must be able to synthesize all es¬sential organic compounds (e.g., amino acids, purines, pyrimidines, sugars, vitamins, and fatty acids).
    - A bac¬terium that can accomplish this remarkable biosyn¬thetic pathways is termed a prototroph.
    - It is said to be wild type for all growth requirements. On the other hand, if a bacterium loses, through mutation, the ability to synthesize one or more organic components, it is said to be an auxotroph.
    - For example, if it loses the ability to make histidine, then this amino acid must be added as a supplement to the minimal medium in order for growth to occur.
    - The resulting bacterium is designated as a his- auxotroph, as opposed to its prototrophic his+ counterpart.
    - In order to study mutant bacteria quantitatively, an in¬oculum of bacteria is placed in liquid culture medium.
    - A characteristic growth pattern is exhibited, as illus¬trated in Figure .

    - Initially, during the lag phase, growth is slow, then, a period of rapid growth, called the logarithmic (log) phase, ensues.
    - During this phase, cells divide many times with a fixed time interval be¬tween cell divisions, resulting in exponential growth.
    - When a cell density of about 109 cells/ml is reached, nu¬trients and oxygen become limiting and cells cease dividing; at this point the cells enter the stationary phase.
    - Because the doubling time during the log phase may be as short as 20 minutes, an initial inoculum of a few thousand cells can easily achieve maximum cell density during an overnight incubation.
    - Cells grown in liquid medium may be quantified by transferring them to semisolid medium in a Petri dish.
    - Following incubation and many divisions, each cell gives rise to a visible colony on the surface of the medium.
    - If the number of colonies is too great to count then a series of successive dilutions (a technique called serial dilution) of the original liquid culture can be made and plated, until the colony number is reduced to the point where it can be counted (Figure ).


    - This technique allows one to calculate the number of bacteria present in the original culture.
    - For example, assume that the three dishes in Figure represent serial dilutions of 10-3, 10-4, and 10-5 .
    - We need only select the dish in which the number of colonies can be counted accurately.
    - Because each colony arose from a single bacterium, the number of colonies multiplied by the dilution factor represents the number of bacteria in each milliliter of the initial inoculum used to start the serial dilutions.
    - In our case, the dish farthest to the right has 15 colonies. The dilution factor for a 10-5 dilution is 105. Therefore, the initial number of bacteria is calculated to be 15 x 105 per milliliter.

    Genetic Recombination in Bacteria:
    Conjugation
    F+ and F- Bacteria
    - Cells of one strain can serve as donors of parts of their chromosomes.
    - Those that do are designated as F+ cells (F stands for "fertility").
    - Recipient bacteria, which un¬dergo genetic recombination by receiving the donor chromosome and exchanging part of it with part of their own, are designated as F- cells.
    - The contact between cells of the two strains is essential to ge¬netic recombination.
    - This physical interaction is the ini¬tial step in the process of conjugation and is established by a structure called the F sex pilus.
    - Bacteria often have many pili, which are tubular extensions of the cell.
    - After contact has been initiated between mating pairs (Figure ), transfer of the chromosome begins.


    - The F factor is in reality an au¬tonomous genetic unit referred to as a plasmid.
    - It is believed that the transfer of the F factor during conjugation involves separation of the two strands of the double helix making up the F factor and the move¬ment of one of the two strands into the recipient.
    - Both strands, one moving across the conjugation tube and one remaining in the donor cell, are replicated. The re¬sult is that both the donor and the recipient cells are F+ as diagrammed in Figure.

    - To summarize, an E. coli cell may or may not contain the F factor.
    - When this factor is present, the cell is able to form a sex pilus and potentially serve as a donor of genetic information.
    - During conjugation, a copy of the F factor is almost always transferred from the F+ cell to the F- recipient, converting it to the F+ state.

    Hfr Bacteria and Chromosome Mapping
    - Subsequent discoveries not only clarified how genetic recombination occurs, but also defined a mechanism by which the E. coli chromosome could be mapped.









    Plasmids
    - When the F factor exists autonomously in the bacterial cytoplasm, this unit takes the form of a double¬ stranded closed circle (or loop) of DNA.
    - These charac¬teristics place the F factor in the more general category of the genetic structures called plasmids.
    - These struc¬tures contain one or more genes and often contain quite a few.
    - Their replication depends on the same en¬zymes that replicate the chromosome of the cell, and they are distributed to daughter cells along with the host chromosome during cell division.
    - Plasmids are generally classified according to the ge¬netic information specified by their DNA.
    - The F factor confers fertility and contains genes that are essential for sex pilus formation.
    - Other examples include the R and the Col plasmids.
    - These plasmids confer multiple resistance to antibiotics and the ability to release toxic substances called colicins, respectively.
    - An R plasmid, illustrated in Figure, contains both r-determi¬nants (for resistance) and RTFs, which facilitate plas¬mid transfer.


    - The R plasmid, which may be present in one to three copies per cell, is significant because it provides the means for rapid multiple drug resistance to cells that were previously sensitive to antibiotics.
    - This phenomenon came to the attention of re¬searchers in the late 1950s in Japan, when multiple re¬sistant strains of the bacterium Shigella (which causes dysentery) were found in the guts of humans.
    - Shigella had acquired this resistance following conjugation with E. coli, where the R factor originated.

    - Interest in plasmids has increased dramatically be¬cause of their role in the genetic technology referred to as recombinant DNA research.
    - Specific genes from any source (e.g., a human cell) may be inserted into a plas¬mid, which may then be inserted into a bacterial cell.
    - As the cell replicates its DNA and undergoes division, the foreign gene is treated like one of the cell's own.

    Bacterial Transformation
    - Transfor¬mation, like conjugation, provides a mechanism for the recombination of genetic information in certain bacte¬ria.
    - This process consists of numerous steps that can be divided into two main categories: (1) entry of DNA into a recipient cell, and (2) recombina¬tion of the donor DNA with its homologous region in the recipient chromosome.
    - In a population of bacterial cells, only those in a particular physiological state, re¬ferred to as competence, take up DNA.
    - Passage across the cell wall and mem¬brane is an active process that requires energy and
    spe¬cific transport molecules.
    - During the process of entry, one of the two strands of the double helix is digested by nucleases.
    - The intact sin¬gle strand of DNA then aligns with its complementary region of the bacterial chromosome.
    - In a process in¬volving several enzymes, the segment replaces its counterpart in the chromosome, which is excised and degraded.
    - For recombination to be detected, the transforming DNA must be derived from a different strain of bacte¬ria that bears some genetic variation, such as a muta¬tion.
    - Once it is integrated into the chromosome, the recombinant region contains one host strand (present originally) and one mutant strand.
    - Because these strands are from different sources, this helical region is referred to as a heteroduplex.
    - Following one round of semiconservative replication, one chromosome is re¬stored to its identical configuration, and the other con¬tains the mutant gene.
    - Following cell division, one nonmutant (untransformed) cell and one mutant (transformed) cell are produced.

  • #2
    شكراً لك أبو رامز

    لكن لو كان أكثر ترتيب يكون أفضل علشان يسهل علينا القراءة

    الله يعطيك العافية على جهدك

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    • #3
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      • #4
        ناقل صفحة كان أحسن تكون في ملف كتابة مشــــــــــــــــــــــــــــــــــــــــــكور
        http://www10.0zz0.com/2011/07/21/15/241483904.jpg

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        • #5
          مهو أنا ما بعرف لو سمحت قلي كيف
          ومشكور جدا

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