” (The Columbia encyclopedia, Sixth Edition 2004) These plasmids are the ones carried forward from one bacteria to the other bacteria. A plasmid is an “extra-chromosomal” piece of bacterial DNA (deoxyribonucleic acid). Plasmids are maintained inside the bacterial cell, replicating fast enough until they are passed on to bacterial progeny as the bacteria divide. Plasmids are just like chromosomes. They are circular and essentially double-stranded DNA.
The distinct characteristics that set apart plasmids from chromosomes are in their size and the genes that they carry. Plasmids are much smaller in size than chromosomes. Plasmids carry only genes that are essential. Bacteria are an interesting group of organism. In order to better comprehend how bacteria multiplies, and replicates its genes, there is a need to understand its structure. Bacteria are “microscopic unicellular prokaryotic organisms characterised by the lack of a membrane-bound organelles.
” (The Columbia encyclopedia, Sixth Edition 2004) Bacteria are remarkably adaptable to diverse environmental conditions: they are found in the bodies of all living organisms and on all parts of the earth --- in land terrain and ocean depths, in arctic ice and glaciers, in hot springs, and even in the stratosphere. Most bacteria are of one of three typical shapes --- rod-shaped (bacillus), round (coccus), and spiral (spirillum). The cytoplasm and plasma membrane of most bacterial cells are surrounded by a cell wall.
In bacteria, the genetic material is organized in a continuous strand of DNA. This circle of DNA is localized in an area called the nucleoid, but there is no membrane surrounding a defined nucleus. In addition to the nucleoid, the bacterial cell may include one or more plasmids. Some bacteria are capable of specialized type of genetic recombination which involves the transfer of nucleic acid by individual contact, that is, the process of conjugation. Recombination involves a “process of shuffling genes by which new combinations can be generated.
” (The Columbia encyclopedia, Sixth Edition 2004) Genetic recombination in bacteria may be mediated by transformation, transduction, or conjugation. In these methods, genetic transfers occur unidirectionally from donor to recipient bacteria and only a fraction of the genetic material of a donor cell is transferred to a recipient, which, on the other hand, contributes its cytoplasm as well as its entire genome. In conjugation, the genetic contribution of the donor (male) is incomplete and is genetically and physiologically determined.
In so doing, “the system of conjugation is well adapted to providing information about the nature and organization of the bacterial chromosome as a whole as well as to the study of nuclear-cytoplasmic interactions. ” (Burdette et. al. 1963) In E. coli bacteria, the transmissible sex factor responsible for the donor state, and thus for fertility, was called F, donor cells being F+ and recipient cell F-. From population of F+ cells, strains of a new type of donor called Hfr (for high frequency of recombination) were occasionally isolated.
Both F+ and Hfr donors share the following characteristics that distinguish with F- recipients: they possess similar surface properties that enable them to pair specifically and to mate with F- cells with comparable efficiency; they have the actual or potential ability to transfer genetic determinants to recipients, although the nature of the determinants so transferred by the two types of donor may be of quite different kinds; each type of donor has the potentiality to mutate to the other or to the F- type; and both types are under the control of a specific genetic structure, the sex factor F.
F functions as a genetic particle insofar as it is stably inheritable by progeny, is transmissible in crosses, and is the determinant of those properties that characterize donor cells. (Burdette et. al. 1963) The facts recounted above and the interpretations they have engendered may now be brought together to form a unified picture of the mating system in E. coli. The sexual differentiation of E. coli into males and females is genetically controlled by the presence or absence of a sex factor, F, that has the properties of an episome and is more akin in its behavior to temperate bacteriophage than to a normal genetic determinant.
The propensities of male cells, in turn, are governed by the state in which the sex factor exists in them. F+ male cells, which harbor the sex factor in its autonomous state, preserve a continuous linkage group and, on conjugation, transfer only their sex factor and other extrachromosomal elements to females. They have the potentiality, however, to generate a spectrum of Hfr male types, each characterized by a linear, transferable chromosome the extremities of which are defined by the integration of the sex factor at one of a variety of chromosomal sites; only the proximal part of the linear chromosome is transferred with high efficiency.
Such modified sex factors serve as efficient vehicles for the transport to female cells of their incorporated segments of male chromosome, with the result that stable, partial diploids for various regions of the chromosome can readily be synthesized. (Burdette et. al. 1963) Formation of the zygote extends from the initial collision between an Hfr and an F- cell to the completion of chromosomal transfer and comprises the stages of collision, effective contact formation, and chromosomal transfer.
In interrupted mating, a number of different Hfr markers are selected, each is found to enter the zygotes at a different time that is specific for each marker under standard conditions. The times of entry of the various markers correspond to their order of arrangement on the chromosome and are proportional to their distances from O where O indicates the extremity (leading locus) which first penetrates the recipient cells during conjugation. The peculiarity of conjugation resides in the mechanism by which genetic transfer is accomplished.
This is expressed by the fact that, when different selections are made, the different genetic characters of a given Hfr strain are transmitted to recombinants with different frequencies depending on their distances from O. The system thus lends itself to an original and convenient method of mapping, in terms of time of transfer. Moreover, mapping is greatly facilitated by the availability of a number of different Hfr strains that transfer different parts of the chromosome at high frequency. In all systems other than conjugation in E.
coli, the only practical way of measuring the distance between genetic loci is by comparing the frequency with which recombination occurs between them. In conjugation two additional methods of measurement are available, in terms of transfer time and of the decay of P32 atoms, both of which are absolute and independent of the recombination process and so provide the means of interpreting recombinational events in physical terms. Therefore, bacterial conjugation’s significance in gene mapping exists in its capability to determine the precise positioning of genes on the genome. Studies concerning a peculiar bacterial strain, E.
coli Hfr, which engaged in conjugation with surprising frequency, paved the way for its use in 21st century genetics. “By sundering conjugal bugs at various times during mating, geneticists Francois Jacob and Elie Wollman were able to determine that the male transferred a complete copy of its genome like one long piece of spaghetti. The implications of the notorious "coitus interruptus" experiment and the resulting "spaghetti hypothesis" were clear: by carefully monitoring the time at which each trait was transferred, the two researchers could determine the precise positioning of genes on the genome.
In this way they plotted the first crude genomic map of a bacterium. ” (Hirsch 1999, p. 145) References Burdette, WJ 1963, Methodology in Basic Genetics, Holden-Day, San Francisco. Hirsch, AE (1999, Spring). Of Flies, Mice and Men”, American Scholar, p. 145. Johnson, AD (2002, Spring). “Living with Microbes”, The Wilson Quarterly, pp. 42+. Rheinberger, HJ (ed. ) & Gaudilliere, JP 2004, Classical Genetic Research and Its Legacy: The Mapping Cultures of Twentieth-Century Genetics, Routledge, New York.