Dr. Sara Abd-Elsabor Department of Biochemistry Medical Research
Total Page：16
File Type：pdf, Size：1020Kb
Dr. Sara Abd-elsabor Department of Biochemistry Medical Research Institute Although genetic stability is crucial for the survival of individuals, in the longer term the survival of organisms may depend on genetic variation, through which they can adapt to a changing environment. Thus an important property of the DNA in cells is its ability to undergo rearrangements that can vary the particular combination of genes present in any individual genome, as well as the timing and the level of expression of these genes. These DNA rearrangements are caused by genetic recombination. Genetic recombination is the process by which a strand of genetic material (usually DNA; but can also be RNA) is broken and then joined to a different DNA molecule. In eukaryotes recombination commonly occurs during meiosis as chromosomal crossover between paired chromosomes. This process leads to offspring having different combination of genes from their parents and can produce new genes. In evolutionary biology this shuffling of genes is thought to have many advantages, including that of allowing sexually reproducing organisms to avoid Muller’s ratchet. In evolutionary genetics, Muller’s ratchet (named after Hermann Joseph Muller and a mechanical device) is the name given to the process by which the genome of an asexual population accumulate deleterious mutations in an irreversible manner. Muller proposed this mechanism as one reason why sexual reproduction may be favored over asexual reproduction. In molecular biology “recombination” can also refer to artificial and deliberate recombination of disparate pieces of DNA, often from different organisms, creating what is called recombinant DNA. Bacteria is the most diversified group of organisms on and they are crucial to the maintenance of Earth’s environment. Various species release oxygen into the atmosphere; recycle carbon, nitrogen and other elements and digest human and other animal wastes as well as pesticides and other pollutants, which would otherwise eventually poison the air, soil and water. Bacteria also cause hundreds of animal and plant diseases but even so, harmful species are a small fraction of all bacteria. Many species, in fact, produce vitamins and other materials essential to the health and survival of humans and other organisms. It is important to mention here that bacteria are the most studied models of extraterrestrial life. A plasmid is a small, circular molecule double- stranded DNA molecules that replicates independently of the bacterial chromosome. Plasmids carry information required for their own replication, and often for one or more cellular traits which are not essential for normal bacterial metabolism, growth, or reproduction, but confer survival advantages to the bacteria that possess plasmids. Bacteria have no sexual reproduction in the sense that eukaryotes do. Reproduction: Bacteria reproduce through binary fission, a process of asexual reproduction in which the bacterium divides, producing an exact copy. When organisms replicate their genomes and provide copies to descendants (future generations), this is considered vertical gene transfer. HGT is a process in which a prokaryotic cell can acquire genes from other microbes of the same generation, which, in some cases, can be a different species, or even a different genus than the donor. There are three types of horizontal gene transfer: . Transformation. Conjugation. Transduction. 1) Insertion of new DNA (e.g., a plasmid) into a cell or organism, such as into competent E.coli. 2) Conversion of a normal cell into one that behaves in many ways like a cancer cell (i.e., unregulated proliferation, anchorage- independent growth in culture). Bacterial transformation is the process by which bacterial cells take up naked DNA molecules [Fig]. If the foreign DNA has an origin of replication recognized by the host cell DNA polymerase, the bacteria will replicate the foreign DNA along with their own DNA. Bacteria transformation may be referred to as a stable genetic change brought about by taking up naked DNA (DNA without associated cells or proteins), and competence refers to the state of being able to take up exogenous DNA from the environment. Two different forms of competence should be distinguished: natural and artificial. Some bacteria (around 1% of all species) are naturally capable of taking up DNA under laboratory conditions; many more may be able to take it up in their natural environments. Such species carry sets of genes specifying the cause of the machinery for bringing DNA across the cell's membrane or membranes. Artificial competence is not encoded in the cell's genes. Instead it is induced by laboratory procedures in which cells are passively made permeable to DNA, using conditions that do not normally occur in nature. Calcium chloride transformation is a method of promoting competence The bacterial cell membrane is permeable to chloride ions, but is non-permeable to calcium ions. As the chloride ions enter the cell, water molecules accompany the charged particle. This influx of water causes the cells to swell and is necessary for the uptake of DNA. The exact mechanism of this uptake is unknown. It is known, however, that the calcium chloride treatment be followed by heat. When E.coli are subjected to 42°C heat, a set of genes are expressed which aid the bacteria in surviving at such temperatures. This set of genes are called the heat shock genes. The heat shock step is necessary for the uptake of DNA. At temperatures above 42°C, the bacteria’s ability to uptake DNA becomes reduced, and at extreme temperatures the bacteria will die. Fig. Bacterial Transformation Plasmids used for the cloning and manipulation of DNA have been engineered to harbour the genes for antibiotic resistance. Thus, if the bacterial transformation is plated onto media containing ampicillin, only bacteria which possess the plasmid DNA will have the ability to metabolize ampicillin and form colonies. In this way, bacterial cells containing plasmid DNA are selected. Bacterial conjugation is the transfer of genetic material between bacteria through direct cell-to- cell contact. Discovered in 1946 by Joshua Lederberg and Edward Tatum, conjugation is a mechanism of horizontal gene transfer – as are transformation– although these mechanisms do not involve cell-to-cell contact. Bacterial conjugation is often incorrectly regarded as the bacterial equivalent of sexual reproduction or mating. It is not actually sexual, as it does not involve the fusing of gametes and the creation of a zygote, nor is there equal exchange of genetic material. It is merely the transfer of genetic information from a donor cell to a recipient. In order to perform conjugation, one of the bacteria, the donor, must play host to a conjugative or mobilizable genetic element, most often a conjugative or mobilizable plasmid. Most conjugative plasmids have systems ensuring that the recipient cell does not already contain a similar element. The genetic information transferred is often beneficial to the recipient cell. Benefits may include antibiotic resistance, other xenobiotic tolerance, or the ability to utilize a new metabolite. Such beneficial plasmids may be considered bacterial endosymbionts. Some conjugative elements may also be viewed as genetic parasites on the bacterium, and conjugation as a mechanism was evolved by the mobile element to spread itself into new hosts. The prototype for conjugative plasmids is the F-plasmid, also called the F-factor. The F- plasmid is an about 100 kb length. It carries its own origin of replication, the ori V, as well as an origin of transfer, or ori T. There can only be one copy of the F-plasmid in a given bacterium, either free or integrated (two immediately before cell division). The host bacterium is called F- positive or F-plus ( denoted F+ ). Strains that lack F plasmids are called F-negative or F-minus ( F– ) [Fig]. Fig. : Schematic Drawing of Bacterial Conjugation. Conjugation diagram: 1- Donor cell produces pilus. 2- Pilus attaches to recipient cell, brings the two cells together. 3- The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell. 4- Both cells recircularize their plasmids, synthesize second strands, and reproduce pili; both cells are now viable donors. Among other genetic information, the F-plasmid carries a tra and a trb locus, which together are about 33 Tk long and consist of about 40 genes. The tra locus includes the pilin gene and regulatory genes, which together form pili on the cell surface, polymeric proteins that can attach themselves to the surface of F– bacteria and initiate the conjugation. When conjugation is initiated, via a mating signal, a relaxase enzyme creates a nick in one plasmid DNA strand at the origin of transfer, or ori T. The relaxase may work alone or in a complex, of over a dozen proteins, known collectively as a relaxosome. transferred into the recipient bacterium in a 5-terminus to 3-terminus direction. Another methods of horizontal gene transfer is called transduction. This process involves the transfer of DNA from one bacterium to another via a replicating virus. Transduction also refers to the process whereby foreign DNA is introduced into another cell via a viral vector. This is a common tool used by molecular biologists to stably introduce a foreign gene into a host cell’s genome. When bacteriophages (viruses that infect bacteria) infect a bacterial cell, their normal mode of reproduction is to harness the replicational, transcriptional, and translation machinery of the host bacterial cell to make numerous virions, or complete viral particles, including the viral DNA or RNA and the protein coat. If the lysogenic cycle is adopted, the temperate phage (nonvirulent type) enters the bacterium, the phage DNA is integrated into the bacterial chromosome, where it can remain dormant for thousands of generations. This process is known as lysogeny. If the lysogen (e.g., UV light) is induced, the phage genome is excised from the bacterial chromosome and initiates the lytic cycle, which culminates in disintegration of the bacterial chromosome, lysis of the cell and the release of phage particles.