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Bacterial genetics Genetic recombination in bacteria
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Bacteria can transfer genes from one strain to another by three different mechanisms: transformation, conjugation and transduction, these events clearly show the universality of sexuality in the living world. The genetic recombination in bacteria has a very vital significance. Some of them are as follow:
Plasmids are the extra chromosomal structures in the cells of bacteria which have the ability to self- replicate. They do not combine with the genetic material of the host cell but stay independent. They are genetically modified and are used in the recombinant DNA technology. Plasmids are usually made up of double stranded, non-chromosomal DNA, but in some cases they are circular. They make their structure circular by combining the two ends of the double stranded DNA together. These ends are combined through covalent bonds.
1.Number and size: A bacterium can have no plasmids at all or have many plasmids (20-30) or multiple copies of a plasmid. Usually they are closed and circular molecules; however they occur as linear molecule in Borrelia burgdorferi. Their size may vary from 1 Kb to 400 Kb.
2.Multiplication : Plasmids multiply independently of the chromosome and are inherited regularly by the daughter cells. Plasmids Functions
Whenever two fragments of DNA come into contact with each other, exchange between the sections of each DNA takes place. This stage is called as crossing over during which, the DNA breaks and is attached
2.Conjugation In conjugation, DNA is transferred through a tube between two bacteria cells. This tube is termed as conjugation tube. In 1946, Joshua Lederberg and Tatum discovered that some bacteria can transfer genetic information to other bacteria through a tube known as conjugal tube. Conjugation involves the transfer of DNA in the form of a plasmid from donor bacterium to recipient bacterium. Conjugation involves the transfer of plasmids from donor bacterium to recipient bacterium. Plasmid transfer in Gram-negative bacteria occurs only between strains of the same species or closely related species. Some plasmids are designated as F factor (F plasmid, fertility factor or sex factor) because they carry genes that mediate their own transfer. The F factor can replicate autonomously in the cell. These genes code for the production of the sex pilus and enzymes necessary for conjugation. Cells possessing F plasmids are F+ (male) and act as donors. Those cells lacking this plasmid are F- (female) and act as recipient. All those plasmids, which confer on their host cells to act as donors in conjugation are called transfer factor.
A.F+ conjugation (F+ x F- conjugation) The plasmid DNA is nicked at a specific site called the origin of transfer. Pair formation - The tip of the sex pilus comes in contact with the recipient and a conjugation bridge is formed between the two cells, through which the plasmid DNA will pass from the donor to the recipient. A single strand of plasmid DNA passes through the conjugation bridge and enters the recipient where the complimentary strand is synthesized by a rolling circle mechanism. This results in the transfer of a F+ plasmid (coding only for a sex pilus), but not chromosomal DNA, from a male (F+) donor bacterium to a female recipient (F-) bacterium. The recipient F- then becomes an F+ (male) and can make a sex pilus. Other genes present on the plasmid, such as those coding for antibiotic( resistance, may also be transferred during this process. B.Hfr (high frequency recombinant) conjugation: Plasmids may integrate into the bacterial chromosome by a recombination event depending upon the extent of DNA homology between the two. After integration, both plasmid and chromosome will replicate as a single unit. A plasmid that is capable of integrating into the chromosome is called an episome. If the F plasmid is integrated into the chromosome it is called an Hfr cell. After integration, both chromosome and plasmid can be conjugally transferred to a recipient cell. Hfr cells are called so because they are able to transfer chromosomal genes to recipient cells with high frequency.
Hfr x F- crosses The DNA of Hfr male (donor) breaks in the middle of the inserted F+ plasmid and one DNA strand begins to enter the F-(recipient) bacterium. The connection usually breaks before the transfer of the entire chromosome is completed so the F+ plasmid seldom enters the recipient.
F’ formation
3.Transduction In transduction, a bacterium transfers its DNA (or a portion of it) to another bacterium (that is not its progeny) through a virus. Bacteriophages are viruses that parasitic bacteria and use their machinery for their own replication. During the process of replication inside the host bacteria the bacterial chromosome or plasmid is erroneously packaged into the bacteriophage capsid. Thus newer progeny of the phages may contain fragments of the host chromosome along with their own DNA or entirely host chromosome. When such phage infects another bacterium, the bacterial chromosome in the phage also gets transferred to the new bacterium. This fragment may undergo recombination with the host chromosome and confer new property to the bacterium. Phage Composition and Structure Composition
1. An infecting virus (bacteriophage) adsorbs to the cell surface of a bacterium.
Transposable elements (Transposons) What are transposons Transposable elements, also known as transposons or “jumping genes,” are DNA elements that move (or transpose) within the genome and between genomes, from bacteria to humans providing new genetic raw material for evolution. Barbara McClintock (1902-1992) Cold Spring Harbor Laboratory, NY Nobel Prize in Physiology and Medicine 1983 “for her discovery of mobile genetic elements” Studied transposable elements in corn (Zea mays) 1940s1950s (formerly identified as mutator genes by Marcus Rhoades 1930s)