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Microbiology
Lecture -
Bacterial genetics
Genetic recombination in bacteria
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:
- Transfer of antibiotic resistance genes: The remarkable spread of resistance to multiple antibiotics may have been aided by the transfer of resistance genes within populations and among species.
- As a tool to study advances of molecular biology and biotechnology: Many bacteria have enzymes that enable them to destroy foreign DNA that gets into their cells. It seems unlikely that these would be needed if that did not occur in nature. In addition, the prime enzymes of bacterial reproduction namely restriction endonucleases have provided the tools of molecular biology. Most of the advances in biotechnology industry directly depend on the use of these enzymes.
- Study of Evolution: The recent completion of the sequence of the entire genome of a variety of bacteria (and archaea) suggests that in the past the genes have moved from one species to another by horizontal gene transfer.
Plasmids
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.
The salient features of a typical plasmid are as follow :
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
- The main function of plasmids is to carry antibiotic resistant genes and spread them in the whole human or animal body. In this way many diseases of humans and animals can be treated.
- The other function of plasmids is to carry those genes which are involved in metabolic activities and are helpful in digesting the pollutants from the environment.
- They are also capable of producing antibacterial proteins.
- Plasmids also able to carry the genes which are concerned with increasing the pathogenicity of bacteria which cause diseases like anthrax and tetanus.
- Plasmids carry genes for resistance/sensitivity to heavy metals such as Hg, Ag, Cd, Pb etc.
- Plasmids carry virulence genes. Eg., vir genes of Agrobacterium tumefaciens. Types of plasmids There are five types of plasmids which are used for different purposes. 1. Resistance Plasmids : This type of plasmids is involved in the bacterial conjugation. They usually carry those genes which code for the resistance of antibiotics or poisons. They also code for those genes which are responsible for the production of conjugation pili. The main role of conjugation pili is to transfer the R plasmid from a donor bacterium to the recipient bacterium. This is how the other bacteria also become antibiotic resistant. 2. Degradative plasmids : This type of plasmids is capable of degrading or digesting the dead organic matter from dead animals or plants. They use this organic matter in the process of biosynthesis and make energy and recycle them. 3. Fertility Plasmids: These plasmids carry the tra-genes which are used in the process of conjugation. They are helpful in transferring the genetic material between bacteria. 4. Col Plasmids : The plasmids of this type produce such antibiotics which are involved in killing the other harmful strains of bacteria by staying in the host bacterial cell. The antibiotics are also called as colicin. 5. Virulence Plasmids : As the name shows, these plasmids have the ability to transform bacteria into a pathogen. So they are responsible for carrying the genes which cause diseases.
Transfer of genetic material
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
- Sometimes the F factor breaks free from the chromosome of an Hfr cell, and takes a segment of the chromosomal DNA.
- The factor is now called F' factor (F prime).
- When the F' factor is transferred during conjugation, the process is called as Sexduction. Mechanism of F’ x F- Crosses
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
- Nucleic acid (DNA/RNA)
- Genome size
- Modified bases (protect from host nucleases)
- Protection
- Infection Structure (T4)
- Size (80 X 100 nm)
- Head or capsid
- Tail (contractile sheath, base plate, tail fibres) Types of Bacteriophage Virulent phage: A phage that multiply within the host cell, lyse the cell and release progeny phage (e.g. T4) – lytic cycle Temperate phage: A phage that can either multiply via the lytic cycle or enter a quiescent integrated state in the bacterial cell. (e.g., lysogenic cycle Expression of most phage genes repressed Prophage: Phage DNA in the quiescent integrated state
1. An infecting virus (bacteriophage) adsorbs to the cell surface of a bacterium.
- The bacteriophage genome gets inside into the cell of bacterium. The phage DNA uses the bacterium's machinery (DNA template and enzymes) to synthesize bacteriophage components and enzymes as well.
- Once reassembled, the bacteriophages kills its host by bursting the cell (hence lysis) and are released to infect other cells.
- The bacteriophage carrying the DNA of the donor bacterium again adsorbs to another recipient bacterium.
- The DNA from donor bacterium thus gets exchanged by recombination of the recipient's DNA and confers new property to the bacterium. Generalized transduction can transfer any gene of donor bacteria to recipient bacteria. During the replication of a lytic phage the capsid sometimes e nclose a small fragment of lysed bacterial DNA, instead of phage DNA, by a "head-full" mechanism. This is a defective phage (b) Specialized transduction (Lysogenic Cycle) In case of specialized transduction or lysogenic cycle, the phage DNA gets incorporated into the bacterium chromosome. This is called a prophage and it behaves as if it were a part of bacterial chromosome. Once incorporated, the genes of the phage DNA also get expressed in the bacterium. Sometimes, the prophage gets detached from the host chromosome during multiplication of lysogenic host bacteria, and in this course, it may carry along fragments of bacterial chromosome with itself. The detached prophage then starts a new lytic cycle. This prophage may have a piece of chromosomal DNA of bacteria. When such phage infects another bacterium, it may incorporate the gene that was picked up from the preceding host into the new host genome. The examples of specialized transduction include λ phage in Escherichia coli. A transduction in which only certain donor genes can be transferred to the recipient. Occur during the lysogenic life cycle of a temperate phage
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)
