Genetics: Genetic is the study of heredity and variation
- Heredity characteristic are encoded in DNA.
- Bacterial DNA is circular double-stranded DNA ; 100 micrometre or 1 mm long.
- Extrachromosomal genetic material: plasmid
- Besides chromosomal DNA, some bacteria may possess extrachromosomal DNA that is plasmid and episode.
- Plasmid
- Extrachromosomal DNA is situated in cytoplasm as a free state.
- It is Circular DNA
- It Replicate independently
- Episomes
- Extrachromosomal DNA integrated with chromosomal DNA of bacteria.
- Plasmid and episode are nonessential for the function of bacterium but may vary properties of drug resistance, toxigenicity, conjugation etc.
- Plasmid has the ability to transfer themselves to other bacterium,(both are same or other species) so plasmids are important vector in recombinant DNA technology/genetic engineering.
Bacterial variation
There is two types of bacterial variation: phenotyping and genotyping.
Phenotypic variation
Expression of various characters in a given environment by a bacterial cell is known as phenotyping variation.
Bacterial cell may exhibit different phenotyping appearance in different environmental condition.
Examples:
- Synthesis of flagella: salmonella typhi are flagellated bacilli but grown in phenol agar, flagella are not formed. This effect is reversed when cultured into broth.
- Synthesis of enzyme: E coli has gene for synthesis of enzyme beta-galactosidase, but actual synthesis takes place only when grown in medium containing lactose i.e. inducible enzyme.
- Inducible enzyme: enzyme which are synthesized only when induced by the substrate are called inducible enzyme.
- Constitutive enzyme
- These enzymes are synthesized irrespective of the presence or absence of substrate.
Genotypic variation
Genetic variation occurs in genetic material as DNA so that is stable heritable
Not influenced by the environment
Occurs by mutation or gene transfer
Mutation
The mutation is a natural spontaneous random heritable variation caused by a change in the nucleotide sequence of the DNA of the cells.
The mutation causes both phenotypic and genotypic.
Mutation frequency: 10-2 – 10-10.per bacterium per division
Read more about Type of Mutation…
GENE TRANSFER IN BACTERIA
Gene transfer in bacteria occurs by following methods
- Transformation (uptake of naked DNA)
- Transduction (through bacteriophage)
- Lysogenic coversion
- Conjugation (plasmid mediated)
1. Transformation
Transformation is a method of genetic recombination in which a naked DNA from a donor bacteria is transferred to a competent recipient bacteria and incorporated into the chromosome of the latter., e.g. in Bacillus, Haemophilus, Neisseria, Pneumococcus.
In Gram+ve bacteria, the DNA is taken up as a single-stranded molecule and the complementary strand is synthesized in the recipient.
In Gram-ve bacteria, double-stranded DNA is transformed.
Mechanism of Transformation
- A bacterial cell dies or is degraded releasing its dsDNA molecule in environment.
- Nuclease enzymes cut the released DNA into fragments of usually about 20 genes long.
- The fragments bind to DNA binding proteins present on the surface of a competent recipient bacterium and subsequently translocated in the cytoplasm of recipient bacteria
- The DNA fragment from the donor is then exchanged for a piece of the recipient’s DNA by means of Rec A proteins.
2. Transduction
Transmission of a portion of DNA from one bacterium to another by a bacteriophage is known as transduction.
Bacteriophage: A virus that infects bacteria and multiplies in it (i.e.parasite in bacteria).
Inside the infected bacteria, during the Assembly of bacteriophage progeny, error may happen occasionally. Besides it’s over nucleic acid, host DNA may accidentally be incorporated into the bacteriophage during the Assembly of bacteriophage progeny in the bacterial cell. This is known as a packaging error.
When this bacteriophage infects another bacterium, post DNA is transferred and the recipient cell acquire new character coded by Donor DNA.
- Transduction is the transfer of gene or portion of DNA from one bacterium (donor) to another (recipient) mediated by a bacteriophage.
- In transduction, the bacteriophage at first infects one bacterium (donor) and some part of donor’s DNA is incorporated in bacteriophage which is carried into the recipient bacterium at the time of infection.
- The phase DNA undergoes lysogenic conversion inside the recipient bacterial cell and may confer pathogenic property in a non-pathogenic bacterium.
- Transduction is not only confined to transfer of chromosomal DNA but also to plasmids and episomes.
- Transduction is of following two types:
- Generalized transduction
- Specialized transduction
Type of transduction
A) Generalized transduction
B) Restricted/specialized transduction
A) Generalized transduction
Transduction may be generalized when it involves any segment of DNA at random.
- If all the fragments of bacterial DNA from any region of the chromosome have the same chance to enter into the transducing bacteriophage, it is called generalized transduction.
- The viral DNA first begins the lytic cycle in the donor bacterium and hydrolyses the host DNA into small fragments.
- During assembly of the virus, any fragment of the host DNA (chromosome) can be incorporated into the viral DNA in its head.
- Such abnormal bacteriophages (those containing host DNA) infect the recipient bacteria and transfer this DNA into the new cell.
- Since this new DNA is not a viral DNA, it doesn’t replicate in the recipient cell but undergoes homologous recombination with the recipient chromosomal DNA.
- This occurs at a frequency of about 1 in every 1000 viruses.
- Generalized transduction can be of further two types:
- Complete transduction:
- It produces stable recombinants that inherit donor genes and retain the ability to express them.
- Abortive transduction:
- There is transient expression of one or more donor genes without the formation of recombinant progeny.
- The donor DNA fragment doesn’t replicate and only one bacterium contains the donor DNA fragment among the progeny of original transductant.
- The donor phenotype is finally unexpressed as the donor gene products become dilute in all progeny after each generation.
- The frequency of abortive transduction is typically 1-2 times more than that of generalized transduction.
- Complete transduction:
B) Restricted transduction
Transduction may be restricted when a specific bacteriophage transduces only a particular genetic trait.
- The bacteriophage transfers only few restricted genes from the donor to the recipient.
- This transduction is carried out by the temperate bacteriophage which undergoes lysogenic cycle in bacteria.
- At first, the temperate bacteriophage enters into the donor cells and then its genome integrates with the host genome.
- Viral genome (DNA) integrates only at certain sites of the host DNA and upon induction from host chromosome, this phage genome sometimes carries host DNA with it.
- Only small parts of genes which lie at the sides of integrated phage DNA are carried with it and transfer that donor DNA into the recipient cell during infection of the new cell.
Role of transduction
Transduction may transfer chromosomal DNA, episode and plasma DNA. Penicillin resistance in staphylococcus is due to the plasmid transferred from one bacterium to another by transduction.
3. Lysogenic conversion
Bacteriophages have two type of lifecycle inside the host bacterium.
a) Violent or lighting cycle
b) Temperate or non-lytic cycle
a) Violent or lighting cycle
Large number of progeny phages are formed and subsequently, these progeny phages are released causing death and lysis of the host bacterial cell.
b) Temperate or non lytic cycle
In contrast to virulent cycle or lytic cycle, the host bacterium is unharmed in temperate or non-lytic cycle.
4. Conjugation
Bacterial conjugation is a way of gene transfer in bacteria by which a bacterial cell transfers genetic material to another bacterial cell, usually mediated by a conjugation pilus or sex pilus.
The genetic material that is transferred through bacterial conjugation is a small plasmid, known as F-plasmid (F for fertility factor). F-plasmid can replicate independently in the cytoplasm separately from the bacterial chromosome.
A cell that already has a copy of the F-plasmid is called an F-positive, F-plus or F+ cell, and is considered a donor cell.
While a cell that does not have a copy of the F-plasmid is called an F-negative, F-minus or F– cell, and is considered a recipient cell.
Conjugation via F-plasmid takes place through a horizontal connection by which the donor cell and the recipient cell directly contact each other to transfer the genetic material.
For transfer of gene by conjugation, a donor cell and a recipient cell must first establish contact via sex pilli.
Recipients of the DNA transferred by conjugation are called transconjugants.
