Bacterial variations are changes in the bacterial characters. They may be Phenotypic or Genotyping (see the table below).
Table 1: Comparison between Phenotypic and Genotyping variations:
| Characteristic | Phenotypic Variation | Genotypic Variation |
|---|---|---|
| Cause | Change in environmental conditions | Change in genetic constitution |
| Reversible | Reversible | Irreversible |
| Heritable | Non-heritable | Heritable |
| Examples |
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Mutation
It results from a change in the nucleotide sequence of DNA that may occur spontaneously as a replication error (at a rate of once every 10000000-100000000 cells), or may be induced by radiation or chemical agents (at a higher rate of once every 10000-100000 cells).
Mutation can be classified according to nucleotide substitution, insertion or deletion into:
Single-base (point) mutations:
Involve the replacement (substitution) of a single nucleotide in the coding sequence. This may result in:
- Same sense (silent) mutations: occur when the resulting base triplet (codon) codes for the same amino acid as the original triplet.
- Missense mutations: occur when the mutant base changes the coding sequence so that a different amino acid is produced.
The resulting protein may be functioning or not, depending on the importance of the area affected by the mutation.
Frame-shift mutations:
Occur when a nucleotide is inserted into, or deleted from the coding sequence, resulting in a shift of the reading frame, e.g. insertion of a transposable element.
Induced mutations may be used to manipulate viral genomes for vaccine production and gene therapy.
Gene Transfer
There are 3 methods for gene transfer among bacteria (see figure 1 below).
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| Fig.1: Methods of gene transfer |
1. Transformation
Dying bacteria release DNA which can be taken up by other bacteria. Such DNA may be chromosomal or plasmid in origin, and may carry genes that "transform" the recipient bacterium.
The transforming DNA may become integrated with the bacterial chromosome or re- established extrachromosomally in the recipient cell.
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| Fig. 2: Gene transfer by transformation |
Transformation depends on competence, which is the ability of the recipient bacterial cell to take up DNA. Competence depends on the presence of proteins in the cell membrane that have a special affinity to bind DNA and transport it into the cytoplasm.
Artificial competence can be induced during recombinant DNA techniques by treating the recipient bacteria with calcium chloride, which alters cell membrane permeability, enabling the uptake of DNA.
2. Transduction
It is the transfer of DNA from one cell to another by means of a bacteriophage. There are 2 types of transduction:
a- Generalized transduction:
During the lytic phage cycle, the bacterial DNA is fragmented and any fragment of DNA (whether chromosomal or plasmid) may be incorporated into the phage head. The phage particle can then transfer the incorporated bacterial DNA into another bacterial host.
b- Specialized transduction:
It takes place when a prophage contained in a lysogenized bacterial cell is induced to detach. Such prophage may carry with it the adjacent piece of the chromosomal DNA and transfer it to another bacterial cell.
Table 2: Comparison between Generalized and Specialized Transduction:
| Characteristic | Generalized Transduction | Specialized Transduction |
|---|---|---|
| Type of phage | Lytic (virulent) | Temperate (lysogenic) |
| Replication cycle | Lytic cycle | Lysogenic cycle |
| Transferred DNA fragments | Any piece of bacterial DNA (chromosomal or plasmid) | A specific piece of chromosomal DNA adjacent to the site of insertion of the prophage |
3. Conjugation
It is the most frequently observed mechanism of DNA transfer. It involves 2 cell types: donors (F+) which possess the fertility (F) factor, and recipients (F-) which lack the F factor.
The F factor carries the genes for the synthesis of the sex pilus which acts as a conjugation tube between the donor and recipient bacterial cells.
The 2 DNA strands of the F factor are then separated, and one strand is transferred from the donor to the recipient cell.
Each strand forms a complementary strand, thus, the recipient cell acquires a copy of the F plasmid and becomes an F+ cell.
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| Fig. 3: Gene transfer by conjugation |