The genome of bacteria are smaller than the eukaryotes. There is less variation in size of the genome among the species when compared to the eukaryotes. The bacterial genomes range in size between 139 kbp to 13x103 kbp.
The architecture of bacterial genome is compact and distinct from that of the eukaryotes. The genes that are functions in the genome of a bacteria are organized into operons. The bacterial genome have non-coding DNA in the form of intergenic regions and introns, this is the main reason for the relative density of the bacterial genome. There is little variation in the genome size of bacteria from species to species.
The free-living bacteria have the largest genomes and they have few psuedogenes.
The genome size of the free-living bacteria smaller than the facultative bacteria and the recently evolved pathogenic bacteria. The facultative bacteria have more pseudogenes than any other form of bacteria.
The symbionts or the pathogens the obligate bacteria have the smallest genomes and the least number of pseudongenes than any other groups of bacteria.
Plasmids are small molecules of DNA that are separate from the chromosomal DNA Of the bacteria. They have ability to replicate independently. The plasmids are found as small circular, double stranded DNA molecules. Plasmids are sometimes also seen in archaea and eukaryotic organisms.
Plasmids are said to carry genes that may benefit the survival of the organism. The plasmids can be transmitted from bacterium to another by the process of horizontal gene transfer. Plasmids are made artificially are used widely as vectors in genetic engineering and molecular clonning. The size of plasmids vary from 1 to over 103 kbp.
Some bacteria show transfer of genetic material between cells. The gene transfer can occur in three main ways.
- Transduction and
Transformation is the process when the bacteria can take up exogenous or external DNA from their environment.
Transduction is the process when there is transfer of genes with the integration of a bacteriophage that introduces a foreign DNA molecule into the bacterial chromosome.
Conjugation is a method of gene transfer where genes are transferred through direct cell contact.
Transformation is one of the three processes by which genetic material may be introduced into a bacterial cell. Transformation is the genetic alteration of the bacterial cell where the there is direct uptake, fusion and expression of exogenic genetic material from its environment, which is taken up to the cell membrane. In some species of bacteria, transformation generally occurs, for the process of transformation to happen the bacteria must be in a state of competence.
Transduction is one of the process of genetic process where the DNA is transferred from one bacterium to another by a virus. It is also a process where foreign DNA is introduced into another cell with help of a virus that acts as a vector. The process of transduction does not require physical contact between the cells in the process. When bacteriophage infects a bacterial cell, the virus that infects the bacteria harness the reproduction the DNA replicating machinery of the host bacterial cell and to make numerous virons or viral particles that includes the genetic material of the virus and the protein coat.
The process of transduction takes place either through the lytic cycle or the lysogenic cycle. In the lysigenic cycle the chromosome of the bacteriophage is integrated into the bacterial chromosome and it can remain dormant for thousands of generations. On introducing a lysogen the genome of the bacteriophage is excised from the chromosome of the bacteria and this initiate the lytic cycle. The lytic cycle results in the lysis of the host cell or the bacterial cell and release of the phage particles. The lytic cycles produces new phage particles and which are released by the lysis of the host cell.
Conjugation in bacteria is the transfer of genetic material or the plasmid between the bacterial cells. In this process the bacterial cells show DNA transfer through cell to cell contact or by a bridge like connection between the two cells. Conjugation is the only mechanism which involves cell to cell contact for gene transfer and is a mechanism of horizontal gene transfer.
This process is regarded as the equivalent of bacterial sexual reproduction or mating as the process involves the exchange of genetic material. During this process the donor cell gives a mobilizable genetic material like the plasmid or transposon. In most plasmids that undergo conjugation ensure that the recipient cell does not contain a similar element. The genes transferred are mostly beneficial to the recipient; these may include antibiotic resistance, xenobiotic tolerance and the ability to use new metabolites.