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Transfer of DNA Within Bacterial Cells
Transfer of DNA within the bacterial cells can occur by (a) trans-posons (b) integrative conjugating elements, and (c) programed rearrangement.
Transposons are a type of mobile DNA of 2000–20,000 bp. They can transfer DNA from one site of the bacterial chromosome to another site or to a plasmid. The idea of trans-posons or jumping genes was first given by Barbara McClintock, a geneticist working in the field of maize genetics.
The mode of genetic transfer by transposon is called transposition. The transposition differs from recombinationin that a segment of DNA can be transferred from one to another molecule that has no genetic homology with either the transposable element or the donor DNA.
Transposons do not occur independently but have the char-acteristic of jumping from one part of a chromosome to another or to a plasmid. They can also jump from one plasmid to another or back to the chromosome, hence, are called as jumping genes. Transposition in prokaryotes usually involves two steps—self-replication and recombination. They jump from one part to another by synthesizing a copy of their DNA and inserting the copy at another site in the bacterial chromosome or the plas-mid. Transposons, unlike plasmids, are not self-replicating and depend on chromosomal or plasmid DNA for replication. Transposons do not require homology with the recipient site for its transfer. Transposons typically consist of four domains:
· The first domain is a short DNA sequence of inserted repeats that are present at the end. This domain mediates the inte-gration of the transposons into the recipient DNA.
· The second domain is the gene for the enzyme transposases.
· These enzymes mediate the excision and integration process.
· The third domain is the gene for the repressor, which regulates the synthesis of both transposase and the gene products of the fourth domain.
· The fourth domain is the gene for the expression of enzymes.
Plasmid can contain several transposons carrying drug-resistance genes. The transposons code for drug-resistance enzymes, toxins, or a variety of metabolic enzymes. These transposons can either cause mutations in the gene into which they insert or alter the expression of nearby genes.
Insertion sequences: The simplest form of transposons is theinsertion sequences. These are a type of transposons that have a few bases varying from 800 to 1500 bp. They are found as multiple copies at the end of larger transposon units. These cause mutation by moving from one side to another in DNA sequence and are believed to control various cellular responses.
Integrative conjugative elements (ICEs) are ways of horizontal gene transfer and self-transmissible mobile genetic elements. The elements exchange by conjugation, but need to integrate into chromosome to propagate. They cannot replicate autono-mously. ICEs integrate into and replicate along with the host cell chromosome, whereas plasmids exist as extra-chromosomal (usually circular) autonomously replicating DNA molecules.
The transfer of DNA within bacteria can also occur by programed rearrangement. In this programed rearrangement, there is a move-ment of a gene from a silent site where the gene is not expressed to an active site where transcription and translation occur.
Many silent genes are present in the DNA that encode vari-ants of the antigens. Presentation of the new gene into the active site occurs in a sequential and repeated manner, which then manifests in antigenic variations in the bacteria and parasites. This mechanism is responsible for antigenic variations seen in Neisseria gonorrhoeae, Borrelia recurrentis, and Trypanosoma brucei.
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