A circular double strand or duplex DNA is folded variously with the help of RNA and polyamines to form bacterial chromosome, nucleoid, or genophore. It has 1/1000 DNA content of a eucaryotic genome. DNA of E. coli has a length of 1100 µm with some 2500 genes. Nucleoid is connected by y-shaped forks to a membranous body/mesosome that develops over the plasmalemma.
A bacterial cell contains one to several circular extrachromosomal DNA segments called plasmids (Hoyes and Lederberg, 1952). They are capable of replication like nucleoid. At times the plasmid integrates with bacterial chromosome. It is men caYleO eplsome. A plasmid carries a few genes, e.g. fertility factor (F+), R-factors (resistance factors against common antibiotics like chloroamphenicol, tetracycline, streptomycin, sulphonamide), Nif (Nitrogen fixation) or colicinogenic factors (for synthesis of bacteriocidal and bacteriostatic chemicals).
Bacteria multiply by binary fission. Generation time is 20 minutes in E. coli to 15-20 hours in Myxobacterium tuberculosis. Nucleoid replicates while attached to mesosome. It behaves like a single replicon (Cairns, 1963). Mesosome divides alongwith replication of nucleoid so that each daughter nucleoid is attached to its own mesosome.
Gene Recombination in Bacteria
Sexual reproduction and haploid-diploid alternation are absent in bacteria. However, one way gene reshuffling or recombination (mesomixis) can occur by three methods-conjugation, transformation & transduction.
It was discovered by Lederberg and Tatum (1946) in E. coli. They found that mixing of two different double (Bio–, Met–) and triple (Thr–, Leu–, Thi–) auxotrophs over minimal medium produced occassional prototrophs. Cell contact was required. Anderson (1957) observed conjugation sare dimorphic, male donor having fertility factor (F+) and sex pili while female or recipient is without both fertility factor (F–) and sex pili. Contact between the two types cause development of ‘conjugation tube’ by means of a pilus of donor.
Meromixis occurs by two methods.
(a) Sterile Male Method:
No gene of the male chromosome is transferred to the recipient cell. Fertility factor replicates. One plasmid passes into recipient/female cell through the conjugation tube to make it donor as well. The phenomenon is called sexduction.
(b) Fertile Male Method:
The plasmid of fertility factor gets integrated to bacterial chromosome and is called episome. The donor F+ cell with episome is called Ffr (High frequency of recombination) or super male as recombination frequency increases to some 1000 times. The integrated F+ factor breaks bacterial chromosome at the end of its attachment.
The freed chromosome end (zero end) becomes linear and passes into recipient cell. The whole male chromosome is rarely transferred (it takes some two hours). The fertility factor is the last to migrate. Conjugation is generally interrupted any time. A few to several genes of male/donor are transferred to female/recipient cell.
The latters are now called merozygote or partial zygote. The donor cell generally dies after conjugation. Some donor genes get integrated to recipient genome while others degenerate.
It is the picking up of genes from DNA of the dead relatives. Transformation was discovered by Griffith (1928) in Diplococcus pneumoniae (transfer of gene for virulence from dead bacteria to nonvirulent bacteria).
Avery et al (1944) found transferring material to be segment of DNA. Transformation is quite common as DNA is released into the medium after death of cells. Competence or ability to pick up foreign DNA is present for a brief period towards the end of active growth.
Additionally calcium chloride in required by E. coli. A competent cell has many receptor sites for attachment of foreign DNAs but only a specific DNA segment gains entry. It attaches to fork of mesosome and gets integrated to host DNA during its replication. The competent recipient cell now becomes recombinant.
Transfer of genes from one organism to another with the help of a virus/vector is known as transduction. It was discovered by Zinder & Lederberg (1952) in case of Salmonella typhimurium (typhoid in mice). Later it was reported in a number of other bacteria. Bacteriophages have two types of multiplication, lysogenic and lytic. In lysogenic/ nonvirulent/temperate phase, the virus genome is integrated to bacterial genome and called prophage.
At times the prophage dissociates from the bacterial genome, becomes virulent and performs lytic cycle. However, the bacterial DNA is not destroyed completely so that some of the phage particles come to have segments of bacterial genome. They are called ‘transducting phages’ as they transfer some genes of the previous host to the new host, (the new host is not harmed as the phage becomes deficient in some of its own genes).
Transduction is of two types, ‘generalised’ (genes not fixed) and ‘restricted’ (same gens, e.g. lac or bio gene in case of lambda phage in E. coli K-12, because the phage genome attaches between the two). Donor gene may integrated between the two. Donor gene may integrate with genome of recipient (becomes recombinant, hence complete transduction) or fails to get associated with recipient DNA and degenerate abortive transduction.)