Genes expression in prokaryotes

J. Lederberg and E. I. Tatum (1946) demonstrated sexuality in bacteria for the first time and this opened a new era of research. Most of the important work on genetics of bacteria was initially done on the colon Bacillus bacteria named Escherichia coli.
Bacteria genome is represented by a circular double stranded DNA. Its DNA is associated with few proteins. E. coli contains 2000-3000 genes. Bacterium can survive on glucose diet giving the idea that these genes have information for synthesis of all organic compounds it needed.
Modes of genetic transfer in bacteria
Bacteria mainly reproduce asexually by binary fission. Meiosis is lacking. They do not show sexual reproduction like eukaryotes. However, some bacteria show primitive form of sexual reproduction to exchange genetic material between two cells. There are three modes of exchange of genetic materials or genetic recombination. They are:
a) Transformation
b) Transduction
c) Conjugation
a) Transformation – A short fragment of naked DNA isolated from one type of bacteria cell is incorporated into other type of bacterial cell. A recombinant or hybrid DNA is, thus, formed. This phenomenon is called as transformation. In this way, DNA of donor cell expresses some of its properties into the recipient cell. Griffith confirmed it with experiment on Diplococcus pheumonia. Normally E. coli does not pick up foreign DNA but can be done in the presence of calcium chloride.
b) Transduction – This is similar to the transformation but transfer of DNA from one bacterium to another is mediated through a vector. This process is called transduction. A vector may be a bacteriophage (virus) or a plasmid or a cosmid.
During this process, bacteriophage is used as a plasmid to transfer a small piece of double stranded DNA from one bacterial cell (so called donor cell) to another cell (so called recipient cell). A recombinant or hybrid DNA is, thus formed which consists of genome of both bacteria and thus expresses both of the properties.
c) Conjugation – Transfer of DNA from one cell to another through a sexual mating is called conjugation. It is similar to sexual mating in eukaryotes. Lederberg and Tatum first demonstrated it in E. coli.
In this process, male bacterium makes sex pili which enables cell to cell contact. The male bacterium is represented as a donor (F?). The female bacterium (recipient F?) lacks pili and receives the DNA from the male bacterium. This result to new genetic recombination. The progeny of the recipient then expresses some of the donor’s traits due to recombination.
Fertility factor and Hfr strain
The ability of transferring genetic material from male is regulated by sex or fertility factor (F genes) present in a plasmid. The bacteria with F genes are said to be F positive (F?) and another F negative (F?). F genes code for producing sex pili and other functions required for transferring DNA. At times F factor integrates into the bacterial chromosome
Such bacteria can transfer their genetic material into female with high frequency (Hfr) in a particular sequence. The are called as Hfr strains. Frequency of recombination was very low in Lederberg’s experiments. Hayes (1952) found a strain of E. coli in which the frequency of recombination was as high as 100 to 1000 times as reported by Lederberg. The strain was called as high frequency recombinant (Hfr) strain.