Sunday, September 27, 2009
Conjugation and Gene Recombination in E. Coli also Transduction and Transformation
Conjugation and Recombination:
J. Lederberg and E. L. Tatum discovered specific type of sexual reproduction E. coli bacteria which was named as Conjugation. Wild type E. coli usually grow on a minimal medium containgin inorganic salts and glucose. Two mutant kinds were found in E. coli bacteria which could not grow on the minimal medium. One mutant strain was named as Y10 and the other mutant strain was known as Y24.
Y24 strain was unable to synthesize three chemical substances i.e. threonine, Leucine (both amino acids) and a vitamin biotin. Various experiments revealed that both the strains can give rise to wild type back mutants but at very low rate i.e. 1/1000000 (one per million) or even 1/10000000 (one per hundred million). The scientists prepared mixed cultures of Y10 and Y24 by providing all six additional substances which were necessary for their growth. They discovered wild type bacteria from the mixed culture. They also discovered various recombinants of six mutations. Because the rate of back mutation was already determined in these bacteria which was very low rate, therefore it was explained that the discovers of wild type bacteria and other recombination of mutations could not have possibly originated due to back mutation.
This only was to explain the origin of coild type bacteria and recombinants could be the explanation that the exchange of hereditary material might have occurred during a process that was named as Conjugation.
SEXUAL MATING TYPE IN BACTERIA:
In E. coli the bacterial strains between which the exchange of hereditary material occur by Conjugation are known as mating types one mating type which donates its hereditary material to the other is usually known as F+; other bacterial strain which receives the hereditary material is called as F-. F+ cannot mate with F+ type; similarly F‑ cannot mate with F- type. Frequency of recombination of hereditary material is relatively low. Period of 430minutes is normal time of Conjugation.
When a phase virus is allowed to cause infection in bacteria, a few bacteria survive and are not destroyed by phage, although the phages chromosome had entered the bacteria. Such resistant bacteria are supposed to carry phage chromosome in inactive form known as prophage of temperate phage; however such passive phage itself is known as temperate phage. Resistant strains of bacteria which escape from Lysis are known as Lysogenic. The presence of phages chromosome in Lysogenic bacteria in an inactive form is useful to bacteria because it enables them to with stand infection and prevents the reproduction of phage particles. In 1952 Zinger and Lederberg discovered that such inactive phage in the bacteria might act as a carrier for the transfer of genes from one bacterium to another bacterium, the process is known as transduction.
Example: In Salmonella (bacteria) there are two mutant strains, one of which is tryptoplan (amino acid) dependent and the other is cysteine dependent. When these two mutant strains were grown together and were allowed to be infected with T4-phage virus, three types of bacteria were discovered from the culture i.e. wild type bacteria, tryplophan dependent bacteria and cysteine dependent bacteria. The rate of occurance of wild type bacteria in this case is much greater than can be expected by back mutation, therefore, it was concluded that the wild type bacteria in this case might have been produced by Transduction i.e. some of the wild type T4-phage virus which were containing some of the genes of the other bacteria would have been responsible to induce the property of synthesizing typtophan and cysteine.
If some living strain of bacteria is exposed to purified DNA extract of some of other strain of bacteria, recombinant type bacteria arise which may show recombination for one gene that are known as single gene transformation or they may show recombination for two genes which are known as double transformants or may show recombination for three genes that are known as triple transformants. Various experiments of transformation on all Mutants have shown that many genes are involved to synthesize and enzyme amylomaltase which is actually responsible to metabolize maltose.Example: There are two strains of bacteria one of which is resistant to sulpha drug and amethopterin (a toxic substance), the other strain is resistant to streptomycin and micrococin. When mixed culture of these two strains were grown in Laboratory or the DNA extract of one strain is added to the living culture of the other strain osme bacteria resistant to all the four substances were produced. Formation of such recombinant type bacteria by adding DNA extract of the other strain was indicative of genetic transformation. Tags: Biology , Botany
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