Genetic Exchange: Transformation and Conjugation
Uploaded by knoxville on May 14, 2005
Mechanisms of Genetic Exchange: Transformation and Conjugation
Introduction
Genetic exchange had been known to play a role in the evolution and survival of microorganisms. Genes of one bacterium can be exchanged to another through several different ways: conjugation, transformation and transduction. However, the focus of this paper is to further examine the characteristics of conjugation and transformation mechanisms.
Transformation was first discovered in 1928 by Frederick Griffith while he was studying pneumococcal infection in mice.1 In 1944, Avery, McCleod and McCarthy discovered that the transforming substance is DNA.1,2 Transformation occurs when a naked, double-stranded DNA was taken from outside into the cell, may be followed by the integration of the DNA into the host’s chromosome, and also replication, transcription and expression of the information encoded in the DNA.1,2,3 However, the recipient cell must be competent- do not secrete DNAase- upon the intake of the DNA fragment (from the donor) in order for the DNA fragments to be recombined into the recipient cell’s chromosome.2 In other words, transformation can be made to be more efficient if the DNAase do not exist since the enzyme acts as a digestor of foreign DNA.4 An advantage of transformation is the fact that the DNA can be subjected to mutagenesis and other treatments or manipulations, enabling the analysis of the bacterial genome. A disadvantage to the transformation method is the fact that this method can only produce small percentage of transformation in most bacteria.3 Due to the much smaller sizes of the transforming fragments and their competition for uptake with many other DNA fragments, the probability that a recipient cell incorporated the desired DNA fragment is very slim.4
Conjugation is another method that is very adaptable and efficient for intra- or inter- species genetic transfer.3 First discovered in 1946 by Lederberg and Tatum in a strain of E.coli called K-12, conjugation requires physical contact between two genetically different cells via a conjugation tube called the F-pilus, which transfers the F factor.1,2,4 The F factor is acquired by conjugation from an F+ to an F-.
The F factor has a size of approximately 1/50 the size of the E.coli chromosome and it has no connection to the bacterial chromosome, thus the F factor and the chromosome replicated independently of each other. Eventhough the F factor is not involved in the normal function of the cell and it is dispensable, it is the determinant in the “organism’s...