Microbial genetics lecture 10

Transformation Transformation: is the uptake of naked DNA from the environment (DNA without associated cells or proteins) Competence: refers to the state of being able to take up exogenous DNA from the environment. Transformation is one of three processes by which exogenous genetic material may be introduced into a bacterial cell, the other two being conjugation (transfer of genetic material between two bacterial cells in direct contact), and transduction (injection of foreign DNA by a bacteriophage virus into the host bacterium).

Conjugation in E. coli Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells. It is a mechanism of horizontal gene transfer as are transformation and transduction although these two other mechanisms do not involve cell-to-cell contact. The donor cell provides a mobilizable genetic element that is often a plasmid or transposon.

Bacterial Conjugation Most conjugative plasmids have systems ensuring that the recipient cell does not already contain a similar element. The genetic information transferred is often beneficial to the recipient. Benefits may include antibiotic resistance, xenobiotic tolerance or the ability to use new metabolites Such beneficial plasmids may be considered bacterial endosymbionts. Other elements, however, may be viewed as bacterial parasites and conjugation as a mechanism evolved by them to allow for their spread.

Transduction by a Bacteriophage Transduction: is the process by which DNA is transferred from one bacterium to another by a virus. It also refers to the process by which foreign DNA is introduced into another cell via a viral vector. Transduction does not require cell-to-cell contact (unlike conjugation), and it is DNAase resistant (unlike transformation). It is a common tool used by biologists to introduce a foreign gene into a host cell's genome.

Transduction by a Bacteriophage When bacteriophages infect a bacterial cell, they harness the replicational, transcriptional, and translation machinery of the host bacterial cell to make numerous virions, or complete viral particles, including the viral DNA or RNA and the protein coat. Transduction and specialized transduction is especially important because they explain how anti-biotic drugs become ineffective due to the transfer of resistant genes between bacteria. In addition, hopes to create medical methods of genetic modification of diseases

Plasmids Plasmid: is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and circular. Naturally in bacteria, but sometimes found in eukaryotes Sizes vary from 1 to over 1,000 kbp. The number of identical plasmids in a single cell range from one to thousands

R Factor, a Type of Plasmid Conjugative plasmid: Carries genes for sex pili and transfer of the plasmid Dissimilation plasmids: Encode enzymes for catabolism of unusual compounds R factors: Encode antibiotic resistance Figure 8.29

Transposons Transposons are sequences of DNA that can move themselves to new positions within the genome of a single cell. transposition can be either "copy and paste" or "cut and paste". Transposons are often considered "junk DNA". Transposons are very useful to researchers as a means to alter DNA inside a living organism. Contain insertion sequences for cutting and resealing DNA (transposase) Complex transposons carry other genes