Transcript chapter 8
CHAPTER 8 Bacterial and Viral Genetic Systems GENE TRANSFER IN BACTERIA Bacteria can transfer genes from one strain to another by three different mechanisms In all three mechanisms, one cell - the donor - provides the genetic material for transfer, while a second cell - the recipient - receives the material. Conjugation - one bacterial cell transfers DNA to another cell by direct cell-to-cell contact Transformation - a bacterial cell acquires DNA from the environment and incorporates this DNA into its own chromosome Transduction - certain bacterial viruses can pick up a piece of DNA from one bacterial cell and inject it into another, where it can be incorporated into the chromosome The recipients of a gene transfer are known as transformants, transconjugants, or transductants, depending on the mechanism of DNA transfer All bacterial gene transfer is asymmetrical Most recipients receive 3% or less of a donor’s DNA; some transconjugants contain a greater percentage of donor material The amount of donor DNA entering the recipient is small relative to the size of the recipient’s chromosome, and the recipient retains most of its own DNA Discovery of the fertility factor (F) Strains that carry F can donate, and are designated F+ Strains that lack F cannot donate and are recipients. These strains are designated F- F is an example of a plasmid that can replicate in the cytoplasm independently of the host chromosome Hfr strains High frequency of recombination On crossing an Hfr strain with F-, the Hfr strain produces 1000 times as many recombinants for genetic markers as did a normal F+ strain In Hfr x F- crosses, virtually none of the F- parents were converted into F+ or into Hfr In F+ x F- crosses, transfer of F results in a large proportion of the F- parents being converted into F+ Hfr strain results from the integration of the F factor into the chromosome During conjugation between an Hfr and a F- cell a part of the chromosome is transferred with F Random breakage interrupts the transfer before the entire chromosome is transferred The chromosomal fragment can then recombine with the recipient chromosome Bacterial transformation The conversion of one genotype into another by the introduction of exogenous DNA is termed transformation Transformation was discovered in Streptococcus pneumoniae in 1928 by Frederick Griffith In 1944, Oswald T. Avery, Colin M. MacLeod, and Maclyn McCarty demonstrated that the “transforming principle” was DNA The field of bacterial viruses is a fine playground for serious children who ask ambitious questions - Max Delbruck