Bacterial Transduction & Transformation Lecture Notes PDF

7.4 Bacterial Transduction Transduction is the transfer of DNA from one bacterium to another via a bacteriophage A bacteriophage is a virus that specifically infects bacterial cells It is composed of genetic material surrounded by a protein coat Depending on the type of bact...

7.4 Bacterial Transduction Transduction is the transfer of DNA from one bacterium to another via a bacteriophage A bacteriophage is a virus that specifically infects bacterial cells It is composed of genetic material surrounded by a protein coat Depending on the type of bacteriophage, it may follow one of two different cycles, or both Lytic Lysogenic © McGraw Hill 27  Transduction 1 Bacteriophages that can transfer bacterial DNA include: P22, which infects Salmonella typhimurium P1, which infects Escherichia coli Figure 7.10 illustrates the process of transduction © McGraw Hill 28  Transduction 2 Donor cell is his + his + gene is on a chromosome fragment that is packaged into a phage particle Phage DNA with his + gene infects recipient cell + Recipient cell becomes recombinant bacterium that is his © McGraw Hill 29  (Figure 7.10) Transduction in bacteria Access the text alternative for slide images. © McGraw Hill 30  7.5 Bacterial Transformation 1 Transformation is the process by which a bacterium will take up extracellular DNA released by a dead bacterium It was discovered by Frederick Griffith in 1928 while working with strains of Streptococcus pneumoniae © McGraw Hill 31 © McGraw Hill 32  7.5 Bacterial Transformation 2 There are two types: Natural transformation DNA uptake occurs naturally Artificial transformation DNA uptake occurs with the help of experimental techniques © McGraw Hill 33  Transformation Natural transformation occurs in a wide variety of bacteria Bacterial cells able to take up DNA are termed competent cells They carry genes that encode proteins called competence factors These proteins facilitate the binding, uptake and subsequent incorporation of the DNA into the bacterial chromosome © McGraw Hill 34  Steps of Transformation DNA in the extracellular environment is lys + Cellular DNA is lys − Extracellular endonuclease cuts the DNA into smaller fragments; one strand is degraded and the single strand is transported into competent bacterial cell Homologous recombination can occur Heteroduplex is a region of mismatch caused by sequence differences between the two alleles Repaired by DNA repair enzymes Transformed cell is now lys + © McGraw Hill 35 © McGraw Hill 36 © McGraw Hill 37  (Figure 7.11) The steps of bacterial transformation Access the text alternative for slide images. © McGraw Hill 38  Recombination If the transformed DNA is homologous to a region in the bacterial chromosome, it can be incorporated by homologous recombination Sometimes, the DNA that enters the cell is not homologous to any genes on the chromosome It may be incorporated at a random site on the chromosome This process is termed nonhomologous recombination © McGraw Hill 39  Taking up DNA from the same species Some bacteria preferentially take up DNA released from dead bacteria of the same or a related species Streptococcus pneumoniae secretes a competence- stimulating peptide which leads to competence only when many cells of the same species are nearby Other species use DNA uptake signal sequences 9 or 10 bp long repeated 1,000 to 2,000 times throughout genome DNA with this sequence is preferentially taken up © McGraw Hill 40  Artificial Transformation Laboratory methods are commonly used to get plasmids into cells Common method is to treat with calcium chloride and a high temperature shock Makes the cells permeable to small DNA molecules Electroporation makes cells permeable with external electrical field © McGraw Hill 41  7.6 Medical Relevance of Bacterial Genetic Transfer Horizontal Gene Transfer Process in which an organism receives genes from another organism without being a direct offspring Conjugation, transformation, and transduction are examples of horizontal gene transfer Can occur within and also between species The types of genes acquired through horizontal gene transfer are quite varied and include genes that confer antibiotic resistance © McGraw Hill 42  Acquired Antibiotic Resistance 1 Horizontal gene transfer has dramatically contributed to the phenomenon of acquired antibiotic resistance Antibiotics are widely used to treat human infections and increase health and size of livestock Bacteria can acquire genes that breakdown the antibiotic, pump it out of the cell, or block its inhibiting effects © McGraw Hill 43  Acquired Antibiotic Resistance 2 Bacterial resistance to antibiotics is a serious problem worldwide Some Staphylococcus aureus strains are resistant to methicillin and all penicillins Resistance probably acquired by horizontal transfer, possibly from Enterococcus faecalis These MRSA strains cause serious skin infections © McGraw Hill 44  (Figure 7.12) Percentage of S. aureus strains that were found to be resistant to the antibiotic, methicillin, over a 20-year period. 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