Lecture 12 June 8 2009 Dr Abumaree

Lecture 12 Viruses, disease and Vaccines Biology, Campbell & Reece. 7th Edn. Ch 18

Dr Mohamed Abumaree Molecular Reproductive Biologist & Immunologist College of Medicine King Saud bin Abdulaziz University for Health Science 2009

Mechanisms of Gene Transfer and Genetic Recombination in bacteria

 Bacteria differ from eukaryotes in the mechanisms used to bring DNA from two individuals together in one cell  In eukaryotes, meiosis and fertilization (sexual processes) combine DNA from two individuals in a single zygote  Meiosis: cell division results in cells with half the chromosome number of the original cell

 Meiosis and fertilization do not occur in prokaryotes, but there are 3 other processes: 1.Transformation 2.Transduction 3.Conjugation, bring together bacterial DNA from different individuals

Transformation  The alteration of the genotype and phenotype of a bacterial cell by the uptake of naked foreign DNA from the surrounding environment  For example….a harmless strain of streptococcus pneumoniae are transformed to pneumonia by taking up a piece of DNA carrying the pathogenic gene from a medium containing dead broken–open cells of the pathogenic strain

 The foreign gene codes for a cell coat that protects the bacterium from the immune system of the host  Then, the foreign gene is incorporated into the chromosome of the nonpathogenic cell, replacing the gene for the “coatless” condition by genetic recombination: an exchange of DNA segments by crossing over  The cell is now a recombinant, its chromosome contains DNA derived from two different cells

 Transformation process is too rare!  E. coli lack the transformation mechanism, but can be artificially transformed DNA 

In biotechnology, transformation technique is used to introduce foreign genes into the E. coli genome—genes coding for valuable proteins, such as human insulin & growth hormone

Transduction In transduction, phages carry bacterial genes from one host cell to another as a result of abnormality in the phage reproductive cycle

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Conjugation and Plasmids  Conjugation is bacterial sex: direct transfer of genetic material between two bacterial cells that are temporarily joined (DNA transfer is one–way) One cell donates (Male) DNA and its mate receives the DNA  The donor uses appendages (sex pili) to attach to the recipient (female)

After contacting a recipient cell, a sex pilus retracts, pulling the two cells together to form a bridge for DNA transfer between the two cells 11

 The ability to form sex pili and donate DNA during conjugation results from the presence of a special piece of DNA called an F (fertility) factor

 F factor exists as a segment of DNA within the bacterial chromosome or as a plasmid: small, circular, self– replicating DNA molecule separate from the bacterial chromosome 13

 A plasmid genes are not required for the survival and reproduction of the

bacterium

under

normal

conditions, but they support bacteria living in stressful environments

 For example, the F plasmid facilitates

genetic

recombination,

which may be advantageous in a changing environment that no longer favors existing strains in a bacterial population 15

R Plasmids and Antibiotic Resistance  Mutation in a gene of the pathogen can cause resistance by reducing the ability of pathogen to transport a particular antibiotic into the cell

 For example, mutation in a different gene may alter the intracellular target protein for an antibiotic molecule, thus reducing its inhibitory effect  Some bacteria have resistance genes coding for enzymes that specifically destroy certain antibiotics, such as tetracycline or ampicillin 17

 Genes confer this type of resistance are carried by resistance (R) plasmids  Resistant strains of pathogens are making the treatment of certain bacterial infections more difficult  Because many R plasmids, like F plasmids, have genes that encode sex pili & enable plasmid transfer from one bacterial cell to another by conjugation

Transposition Of Genetic Elements 19

 Transposition: the movement transposable elements

of

the

 Unlike a plasmid or prophage, transposable elements never exist independently, they are part of chromosomal or plasmid DNA  Recombination mechanism: the movement of transposable elements from one site in the DNA of a cell to a target site, without being lost from the old site

Recombination mechanism includes 1. DNA folding or 2. A cut & paste mechanism or 3. A copy & paste mechanism 21

 Transposable elements are jumping genes but they never completely detach from the DNA of a cell  In a bacterial cell, a transposable element may move within the chromosome  From a plasmid to the chromosome (or vice versa), or from one plasmid to another 22

 Transposable elements vary in their selectivity for target sites, but most can move to many alternative locations in the DNA  So the ability to scatter certain genes throughout the genome makes transposition fundamentally different from other mechanisms of genetic shuffling 23

During bacterial transformation, transduction, conjugation & meiosis in eukaryotes…… Recombination occurs between homologous regions of DNA (regions of identical or very similar base sequence that can undergo base pairing)

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 In contrast, the insertion of a transposable element in a new site does not depend on complementary base sequences  A transposable element can move genes to a site where genes of that sort have never before existed

Insertion Sequences  The simplest transposable elements, exist only in bacteria  Contain a single gene coding for transposase

26



Transposase

movement

of

the

catalyzes insertion

sequence from one site to another within the genome

27

 The transposase gene is bracketed by a pair of noncoding DNA sequences, which are called inverted repeats because the base sequence at one end of the insertion sequence is inverted at the other end 28

Transposons Transposable elements longer complex than insertion sequences

and

more

29

In addition to the DNA required for transposition, transposons include antibiotic resistance gene sandwiched between two insertion sequences 30

 Insertion sequences are not known to benefit bacteria in any specific way  Transposons may help bacteria adapt to new environments by adding a gene for antibiotic resistance to a plasmid already carrying genes for resistance to other antibiotics  The transmission of this complex plasmid to other bacterial cells by cell division or conjugation can then spread resistance to a variety of antibiotics throughout 31 a bacterial population

 In an antibiotic–rich environment, natural selection favors bacteria that have built up R plasmids with multiple antibiotic resistance genes through a series of transpositions  Transposons are not unique to bacteria and are important components of eukaryotic genomes as well 32