Lecture 8: Bacterial Sex PDF

Lecture #8- Bacterial Sex- This lecture covers mechanisms that introduce genetic variation into bacterial populations. Lecture Objectives After this lecture and the readings you should be able to: • distinguish horizontal gene transfer from vertical gene transfer • compare and contrast conjugation...

Lecture #8- Bacterial Sex- This lecture covers mechanisms that introduce genetic variation into bacterial populations. Lecture Objectives After this lecture and the readings you should be able to: • distinguish horizontal gene transfer from vertical gene transfer • compare and contrast conjugation, transformation, and transduction • the Ame’s test Microorganisms and Concepts Streptococcus pneumonia • transformation, conjugation, transduction • plasmid • competence • Griffith’s experiment • capsule • F , F’ and Hfr • F plasmid and R plasmid • sex pilus • bacteriophage • Ame’s Test Patterns of DNA Synthesis • DNA in most Bacteria is circular – bidirectional replication from a single origin – replication fork is where DNA is unwound • replicon – origin and is replicated as a unit (entire genome in bacteria) • Archaea is also circular but may have more than one origin • eukaryotic – linear chromosome with many replication forks 4 5 Genetic architecture of a bacterium. Plasmid Could also be…. Plasmids Bacterial chromosome is typically circular. Generally one chromosome but some bacteria such as choleracausing bacteria have two. No nucleus; have a nuclear region or nucleoid. Approximately 1 mm long when stretched out. Supercoiled. Plasmid: small, extrachromosomal DNA molecules, autonomously replicating. Bacterial cells may carry several different plasmids each having few to many copies in the cell. So, how do bacteria have sex? How is genetic information exchanged? Effects of Gene Transfer • Spreads useful genes among bacteria – Antibiotic-resistance genes • Spread wherever antibiotics are overused – Hospitals, farms – Pathogenicity islands • Encode genes for cell to act as pathogen – Difference between typical E. coli in gut » and pathogenic E. coli O157:H7 – Genes to degrade special metabolites • e.g. oil slicks Methods of genetic exchange: 1. Transformation 2. Conjugation 3. Transduction Bacteria have more than one way of having sex! Transformation: Capturing DNA The non-specific acceptance by a bacterial cell of small fragments of DNA from the surrounding environment. Facilitated by special DNA-binding proteins on the cell wall that capture DNA. Competent cells. The new DNA incorporated into chromosome. Transformation is very useful for biotechnology and recombinant DNA technology. Foreign genes from completely unrelated organism inserted into a plasmid which is then introduced to the competent bacterial cell through transformation. http://www.youtube.com/watch?v=MRBdbKFisgI&feature=related How was transformation discovered? Streptococcus pneumoniae (pneumococci) growing as colonies on the surface of a culture medium. Left: The presence of a capsule around the bacterial cells gives the colonies a glistening, smooth (S) appearance. Right: Pneumococci lacking capsules have produced these rough (R) colonies. Capsule is a gel-like Covering made chiefly of polysaccharides. Associated with virulence. Helps resist phagocytosis. Genetically encoded. Griffiths (1928) experiment of genetic transformation with Streptococcus pneumoniae. Transformation Conjugation: Transfer of DNA from one bacterium to another by cellular contact. DNA moves unidirectionally from one bacterium (the donor), to another bacterium, (the recipient). Genes involved in genetic transfer exist in three states in the donor: (1) on an F plasmid, (2) on an F’ (F prime) plasmid, (3) integrated into bacterial chromosome Hfr. Conjugation between F+ (donor) and F- (recipient). Gene Transfer: Conjugation • “Bacterial sex” – Type IV secretion system = pilus – Pilus proteins encoded on F-factor – Transfers DNA on F factor • Starts at oriT site – In recipient cell, transferred DNA forms a new F-plasmid – “Female” cells become “male” • Able to transfer DNA https://www.youtube.com/watch?v=KLIo3wYVKiE Transfer genes exist in three states. Genetic transfer occurs in three forms; F+ -----> FF’-------> FHfr------>F- Gene Transfer: Conjugation • F’ factor – F plasmid contains extra genes lac genes oriT F’lac • In addition to genes for pilus, transfer – Transfers extra genes to recipient • Hfr – F-factor integrates on bacterial chromosome – Tries to transfer entire chromosome • Requires 100 minutes for E. coli • Transfers genes in order • Can determine order of genes on chromosome F-factor bacterial chromosome Production of an F’ and Hfr: A/ region of chromosome where F factor has integrated. This is an Hfr(high frequency of recombination). B/ The F factor loops out and takes a portion of the chromosome with the lac genes. C/ The resultant F factor contains lac genes and is called F’. Medical Significance of Gene Transfer F plasmids responsible for antibiotic resistance. F plasmids that harbour genes for antibiotic resistance are called R factors. A single F’ plasmid may harbour several antibiotic resistance genes. Bacteria resistant to several antibiotics are called MDRs or multidrug-resistant bacteria. F’ plasmids may also carry genes encoding traits that play a role in disease such as toxins. Transduction Gene Transfer: Transduction • Viruses inject DNA into cell – Bacteriophage • Package DNA into viral capsid – Viral DNA – Sometimes package bacterial DNA by mistake • Transfer DNA to new host – Can bring new bacterial genes to host DNA transfer in bacteria: Transformation Transduction Conjugation Horizontal Gene Transfer (HGT) in Bacteria and Archaea • important in evolution of many species – expansion of ecological niche, increased virulence – • genes can be transferred to the same or different species 31