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Horizontal Gene Transfer in Bacteria

Horizontal Gene Transfer in Bacteria. By: Daanyaal A. Lodhi #1205, MD3. Table of Contents. Introduction Nomenclature Conjugation Hfr Cells Transduction Transformation Griffith’s Experiment Recombination. Introduction.

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Horizontal Gene Transfer in Bacteria

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  1. Horizontal Gene Transfer in Bacteria By: Daanyaal A. Lodhi#1205, MD3

  2. Table of Contents • Introduction • Nomenclature • Conjugation • Hfr Cells • Transduction • Transformation • Griffith’s Experiment • Recombination

  3. Introduction • Horizontal Gene Transfer (HGT) refers to the transfer of genes between organisms in a manner that is not through reproduction. • It contrasts to Vertical Gene Transfer, which refers to the transfer of genes from a parental generation via sexual or asexual reproduction. • HGT is the primary cause of antibiotic resistance.

  4. Nomenclature • F+ : A bacteria that contain the Fertility plasmid (F Factor) • F- : A bacteria that does not contain the Fertility plasmid (F Factor) • Hfr: A High Frequency of Recombination cell.

  5. Conjugation • Refers to the mating of two bacteria in which DNA is always transferred from a Donor to Recipient cell. • An F+ cell will always transfer DNA to an F- cell and the end result will always be two F+ cells. • The F Factor (fertility plasmid) has all the necessary genes to code for protein required for conjugation, it most importantly codes for: • Pilin: Forms the Sex Pilus

  6. Conjugation - Mating • Conjugation begins when the Sex Pilus of the F+ cell attaches to a receptor on the F- cell. After which the cells are drawn together and put into direct contact by the sex pilus. • The multi-protein complex called the Relaxosome enzymatically cleaves the DNA of the F+ cell. The strand will travel to the recipient cell down the conjugation tube in a 5’ to 3’ fashion. • Afterwards, the strand will replicate within the F- cell. The DNA will integrate itself into the new cell, producing two F+ cells.

  7. Conjugation – Hfr Cells • Hfr (High Frequency Recombination) cells. Some cells (Hfr cells) have their F factor directly integrated into the bacterial DNA, as a result F+ cells gain the ability to transfer parts of the bacterial chromosome. • During conjugation the first part of the F factor is transferred, followed by a piece of bacterial DNA, and finally the last part of the F factor is transferred. To transmit an entire bacterial chromosome is 100 mins.

  8. Transduction • Transduction describes the process in which Bacterial DNA is transferred via a bacterial virus (phage or bacteriophage). • Transduction can occur via two cycles, the Lysogenic and the Lytic cycle. • The Lysogenic cycle is when viral DNA becomes integrated but no progeny phages are produced, and there is no destruction of the host cell. • Lysogenic Conversion: Phage DNA becomes integrated into host DNA and host gains a new trait. • The Lytic cycle is when progeny phages are produced and there is destruction of the host cell.

  9. Transduction • During Transduction, a bacteriophage will attach to a bacteria and inject its phage DNA into the cell. • The Phage DNA will replicated and the host DNA will fragment. • Progeny phages assemble, some containing phage DNA and some mistakenly have bacterial DNA. • The bacterial cell fragments and the new phages DNA will be injected into different bacteria. • These phages inject their DNA which is integrated into the host.

  10. Transduction • Transduction can be divided into two different types. Generalized and Specialized. • Generalized occurs when the phage contains any type of bacterial chromosome. • Specialized occurs when only specific host sequence is transferred.

  11. Transformation • Transformation is defined as a genetic change brought about by the uptake of exogenous DNA. • As a result, Bacteria can change both their genotype and phenotype, allowing some Bacteria to gain new traits. • This was proved by Griffiths Experiment, which proved that nonvirulent strains of Strep. Pneumoniae could become virulent via the process of Transformation.

  12. Pulled from Lange Review of Medical Microbiology & Immunology

  13. Recombination • After DNA has been transferred from the Donor to the Recipient cell, it can be integrated into the host DNA by a process called recombination. • The two types of recombination are Homologous and Nonhomologous recombination. • Homologous Recombination is when there are extensive homologous regions that pair up and integrate. • Nonhomologous Recombination occurs when little to no homology is necessary for integration.

  14. References • Lange Review of Medical Microbiology & Immunology, Twelfth Edition • Medical Microbiology Made Ridiculously Simple • Dr. Baby’s PM3 Microbiology Lecture Notes

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