Unit Three : (CH 9) Bacterial Genomes - Horizontal Gene Transfer (HGT) Flashcards
what is horizontal gene transfer?
a process in which an organism transfer genetic materials to another coexisting organism
explain how horizontal gene transfer differs from vertical gene transfer
horizontal gene transfer (hgt) transfer between organisms within the same generation, whereas vertical gene transfer (vgt) transfer to genetic material from one organism to its offspring
why do bacteria hgt? (how does it benefit them?)
horizontal gene transfer enables bacteria to respond and adapt to their environment much more rapidly by acquiring large dna sequences from another bacterium in a single transfer
describe fredrick griffith’s experiments that discovered bacterial transformation
- griffith experimented with two strains of s. pneumoniae: smooth bacteria (virulent) & rough bacteria (nonvirulent)
- when injected into a live mouse, the smooth bacteria proliferated within the mouse’s bloodstream and killed the mouse whereas when the rough bacteria were injected into a mouse, the mouse lives
- to verify that the proliferation of the smooth bacteria was causing the death of the mouse, griffith killed the smooth bacteria with heat treatment before injecting them into the mouse. in this case, the mouse also survived
- when streaked onto petri plates containing a solid growth medium, capsule-secreting strains have a smooth colony morphology, whereas those strains unable to secrete a capsule have a rough appearance. the different forms of S. pneumoniae also affect their virulence, or ability to cause disease
- when smooth strains of S. pneumoniae infect a mouse, the capsule allows the bacteria to escape attack by the mouse’s immune system. as a result, the bacteria can grow and eventually kill the mouse. in contrast, the nonencapsulated (rough) bacteria are destroyed by the animal’s immune system
describe the experiments by avery, mccarty, & macleod. what did they conclude about the “factor” responsible for transformation?
the thee scientists that provided additional evidence that DNA was the transfoming factor. used type s extract exposed to various agents to see if it would still transform the type r bacterial cell
describe the steps of transformation
- uptake of dna from the environment
- is a horizontal gene transfer that occurs when a bacterial cell uptakes dna from a lysed bacterial cell and integrates this new dna into its chromosome
- outline of steps
1. dna fragment binds to a cell surface receptor of a competent bacterium
- an extracellular endonuclease cuts the dna into smaller fragments
- one strand is degraded and a single strand is transported into the cell via an uptake system
- the dna strand aligns with a homologous region on the bacterial chromosome
- the dna strand s incorporated into the bacterial chromosome via homologous recombination
- the heteroduplex dna is repaired in a way that changes the lysis- strand to create a lys+ gene
what is the competent cell? when or why does a cell become competent?
- term used to describe bacterial cells that can uptake dna via transformation
- several factors affect competency including temperature, iconic conditions, and nutritional growth conditions
what is (are) source of dna in extra-ceullualr environment?
how can a cell be “forced” to become competent in a laboratory setting?
- being chemically treated with calcium chloride (Cacl2) and heat shock, which alters the cell membrane
- using brief electrical pulses (electroporation) to shoot dna across the membrane
describe the steps of conjugation
- direct physical contact between bacteria required to transfer
- conjugation is a dna transfer process mediated by a transferable plasmid that requires cell-to-cell contact and formation of a protein complex between mating cells
- in bacteria, pili draw together the donor and recipient cell envelopes, and a protein complex transmits dna across
- in ciliated eukaryotes, a conjugation bridge forms between two cells, connecting their cytoplasm and allowing the exchange of micronuclei
what is an f factor plasmid? what is an r plasmid? explain what they have in common and what is different
what “mating types” are necessary for conjugation?
- f+ cell - the dna dno
what is a sex pilus? which cell produces this structure?
explain what a plasmid is and how plasmids differ from the bacterial chromosome
what is an hfr cell?
a bacterial call capable of high-frequency recombination caused by the presence of a chromosomally integrated f factor
what is a bacteriophage?
a virus that infects bacteria (aka phage)
what are the basic parts/structure of a bacteriophage?
describe the steps of transduction
- viral-mediated transfer of genetic material
- transduction, a process of genetic recombination in bacteria in which genes from a host (a bacterium) are incorporated into the genome of a bacterial virus (bacteriophage) and then carried to another host cell when the bacteriophage initiates another cycle of infection
- in general transduction, a phage preparation can move any gene in a bacterial genome to another bacterium
- in specialized transduction, a phage can only move a limited number of bacterial genes
what is generalized transduction vs specialized transduction?
generalized transduction is a transduction in which any bacterial donor gene can be transferred to a recipient cell
specialized transduction is a trandsuction in which the phage can transfer only a specific, limited number of bacterial donor genes to the recipient cell
what is a prophage?
a phage genome that has integrated into a bacterial host genome
what defense do bacterial cells have against invading viral dna?
restriction endonuclease - which protects bacteria from invasion by foreign dna
what is homologous recombination, and what are the basic steps involved?
refers to the recombination between two dna molecules that share long regions of homology (aka generalized recombination)
what is the role of reca enzyme?
what happens to dna that is not integrated?
