Microbiology - Microbial Genetics - Quick Guides

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Study GuideMicrobiology–Microbial Genetics1.Bacterial RecombinationBacterial recombination is the process by which bacteria change their genetic material. When DNAchanges, bacteria may gain new abilities, such as resistance to antibiotics or the ability to causedisease. These changes happen when new genes enter a bacterial cell and are expressed asproteins.There arethree main types of bacterial recombination:1.Conjugation2.Transformation3.Transduction

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Study Guide1.1 Bacterial ConjugationBacterial conjugationis a process in which DNA is transferred directly from one bacterial cell toanother through physical contact. This process was first described in the 1940s byJoshuaLederberg and Edward Tatum.How conjugation works•One bacterium acts as thedonor (F⁺cell)and the other as therecipient (F⁻cell).•The donor cell contains a small circular piece of DNA called theF factor (fertility factor).•A special structure called thesex pilusforms a connection between the two cells.•Through this connection, acopy of the F factor DNAis transferred to the recipient cell.•After transfer, the recipient becomes anF⁺cell.This DNA transfer isone-way (nonreciprocal)—only the donor gives DNA.High-Frequency Recombination (Hfr)Sometimes, the F factor becomes part of the bacterial chromosome. When this happens:•The bacterium is called anHfr (high-frequency recombination) cell.•During conjugation,chromosomal genes(not just the F factor) are transferred to therecipient.•Usually, the entire chromosome is not transferred, so the recipient remainsF⁻.•Only if theentire chromosomeis transferred (a rare event) does the recipient becomeF⁺.F′ Plasmids and Sexduction•Occasionally, when the F factor leaves the chromosome, it carriesextra bacterial geneswithit.•This modified plasmid is called anF′ plasmid.•When transferred, it gives the recipient extra genes.•This process is calledsexductionand helps spread useful traits in bacterial populations.

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Study Guide1.2 Bacterial TransformationBacterial transformationoccurs when a bacterium takes upfree DNA from its surroundings.Discovery•First discovered in1928 by Frederick Griffithwhile studying pneumonia-causing bacteria.•He found that harmless bacteria could become harmful after taking DNA from deadpathogenic bacteria.How transformation works•When bacteria die, they release DNA into the environment.•Some bacteria arecompetent, meaning they can take up this DNA.•Competence usually occurs during thelogarithmic (log) growth phase.•The bacterium takes in the DNA, destroys one strand, and integrates the other strand into itschromosome.•The new DNA replaces a similar section of the original DNA.Transformation has been studied in bacteria such as:•Streptococcus pneumoniae•Haemophilus influenzaeIn laboratories, transformation can be encouraged usingheat and calcium chloride, which increasemembrane permeability.1.3 Bacterial TransductionTransductionis the transfer of bacterial DNA from one cell to another usingbacterial viruses, calledbacteriophages(or phages).What is a bacteriophage?•A virus that infects bacteria•Contains DNA surrounded by a protein coat

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Study GuideThe Lytic and Lysogenic CyclesOnce a phage enters a bacterium, two outcomes are possible:Lytic cycle•Phage DNA takes over the bacterial machinery•New phages are produced•The bacterial cellbursts (lysis)and releases phagesLysogenic cycle•Phage DNA integrates into the bacterial chromosome•The virus becomes aprophage•The bacterium survives and reproduces normally•The phage may later exit and enter the lytic cycleGeneralized TransductionIngeneralized transduction:•During phage assembly,random pieces of bacterial DNAare mistakenly packaged intophages.•These phages infect another bacterium.•The donor bacterial DNA becomes integrated into the new host’s chromosome.•The recipient bacterium now expressesnew genetic traits.

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