Unit 6 - Bacterial Genetics

Broad scale gene mapping in bacteria

- Conjugation with interrupted mating (HfrxF-) --- mapping by time of gene transfer - co transformation or cotransduction of genes --- genes that are close to each other often get transferred together --- mapping based on frequency of co-transfer

What is the difference between Hfr and F+ Strains?

- Hfr: F factor is integrated into the chromosome of ALL cells prior to the mating exception - F+ strain: F factor is not integrated into chromosome prior to the mating exception, but in a small fraction of the cells in the culture, the F factor becomes integrated. Converts only a FEW cells from F+ to Hfr, resulting in low frequency transfer of chromosomal genes

Gene mapping by co-transformation or co-transduction

- based on frequency of co-transformation or co-transduction - used to determine map order - if genes are close, they can be acquired at the same time. if they are far apart, they usually won't be acquired together

Mapping by interrupted mating

- cross of Hfr and F- strains - transfer of DNA is initiated at Ori within the F- factor. DNA is nicked (ss break) at this point - site defines direction of transfer - different Hfr strains differ in chromosome location and orientation of F-factor, which determines the direction of transfer - genes closest to F factor are transferred first during conjugation - different genes get transferred in order at different times after initiation of conjugation

Gene mapping by conjugation

- longer times of mating needed to transfer genes farther from origin - genes farther from origin have lower frequency of transfer - distance between genes given in minutes - not useful for determining order of very close genes

Hfr x F-

- transfer begins with a single stranded nick in the F factor. Part of F factor begins to enter recipient and chromosomal genes of donor follow. - transfer is usually incomplete - recipient does not get entire bacterial chromosome or all of the F factor, so recipients stay F-

What are the four ways bacterial DNA can be transferred from cell to cell?

1. Conjugation with plasmid transfer 2. Conjugation with partial genome transfer 3. Transformation 4. Transduction

Principles for mapping genes by co-transformation or co-transduction

1. plate cells on medium that selects for particular class of recombinants and then use some other medium to see whether co-transformation or co-transduction occurred 2. use lowest frequency of co-transfer to determine which loci are farthest apart 3. use highest frequency of co-transfer to determine which loci are closest together

Transformation

Bacterial cell takes up a piece of DNA from the external environment and incorporate this DNA into its own chromosome - used to measure how closely two genes are linked on a bacterial chromosome

Transduction

Bacterial viruses (bacteriophage) transfer genes from one bacterium into another

Gene Transfer to Recipient Cell Involves Recombination

Conjugation, transformation, and transduction involve transfer of a linear piece of DNA to recipient which has to recombine with the circular chromosomal DNA - requires an even number of cross overs to generate a viable recombinant chromosome

F+

Donor cell - has the F factor (fertility factor plasmid) and can transfer it to recipient strains. Can transfer chromosomal DNA at a low frequency

Hfr

Donor cell - has the F factor and transfers chromosomal DNA at a high frequency to recipient strains - F factor is integrated into bacterial chromosome rather than as an independent plasmid

F+ x F-

F factor has genes for making pilus and initiating transfer. Entire F plasmid gets copied and transferred to recipient during conjugation. Plasmid DNA is replicated during transfer, so that donor keeps a complete copy and recipient gets a complete copy. F- becomes F+

F' Factors

F factors that have acquired a little bit of chromosomal DNA - occurs via F factor leaving chromosome of an Hfr strain - cell that contains an F' factor is a partial diploid

Prophage

Phage integrated into bacterial genome

F-

Recipient cell - does not have the F factor but can received it from F+ donor cell. It can mate with Hfr strains and receive chromosomal genes

Temperate phages

Remain within host cell for period of time without killing it. Undergoes lysogenic pathway.

To produce a viable recombinant between the circular genome of an F- cell (the endogenote) and the linear segment of DNA (exogenote) donated form an Hfr cell, what must occur? a) a double crossover between the endogenote and the exogenote b) a single crossover between the endogenote and the exogenote c) selection between the endogenote and the exogenote d) phage progeny must be created

a

Typically, in a cross of an F+ cell with an F- cell, the: a) F- cell becomes F+ b) F+ cell becomes F- c) F+ cell becomes Hfr d) F- cell becomes Hfr

a

Which of the following is TRUE for an Hfr strain? a) The F factor has integrated into the bacterial chromosome in Hfr strains b) The strain transfers F factors to F- cells at high frequency c) An Hfr strain is a very common derivative of an F+ strain d) in a cross of Hfr x F-, the F- strain always becomes F+

a

Plaques

areas where phage infected and killed (lysed) bacteria

The main bacterial model species for bacterial genetics is a) Bacillus coli b) Escherichia coli c) Saccharomyces cerevisiae d) Bacteriophage lambda

b

You are given a strain of bacteria and told it cannot grow on minimal medium unless the medium is supplemented with the nutrient arginine. This is an example of a/an _____ strain. a) minerotrophic b) auxotrophic c) prototrophic d) wild type

b

Lytic pathway

bacteria are lysed and destroyed immediately after phage replication. Host DNA is not broken down. Phage inject their DNA into the bacterial cell, which then replicates in the cytoplasm and is not incorporated into the bacterial genome.

Bacterial strains that carry a cytoplasmic plasmid known as the F (fertility) factor are also referred to as _____ strains. a) diploid b) recipient c) donor d) exconjugant

c

In the process of bacterial conjugation, two bacterial cells: a) fuse together to become a single diploid cell b) undergo reciprocal exchange of chromosomal DNA c) temporarily make physical contact via a structure called a pilus d) always become resistant to antibiotics

c

Two bacterial strains are mixed together: F+ met+ arg+ str^s and F- met- arg - str^r. What medium should you plate the cell mixture on to isolate met+ arg+ str^r recombinants from the parent cells? (met = methionine, arg = arginine, str = streptomycin antibiotic) a) medium containing all three chemicals b) medium lacking all three chemicals c) medium lacking methionine and arginine and containing streptomycin d) medium with methionine and arginine and lacking streptomycin

c

All of the following are general characteristics of most bacteria EXCEPT: a) they can transfer DNA via horizontal transmission b) they may contain extra-chromosomal DNA called plasmids c) they are almost always haploid d) they cannot undergo any form of genetic recombination

d

The process by which genes can be transferred between bacterial cells is called a) transformation b) conjugation c) transduction d) all of the above

d

Lysogenic pathway

does not result in immediate lysing of cell. Phage injects its DNA into cell, where it is incorporated into bacterial chromosome. Bacteria itself will undergo multiple divisions. The start of the lytic cycle begins with the excision of the phage DNA from the bacterial chromosome. The phage DNA replicates, forms phages, and lyses the cell

Which of the following are reasons that microorganisms and their viruses are popular subjects of genetics research? a) they are excellent models for the genetic processes of all organisms b) they can be easily cultured in very large numbers c) they allow the detection and study of very rare genetic events d) their basic biology is interesting and important e) all of the above

e

Generalized transduction

generalized transducing phages carry any part of the bacterial chromosome

Bacteriophage

have a single molecule of DNA or RNA surrounded by a protein coat. viruses that parasitize bacteria. phage takes over and destroys host cell

Concatemeric DNA

multiple copies of same DNA sequence linked in series - concatemers get processed into lengths that fit into phage head. Phage takes a headful of DNA; no more, no less - amount packaged is slightly more than a-z lenght

Specialized transduction

specialized transducing phages carry only certain specific parts of bacterial chromosome

Conjugation

the contact and fusion of two different bacterial cells. After fusion, one cell (donor) sometimes transfers DNA to another cell

Virulent phages

those that immediately lyse and kill the host. Undergo lytic cycle. Cannot become prophages

R Plasmids

vectors carrying multiple resistances. Transferred rapidly on cell conjugation