Lecture 7 Flashcards
What is a bacteriophage?
Viruses that parasitize bacteria
What makes bacteria a good model organism?
They grow quickly (fast dividing) in huge number, take up little space, and allow us to see rare events
How does heredity work in bacteria?
Fundamentally the same as eukaryotes: DNA codes for proteins via RNA, a copy of DNA is passed on to the daughter cell.
- Haploid
- Reproduce asexually
- May exchange DNA with other organisms by accident through transformation, transduction, or conjugation
- Evolution is through mutation and other “accidents”
What is spread plating?
A stock of bacterial cells can be grown in liquid medium containing nutrients, then a small number of bacteria from the liquid suspension can be spread on a solid agar medium. Each cell will give rise to a colony of cells that have the same genotype and phenotype. When the cells reach more than 10^7 they are visible as a colony.
What is a prototroph?
Wild type bacteria that can grow on minimal medium (inorganic salts, carbon source for energy, and water). It has the functional enzymes necessary to make all needed macromolecules.
What is a auxotroph?
Mutant bacteria from prototroph, they are unable to grow on minimal medium. They require supplementation with product the mutant cannot synthesize for itself in order to grow.
What are resistant mutants?
Mutant bacteria that can divide and form colonies in the presence of an inhibitor whereas the wild type bacteria would be susceptible to the inhibitor.
What is transformation?
Direct uptake of DNA from the environment generally from dead or lysed cells. Unless this DNA has a origin of replication (ori), it must recombine into the chromosome. The transforming DNA is incorporated into the bacterial chromosome by a process analogous to the double-recombination events observed in Hfr × F−
crosses.
How can double transformation be used for mapping genes?
Genes located close together may be carried on the same piece of transforming DNA. When the DNA is taken up it causes double transformation. If genes are linked, then the proportion of double transformants will be greater than the product of single-transformant frequencies.
How was conjugation discovered?
Lederberg and Tatum demonstrated that genetic recombination between bacterial genotypes is possible. The basic concept: two auxotrophic cultures (A− and B−) are mixed, yielding prototrophic wild types (WT). Cells of type A− or type B− cannot grow on an unsupplemented minimal medium because A− and B− each carry mutations that cause the inability to synthesize constituents needed for cell growth. When A− and B− are mixed for a few hours and then plated, however, a few colonies appear on the agar plate. These colonies derive from single cells in which genetic material has been exchanged; they are therefore capable of synthesizing all the required constituents of metabolism.
What is conjugation?
Formation of pilus between donor cell containing sex factor and recipient cell. Donor replicates its plasmid and donates copy to recipient. Transfer is unidirectional. Plasmid contains sex factor. If F factor is integrated into chromosome, the entire chromosome will attempt to transferred to the recipient.
What is cross-feeding? U-tube experiment?
Cross-feeding is the idea that strains don’t actually exchange genes but instead leak substances that the other cells can absorb and use to grow. It was disproved by the U-tube experiment where filter separated A- and B- mutants so that no contact was possible but diffusion of molecules was. After incubation and plating, no protrophic cells grew. This proved that cell to cell contact is necessary.
What did Hayes discover how about the sharing of genetic material?
Sharing is unequal. One strain donates it alleles to convert the other. Donors are F+ which contain the fertility factor. F- are converted to F+ and can then pass on the fertility factor themselves. F+ is immune to superinfection (can’t put a second F plasmid into F+).
What are some important properties of the F factor?
F factor is a plasmid that can replicate in the cytoplasm independent of the host chromosome.
- Directs the synthesis of sex pilus
- Codes for pilus proteins, proteins that mobilize plasmid into new cells
- Has genes that inhibit superinfection
- contains oriV for plasmid replication and oriT for conjugative transfer
- F+ has pili that stick out from surface, when a pilus contacts a F- cell, it contracts and proteins facilitate fusion of membranes
How is the F factor transferred?
Rolling circle replication: A single-stranded copy of plasmid DNA is produced in the donor cell and then passes into the recipient bacterium, where the single strand, serving as a template, is converted into the double-stranded helix.
