Chapter 8 Flashcards
Advantages of using bacteria and viruses for genetic studies
- reproduction is rapid
- many progeny are produced
- the haploid genome allows all mutations to be expressed directly
- asexual reproduction simplifies the isolation of genetically pure strains
- growth in the laboratory is easy and requires little space
- genomes are small
- techniques are available for isolating and manipulating the genes
- they have medical importance
- they can be genetically engineered to produce substances of commercial value
What bacteria is known as the workhorse
- Escherichia coli
Prototroph
- wild type bacteria that can synthesize all compounded needed for growth from simple ingredients
Auxotroph
- mutant strain that lacks one or more enzymes required for metabolizing nutrients
- will grow on supplemented media
Plasmids are ______ DNAs
- extrachromosomal
- replicates independently of chromosome
________ found that genetic exchange requires direct contact between bacterial cells by mixing two auxotrophic strains and observing that the bacteria could not pass through a filter
- Lederberg and Tatum
How did the genetic exchange in Lederberg and Tatums experiment take place?
- the filter did not allow bacteria to pass through
- this drew them to their conclusion that genetic exchange requires direct contact between bacterial cells
- generalized transduction experiment
Conjugation
- can transfer F factor
- direct transfer of DNA from one bacterium to another (donor and recipient only)
- one strand on DNA on F factor is nicked, replication occurs replacing the nicked strand
Hfr cells (high frequency) role in conjugation
- F is integrated into the bacterial chromosome
- Hfr cells behave like F+ forming pili and conjugating with F-
- because F factor is in the chromosome some of chromosome is pulled into the recipient cell (F factor plus some bacterial genes results)
- remains F- bc does not get whole F factor
- amount depends on length of time of conjugation
F+ cells vs F- cells
- F+ cells are donor bacterium
- F- cells are recipient bacterium
F’ cells
- cells with F plasmid that contain some bacterial genes
- acts as a donor
Merozygote
- bacteria that is now partially diploid
- the F factor is excised from Hfr- and some genomic genes on the episome
Results of conjugation between cells with different F factors
- F+ x F- = two F+ cells
- Hfr x F- = one Hfr cell and one F- (no change)
- F’ x F- = two F’ cells
_______ found that gene mapping was interrupted by conjugation
- Francois Jacob and Elie Wollman
Gene transfer differs in bacteria than eukaryotes because
- DNA exchange and reproduction are NOT coupled in bacteria
2. Cell remains haploid
Transformation
- DNA take up the medium
Transduction
- Bacterial viruses (Phages) carry DNA from one bacterium to another
Steps in gene transfer of bacterium and ways to map
- Interrupted Conjugation
- Transformation
- Transduction
What is used to map in gene transfer?
- the transfer times indicate the order and relative distances between genes and can be used to construct a map
How do the different Hfr strains affect the F factor in mapping?
- different Hfr strains have the F factor integrated at different sites and in different orientations
Bacterial transformation
- DNA is external and then taken into the cell
- the DNA is taken up from surrounding and incorporated into the genome
- can occur naturally, DNA from dead cells in the environment
Competent cells
- cells that can take up DNA
What is a major key to modern molecular biology?
- introduction of artificial plasmids
Gene mapping using transformation
- rate of cotransformation is inversely proportional to distances between genes
aka the faster the further apart
Virsuses
- all organisms are infected by them
- nucleic acid coated in proteins
- linear or circular genomes
Bacteriophages (phage)
- have two alternative life cycles
Virulent Phage
- reproduce only through lytic cycle
Temperate Phage
- use either lytic cycle or lysogenic cycle
Transducing Phage
- requires that the phage degrade bacteria chromosome
- packaging DNA into phage can not be specific for phage DNA
- recombination must occur within recipient chromosome
Mapping genes using generalized transduction
- rate of cotransduction is inversely proportional to distances between genes
Lytic Cycle
- Lysis the cell to spread its viral DNA
Lysogenic Cycle
- Becomes a part of the plasmid for a cell and duplicates with the newly introduced virus
Plaque
- Where a colony used to be but was killed off by a virus (clear areas)
_________ studied phage recombination using the T2 bacteriophage
- Alfred Hershey and Raquel Rotman
Alfred Hershey and Raquel Rotman conlcusions
- one strain could infect type B E.coli but not type B/2 (h+)
- also produced abnormal plaques (r-)
- allowed for the genotypes to be calculated
Mixed infection experiment
- Allowed both viruses to infect a cell then observed the results
