Bacterial Genetics Flashcards
benefits to using bacteria for genetic studies
- rapid reproduction
- many progeny
- haploid
- asexual reproduction
- easy to grow in lab
- small genome
- able to isolate/manipulate genes
- medically important
- can be genetically engineered to produce substances of commercial value
prototroph
WT
can grow on minimal media
auxotroph
Mutant
- require additional nutrients from the standard media as they cannot produce them
- some cannot metabolize certain carbon sources
- some cannot produce amino acids, vitamins, other nutrients
replica plating
Joshua Lederberg
- used to make a copy of the bacteria growing on a plate
- helpful to transfer specific colonies to multiple types of media
mechanism for gene transfer in bacteria
- conjugation
- transformation
- transduction
Lederberg and Tatum
- mixed to auxotrphic strains on complete media then transferred them to minimal media
- found some prototrophic strains
- thought this was not due to mutations but must be from gene transfer
Davis’s U-tube experiment
- do cells need to touch for gene transfer?
- U tube with filter with one strain on each side
- a pump moved media back and forth through the filter but the whole cells could not get through
- no phototrophs resulted
therefore, cells must touch for this type of gene transfer to occur
conjugation
temporary fusion of 2 single-cell organisms for the sexual exchange of genetic material
cell types in conjugation
F+ - contain episome called F factor and is the donor cell; have extensions called pili that contact the other cell
F- - the receipient cell
describe conjugation
- a pili extends from F+ cell to make contact with F-
- a conjugation tube is formed connecting the cells
- the F factor is nicked and begins to transfer 1 strand to F- cell
- DNA replication occurs so that both cells end up with a double stranded F factor
- now both cells are F+
Hfr
High frequency recombination
cells that transfer genes from bacterial chromosomes at high frequency due to an integrated F factor
results from crossing over between the F factor and the bacterial chromosome in an F + cell
F+ x F- mating
- no bacterial chromosome transferred
- only F factor is transferred
Hfr x F- mating
- Hfr is still donor
- rarely transfers the entire F factor
- transfers some bacterial chromosome
- the F- will have some genetic change due to recombination between new DNA and its chromosome but it will not be a F+ cell
Interrupted Mating Experiments
- used to map bacterial chromosomes
- different Hfr strains integrate in different places and begin transfer at different places and in different directions
- stop mating at different times and see which have transferred
- this information can be used to deduce the map of the chromosome
F’ cell
- have some genes from the bacterial chromosome on the F factor
- orginate from an Hfr cell that had F factor pop out of the bacterial chromosome taking some bacterial chromosome with it
- acts as donor cell in conjucation
F’ + F-
- allows full transfer of the F factor plus some bacterial chromosome
- recipient has two bacterial gene copies
F’ merozygote
- resulting recipient cell from a F’+F- transfer
- partial diploid
- contains two copies of bacterial genes that were on the F’ cell’s factor
F+
F factor characteristics
Role in conjugation
present as separate circular plasmid
Donor
F-
F factor characteristics
Role in conjugation
absent
recipient
Hfr
F factor characteristics
Role in conjugation
present, integrated into bacterial chromosome
high-frequency donor
F’
F factor characteristics
Role in conjugation
present as separate circular plasmid carrying some bacterial genes
donor
Results of
F+xF-
two F+ cells
Results of
Hfr x F-
One Hfr and one cell that may have altered bacterial chromosome genes but will almost never become an F+
Results of
F’ x F-
one F’ cell and one F’ merozygote
Transformation
exogenous DNA transfers genes to competent bacterial cell and brings about heritable change in the cell
competent cell
cell is in proper state to take up DNA
Heteroduplex DNA
different alleles on the 2 strand
describe transformation
- single strand of DNA extrs and is recombined into bacterial chromosome by two crossover events
- now has two strands with different alleles
- the two strands separate during replication and each replicates to form perfect partner strand
- one goes into each daughter cell
- this forms one a+ and one a- daughter cell
- only the a+ is transformed
cotransformation
- used to determine if genes are located near each other
- only those close together will cotransform - the closer together they are, the more frequently they will
transduction
- viral mediated gene transfer
virulant
- goes through the lytic cycle
lytic cycle
phage infects, takes over, replicates, and lyses cell
temperate
- infects cell and becomes part of its chromosome
- can then reproduce into multiple daughter cells and not impact the cells until ready to enter lytic cycle
plaques
cleared areas in a plate covered in bacteria where the cells have been lysed
get bigger as more cells are infected
lysogenic
- phage enters the DNA of the bacterial chromosome without doing harm
- is passed down to daughter cells as they divide
- can eventually jump out of the chromosome, entering the lytic cycle
prophage
integrate phage DNA in the lysogenic cycle
general transduction
- occurs through the lytic cycle of phage infection
- bacterial chromosome DNA is degraded into pieces
- the virus pick us bacterial DNA instead of viral
- it then infects another cell, injecting bacterial DNA into the cell
- recombination of this DNA into the recipient’s chromosome can alter the genotype
- random bacterial chromosome gene transfer
transducing phage
bacteriophage that has picked up some of the degraded bacterial DNA
What type of transduction is useful in gene mapping. Why?
general transduction
if 2 genes co-transduce, they are probably close together on the bacterial chromosome
specialized transduction
- prophage pops out of bacterial chromosome and take one or a few genes with it
- the progeny virus transfer these genes when they infect other cells
Does conjugation require cell contact
yes
Is conjugation sensitive to DNase
no
Does transformation require cell contact
No
Is conjugation sensitive to DNase
yes
Does transduction require cell contact?
no
Is transduction sensitive to DNase?
no
DNase
enzyme that degrades DNA
cis
two mutation on the same chromosome
trans
two mutations on different chromosomes
complementation
the production of a wild-type phenotype when two mutant types are combined in the trans configuration
cistron
region of DNA in which two mutation cannot compliment each other in trans configuration
functional definition of a gene
What will restore the normal phenotype
crossing over between two point mutations
crossing over between two deletions
crossing over between a point mutation and a deletion mutation within the same gene
all of the above assume that the recombinant type has a normal sequence along the entire gene length
When can two point mutations restore normal phenotype?
if they are at different positions within the gene
When can point mutation and a deletion restore normal phenotype?
when the point mutation is not located within the deletion
When can two deletions restore normal phenotype?
if they do not overlap
