Adaptation vs. Variation Controversy: Bacterial Mutations
used to be thought that bacteria adapted due to environmental pressure, not genetic mutation
bacteria are haploid
Þ mutations show up immediately
proliferation is extremely rapid
Þ mutations can quickly become the dominant form, especially if they are environmentally favored (this gives the appearance of environmentally directed genetic change rather than random chance and selection)
bacteria alter gene function much more than higher organisms; phenotype and genotype alterations are often confused
several experiments proved that adaptation was due to mutation followed by selection
Luira Delbruck experiment
Lederberg experiment: streptomycin resistance; mutations arise spontaneously at a finite rate; proven by isolating a resistant strain from a sensitive population (see figure)
Types of Mutations:
prototroph
: wild-type; has ‘normal’ function
resistance mutation
: change susceptibility to toxin or antibiotic
auxotrophs
: affect production of an enzyme required for normal metabolism
no longer manufacture a nutrient it needs Þ must get from environment (i.e. his-; leu-)
mutants that cannot use something wild-type can: (i.e. lac-; mal-; glu-)
lethal mutations
: cannot compensate for loss of gene product Þ growth stops
conditional mutations
: mutated gene expressed depending on circumstances
leads to bugs that my only be able to grow in high temp, low temp, specific pH, etc)
mutants with defects in nonvital and vital processes
Bacterial Genetic Material
chromosome
accessory genetic elements: plasmids; phage (lysogens); transposons and insertional elements (pieces of DNA that insert into genome and move around)
consequences of location
operon
model (requires simultaneous transcription and translation)
one operon (transcription unit) contains many cistrons (unit of protein expression; genes)
all cistrons of an operon are normally transcribed together
regulon
: selected genes on different operons can be controlled together and independent of their operons
may be in response to an environmental change
a common regulator accomplishes this
the group of genes controlled by the regulator is called a regulon
