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Lecture 35
Brock
40
Microbiology
Undergraduate 3
04/24/2014

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Term
mutation
Definition
heritable change in the DNA sequence of an organism's genome
Term
mutant
Definition
strain of any cell or virus that differs from its parental strain in genotype
Term
phenotype
Definition
observable properties of the mutant
Term
wild-type strain
Definition
that which was isolated from nature

can refer to an entire organism or a particular gene
Term
selection
Definition
powerful genetic tool to isolate mutants from a large population via screening
tests to see if mutagen leads to increased reversions
Term
replica plating
Definition
useful for identifying cells with a nutritional requirement for growth
Term
auxotrophs
Definition
cells that have a nutritional requirement for growth
Term
induced mutations
Definition
mutations due to agents in the environment, including mutations made deliberately by humans and from chemical or radiation exposure
Term
spontaneous mutations
Definition
mutations that occur without external intervention, largely result from occasional errors during DNA replication
Term
point mutations
Definition
change only a single base pair, caused by base-pair substitutions or by the loss or gain of a single base pair
Term
silent mutation
Definition
mutation that does not affect the amino acid or the phenotype of the cell, often in the wobble position
Term
missense mutation
Definition
mutation often in the 1st or 2nd base of a codon, that changes the amino acid and alters the protein
Term
nonsense mutation
Definition
changes a codon to a stop codon preventing the entire protein from being synthesized
Term
transition
Definition
mutation where one purine is substituted for the other purine or pyrimidine is substituted for other pyrimidine
Term
transversion
Definition
mutation where a purine base is substituted for a pyrimidine base or vice versa
Term
frameshift mutations
Definition
deletions or insertions that result in a shift in the reading frame
Term
insertion sequence
Definition
transposable elements usually 700-1400 bp in length
Term
reversion or back mutation
Definition
point mutation being changed back to its original form
Term
revertants
Definition
strains in which the original phenotype is restored from a point mutation
Term
same-site revertant
Definition
mutation that restores the phenotype in the same location as the original mutation (point-mutation)
Term
second-site revertant
Definition
mutation is at a different site in the DNA than original mutation
functions as a suppressor mutation and compensates for the effect of the original mutation
Term
mutagens
Definition
chemical. physical, or biological agents that increase the mutation rate
Term
nucleotide base analogs
Definition
resemble purine and pyrimidine bases yet display faulty pairing
Term
alkylating agents
Definition
powerful mutagens that induce mutations more often that base analogs
Term
intercalating agents
Definition
include the acridines, inserted between 2 bases and push them apart, leading to single insertions or deletions
Term
nonionizing
Definition
type of mutagenic electromagnetic radiation that includes UV
purine and pyrimidine bases strongly absorb radiation, producing direct damage to DNA such as pyrimidine dimers
Term
ionizing
Definition
type of mutagenic electromagnetic radiation that includes X-rays, cosmic rays, and gamma rays
produces free radicals that damage macromolecules in cells and can fragment DNA
Term
thymine dimer
Definition
mutation caused by UV light with two T bases connected
Term
direct reversal
Definition
bases that although altered can still be identified and can be repaired without referring to the other strand
DNA repair process
Term
repair of single strand damage
Definition
DNA repair process
damaged DNA is removed and repaired using the opposite strand as the template
Term
repair of double strand damage
Definition
a break in the DNA repaired by recombinatorial mechanisms and often needs more error-prone repair mechanisms
DNA repair process
Term
error-prone methods
Definition
part of SOS system that allows replication to proceed (survival) but organism has many mutations
Term
lesions
Definition
stall DNA replication which can be fatal
Term
stalled replication or other major DNA damage
Definition
what activates the SOS repair system?
Term
damage tolerance
Definition
lesions remain but are bypassed by special DNA polymerases that perform translesion synthesis
Term
SOS system
Definition
regulon that is regulated by two proteins, LexA and RecA
Term
LexA
Definition
a repressor that normally prevents SOS regulon expression
Term
RecA
Definition
activated by presence of DNA damage, stimulates LexA to inactivate itself by self-cleavage, allowing expression of SOS regulon
Term
mutator strains
Definition
organisms that benefit from increased mutation rates in rapidly evolving environments
Term
Ames test
Definition
permits screening of chemicals to determine which may serve as mutagens
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