Genetics PPT 2 Flashcards
Bacteria use three different mechanisms to adapt to
changing environments
Natural Selection
Mutation
Gene transfer
A genetic change alters an organism’s
genotype
a genetic change in bacteria can have a profound impact on bacteria because they are
haploid
A change in genotype can easily alter the observable
characteristics of an organism
phenotype
stable heritable changes in the base
sequence of DNA
mutations
mutations can occur due to (3)
Base substitutions
Removal or addition of nucleotides
Transposable elements
how can mutations occur
spontaneously or be induced through the
use of a mutagen
occur in natural environment
Occur infrequently and randomly
spontaneous mutations
the probability that a mutation will be observed in a
given gene each time the cell divides
rate of mutation
how are mutations classified
in terms of their effect on phenotype
change colonial or cellular morphology
morphological mutations
kill the organism
lethal mutations
expressed only under certain conditions (e.g.,
high temperature)
conditional mutations
changes in metabolic capabilities
auxotrophs vs prototrophs
biochemical mutations
have mutations in biosynthetic pathways
cannot synthesize product of pathway
require product of pathway as nutrient in minimal
growth media
auxotrophs
grow in minimal media without supplements
prototrophs
resistance to pathogen, chemical, or antibiotic
resistance mutations
Most common type of
mutation
Result from mistakes
during DNA synthesis
base substitutions
Occur when one base pair is changed
point mutations
• Results from a nucleotide change that generates a codon which still specifies the wild type amino acid
silent mutation
Mutations resulting from nucleotide substitution leading
to a change in the amino acid incorporated into the protein
missense mutation
Mutations that change an amino acid codon to
a stop codon
nonsense mutation
Deletion or addition of nucleotide base pairs (Shifts the codons)
Alters reading frame
Affects all amino acids downstream from addition or deletion
frameshift mutations
- changes in regulatory sequences
* alter control of gene expression
regulatory mutations
can disrupt protein synthesis
some tRNA mutations are suppressor mutations
rRNA and tRNA mutations
Special segments of DNA that move spontaneously
from gene to gene
Transposable Elements
disrupt proper function of gene into which they insert
transposons
Mutations can be intentionally produced to
demonstrate function of particular gene or set of
genes
induced mutations
what are the three ways to induce mutationsf
Chemical mutagens
Transposition
Radiation
Gene that receives transposon will undergo an
insertion mutation
knock out mutation
what are the two types of radiation
ultraviolet light and x rays
Causes covalent bonding between adjacent thymine bases forming thymine dimers which distorts DNA
ultraviolet light
Cause breaks and alterations in DNA
• Breaks that occur on both strands are often lethal
x rays
Largest group of chemical mutagens
Alkylating agents
Chemicals that are structurally similar to the nitrogenous bases but have slightly altered base pairing properties
base analogs
what do the base analogs include
5-bromouracil and 2 -aminopurine
Planar molecules that insert themselves between adjacent bases (create space between bases)
Intercalating agents
Cells developed two methods of repair
proofreading and post replication repair
correction of errors in base pairing made during
replication
• DNA polymerase has this function
• Able to excise incorrect base and replace with
correct one
proofreading
Corrects damage that causes distortions in double helix
• e.g., thymine dimers
• e.g., damaged bases
Excision Repair
type of excision repair
Endonuclease removes short stretch of nucleotides
• DNA polymerase fills gap
• DNA ligase joins ends
mismatch repair
Used by E. coli postreplication repair system to
distinguish old DNA strands from new DNA strands
DNA Methylation
what DNA is methylated
old DNA methylated; new DNA not methylated
what are the two repair mechanisms of thymine dimers
Photoreactivation and excision repair
Removes the nitrogenous base without removing the sugar phosphate backbone
glycosylase
Last ditch effort to bypass damage Inducible repair system Used to repair excessive damage that halts replication, leaving many gaps Produces new DNA polymerase
SOS repair
finding mutants requires what
sensitive detection methods
• Direct selection
• Indirect selection
Observation of changes in phenotype
Involves inoculating population of bacteria on medium on which only mutants will grow
direct selection
Used to detect auxotrophic mutants
indirect selection
