Chapter Seven: Mutations Flashcards
heritable changes in the base pair sequence of DNA
mutations
changes wild-type allele to a different allele
forward mutation
changes a mutant allele back to wild-type
reverse mutation (reversion)
replacement of a base by another base
substitution
purine replaced by another purine or pyrimidine replaced by another pyrimidine
transition
purine replaced by a pyrimidine
transversion
block of one or more base pairs lost from DNA
deletion
block of one or more base pairs added to DNA
insertion
when both insertion and deletion occur
indel
affect one or a few base pairs, altering one gene at a time
point mutations
four examples of a point mutation
transitions, transversions, small deletions, small insertions
spontaneous mutations occur at a ___
very low rate
different ___ have different mutation rates
genes
mutation rates depend on ___ and ___
gene size and susceptibility of a gene to mutagens
bigger genes are ___ likely to have mutations
more
average mutation rate in gamete-producing eukaryotes is ___ than that of prokaryotes
higher
why is the mutation rate higher in gamete-producing eukaryotes
many cell divisions take place between zygote formation and meiosis and germ cells, more chances to accumulate mutations
human sperm have a ___ mutation rate them human eggs
higher
human mutation rate
1x10^-8
human mutation rate is estimated by ___
sequencing genomes of parents and their children
most mutations do not influence ___
phenotype
sperm mutation rate
2^-4x10^-8
revertants are more ___ than forward mutations
rare
many mutations ___ gene function, only a few ___ gene function
disrupt
restore
fluctuation test provided evidence that ___
mutations in bacteria occur spontaneously
two hypotheses for the origin of bacterial resistance in the fluctuation test
- resistance is a physiological response to bactericide
- resistance arises from random mutation
___ verifies that bacterial resistance is the result of preexisting mutations
replica plating
bacterial resistance arises from mutations that occurred ___ exposure to bactericide. bactericide becomes a ___.
before
selective agent
once random mutations occur they usually ___
remain stable
base changes are often corrected by ___
DNA repair
if a base change does not get corrected before replication, ___
both DNA strands contain the base change and a heritable mutation occurs
hydrolysis of a purine base, loss of a purine
depurination
removal of an amino group
deamination
deamination can change a ___ to a ___
C to a U
cosmic rays and x-rays cause DNA damage by ___
breaking the sugar phosphate backbone
causes adjacent thymines to form abnormal covalent bonds (thymine dimers)
Ultraviolet rays
___ decreases mistakes during replication
proofreading
incorporation of incorrect bases by DNA polymerase is __
exceedingly rare
proofreading portion of DNA polymerase that recognizes and excises mismatches
3’ to 5’ exonuclease
base tautomerization results in ___
replication mistakes
each base has two ___
tautomers
addition of 3 nucleotides
expansion
deletion of 3 nucleotides
contraction
___ and ___ are caused by unstable trinucleotide repeats
fragile X syndrome and Huntington disease
expansion and contraction of triplet repeats occurs by ___
slipped mispairing
HJ Muller used x-ray dose to ___ in Drosophila
increase mutation rate
agents that raise frequency of mutations above spontaneous rate
mutagens
chemical structure almost identical to normal base
base analog
addition of an OH group
hydroxylation
addition of ethyl or methyl groups
alkylation
removal of an amine group
deamination
insert molecules in between bases
intercalators
mutations that occur in non-germ cells
somatic mutations
are somatic mutations heritable
no
test used to screen for chemicals that cause mutations and therefore might cause cancer
Ames test
four accurate DNA repair mechanisms
- reversal of DNA base alterations
- homology-dependent repaid of damaged bases or nucleotides
- double-strand break repair
- mismatch repair of DNA replication errors
two error prone DNA repair systems
- SOS system
- microhomology mediated end joining (MMEJ)
base excision repair mechanism
- DNA glycosylases remove altered base
- AP endonuclease cuts backbone
- DNA exonuclease removes nucleotides near the cut, creating a gap
- DNA polymerase synthesizes new DNA to fill the gap
- DNA ligase seals the cut
nucleotide excision repair mechanism
- UvrA and UvrB scan for distortions to the double helix (thymine dimers for example)
- UvrB and UvrC endonuclease cut strand containing the distortion
- damaged fragment is released from DNA
- DNA polymerase fills in the gap with new DNA
- DNA ligase seals the cut
unrepaired double-strand breaks can lead to ___ and ___
deletions and chromosome rearrangements
two repair mechanisms for double-strand breaks
homologous recombination and nonhomologous end-joining
in bacteria, ___ corrects mistakes in replication
methyl-directed mismatch repair
the SOS system in bacteria is used at ___
replication forks that stalled because of unrepaired DNA damage
in the SOS system, DNA polymerase adds ___ opposite damaged bases
random nucleotides
xeroderma pigmentosa is caused by ___
mutations in any one of seven genes involved in nucleotide excision repair
hereditary forms of colorectal cancer are caused by ___
mutations in mismatch repair genes
hereditary forms of breast cancer are caused by ___
BRCA 1 and BRCA 2 involved in double - strand break repair by homologous recombination
without ___, life would have died out long ago because it could not have responded to changes int he environment
mutations
DNA repair walks a fine line between
being efficient enough to protect genomes and propagation of life
