Chapter 18: Gene Mutations and DNA Repair

Question 1

mutation

Answer 1

inherited change in the DNA sequence of genetic information

Question 2

somatic mutations

Answer 2

mutation that arises in somatic cells and does not give rise to gametes
- mutation is passed to daughter cells, leading to a population of genetically identical cells

Question 3

germ line mutations

Answer 3

mutation in a germ line cell, ultimately gives rise to gametes
- can be passed to future generations, producing offspring that carry the mutation in all their somatic and germ line cells

Question 4

gene mutation

Answer 4

relatively small DNA lesion that affects a single gene

Question 5

chromosome mutation

Answer 5

large scale genetic alteration that affects chromosome structure or the number of chromosomes

Question 6

base substitution

Answer 6

alteration of a single nucleotide in the DNA
two types: transition and transversion

Question 7

transition

Answer 7

type of base substitution mutation
- a purine is replaced by a purine or a pyrimidine is replaced by a pyrimidine
- more frequent

Question 8

transversion

Answer 8

type of base substitution mutation
- purine is replaced by a pyrimidine or a pyrimidine is replaced by a purine

Question 9

insertions and deletions

Answer 9

the addition or removal of one or more nucleotide pairs
- most common type of mutation
- lead to frame shift mutations

Question 10

frame shift mutations

Answer 10

changes in the reading frame
- usually alter all amino acids encoded by the nucleotides following the mutation
- can introduce premature stop codons, terminating transcription early

Question 11

in-frame insertions and in-frame deletions

Answer 11

indels that do not affect the reading frame

Question 12

expanding nucleotide repeats

Answer 12

mutations in which the number of copies of a set of nucleotides increases
- cause of ALS
- number of copies increases in succeeding generations
- number of copies correlates with severity of the disease or age of onset
- can lead to anticipation; diseases become more severe in each generation

Question 13

strand slippage in trinucleotide repeats

Answer 13

can cause nucleotide repeats
- DNA separates and replicates
- a hairpin forms during replication, causing part of the template strand to be replicated twice, increasing the number of repeats
- new strand separates and replicates
- resulting DNA molecule has 5 additional copies of the repeated part

Question 14

forward mutation

Answer 14

mutation that alters the wild-type phenotype

Question 15

reverse mutation

Answer 15

changes a mutant phenotype back into the wild type

Question 16

missense mutation

Answer 16

base substitution that results in a different amino acid in the protein

Question 17

nonsense mutation

Answer 17

changes a sense codon into a nonsense codon

Question 18

sense codon

Answer 18

codon that specifies an amino acid

Question 19

nonsense codon

Answer 19

codon that terminates translation

Question 20

silent mutation

Answer 20

mutation that changes a codon to a synonymous codon that specifies the same amino acid, altering the DNA sequence without changing the amino acid sequence of the protein
- some have phenotypic effects

Question 21

neutral mutation

Answer 21

missense mutation that alters the amino acid sequence of a protein but does not significantly change its function
- occur when one amino acid is replaced by another that is chemically similar or when the affected amino acid has little influence on protein function

Question 22

loss-of-function mutations

Answer 22

causes the complete or partial absence of normal protein function
- alters the structure of the protein so that it no longer works correctly
- frequently recessive

Question 23

gain-of-function mutation

Answer 23

causes the cell to produce a protein or gene product whose function is not normally present
- could result in a new gene product
- frequently dominant

Question 24

conditional mutation

Answer 24

mutation expressed only under certain conditions
- ex: some affect phenotype only at elevated temperatures

Question 25

lethal mutations

Answer 25

mutations that cause premature death

Question 26

suppressor mutation

Answer 26

genetic change that hides or suppresses the effect of another mutation
- occurs at a site distinct from the original mutation
- two classes: intragenic and intergenic

Question 27

intragenic suppressor mutation

Answer 27

suppressor mutation that takes place in the same gene that contains the mutation being suppressed
- suppressor may change a second nucleotide in the same codon altered by the original mutation, producing a codon that specifies the same amino acid that was specified by the original, non mutated codon
- can also work by suppressing a frameshift mutation
- can make compensatory changes in the protein

Question 28

intergenic suppressor mutations

Answer 28

suppressor mutation that occurs in a gene other than the one bearing the original mutation it suppresses
- sometimes work by changing the way the mRNA is translated
- tRNA and gene interactions

Question 29

mutation rate

Answer 29

the frequency with which a wild-type allele at a locus changes into a mutant allele
- bacteria: 10^-8 to 10^-10
- eukaryotes: 10^-5 to 10^-6
- kept low by proofreading and DNA repair

Question 30

adaptive mutation

Answer 30

stressful environments where mutations may be necessary to survive can induce more mutations in bacteria

Question 31

factors affecting mutation rates

Answer 31

1. the frequency with which changes in DNA take place
2. probability that when an alteration in DNA takes place, it will be repaired
3. probability that a mutation will be detected

Question 32

spontaneous mutations

Answer 32

mutations that occur under normal conditions

Question 33

induced mutations

Answer 33

mutations that result from changes caused by environmental chemicals or radiation

Question 34

strand slippage

Answer 34

occurs when one nucleotide strand forms a small loop
- casuses insertions, deletions, and nucleotide repeats
- if looped out nucleotides are on the newly synthesized strand, an insertion results
- if looped out nucleotides are on the template strand, deletion results on the replicated strand

Question 35

unequal crossing over

Answer 35

process that produces insertions and deletions through misaligned pairing
- unequal crossing over causes one DNA molecule to have an insertion and the other to have a deletion

Question 36

depurination

Answer 36

the loss of a purine base from a nucleotide
- results from spontaneous chemical change in DNA
- covalent bond connecting the purine to the 1’-carbon atom of the deoxyribose sugar breaks, producing an apurinic site,
- leads to an incorporated error that is transformed into a replicated error

Question 37

deamination

Answer 37

loss of an amino group (NH2) from a base
- may be spontaneous or induced by mutagenic chemicals
- can alter the pairing properties of the base

Question 38

mutagen

Answer 38

any environmental agent that significantly increases the rate of mutation above the spontaneous rate

Question 39

base analogs

Answer 39

class of chemical mutagens; chemicals with structures similar to those of any of the four nitrogenous bases of DNA
- DNA polyermases cannot distinguish these analogs from the standard bases, and they may be incorporated into newly synthesized DNA molecules
- causes mispairings and transition mutations

Question 40

intercalating agents

Answer 40

produce mutations by sandwiching themselves between adjacent DNA bases, distorting the 3D structure of the helix and causing single-nucleotide insertions and deletions in replication
- proflavin, acridine orange, ethidium bromide, dioxin
- frequently cause frameshift mutations
- can reverse the mutations they produce bc they can cause insertions and deletions

Question 41

radiation

Answer 41

many forms that all damage DNA
- can alter DNA structure, break bonds in DNA, alter bases
- pyrimidine dimers

Question 42

pyrimidine dimers

Answer 42

bulky lesions that distort the configuration of DNA
- made by pyrimidine bases absorbing UV light, causing chemical bonds to form between adjacent pyrimidine molecules on the same strand of DNA
- most are immediately repaired

Question 43

transposable elements

Answer 43

DNA sequences that can move about in the genome
- are often a cause of mutations

Question 44

short flanking direct repeats

Answer 44

short, directly repeated sequences present on both sides of most transposable elements
- not part of the transposable element

Question 45

terminal inverted repeats

Answer 45

sequences at the ends of many transposable elements that are inverted complements of each other

Question 46

mismatch repair

Answer 46

incorrectly paired bases are detected and corrected by mismatch repair enzymes
- corrects small loops in DNA

Question 47

direct repair

Answer 47

restores correct nucleotide structures
- does not replace altered nucleotides

Question 48

base excision repair

Answer 48

a modified base is excised, then the entire nucleotide is replaced
- catalyzed by a set of enzymes called DNA glycosylases

Question 49

nucleotide excision repair

Answer 49

removes bulky DNA lesion that distort the double helix

Question 50

cancer and faulty DNA repair

Answer 50

cancer can arise from mutations in the proteins that carry out mismatch repair