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mutation
a permanent change in the genetic material that can be passed from cell to cell or, if it occurs in reproductive cells, from parent to offspring
homologous recombination
the exchange of identical or similar DNA segments between homologous chromosomes
point mutation
a change in a single base pair within DNA
base substitution
a point mutation in which one base is substituted for another
transition
a point mutation involving a change of a pyrimidine to another pyrimidine (e.g., C to T) or a purine to another purine (e.g., A to G)
transversion
a point mutation in which a purine is interchanged with a pyrimidine, or vice versa
silent mutation
a mutation that does not alter the amino acid sequence of the polypeptide coded by a gene even though the base sequence has changed
missense mutation
a base substitution that leads to a change in the amino acid sequence of the polypeptide that is coded by a gene
nonsense mutation
a mutation that involves a change from a codon that specifies an amino acid to a stop codon
frameshift mutation
a mutation that involves the addition or deletion of a number of nucleotides not divisible by 3, which shifts the reading frame of the codons downstream from the mutation
neutral mutation
a mutation that has no detectible effect on protein function or no detectable effect on the survival of the organism
up promoter mutation
a mutation in a promoter that increases the rate of transcription
down promoter mutation
a mutation in a promoter that decreases the rate of transcription
wild type
a relatively prevalent genotype in a natural population
mutant allele
an allele that has been created because a wild-type allele has been altered by mutation
reversion
a mutation that changes a mutant allele back to a wild-type allele
deleterious mutation
a mutation that is detrimental with regard to its effect on phenotype; it decreases the chances of survival and reproduction
lethal mutation
a mutation that produces an allele that results in the death of a cell or an organism
beneficial mutation
a mutation that enhances the survival or reproductive success of an organism
conditional mutant
a mutant whose phenotype depends on the environmental conditions, such as temperature-sensitive (ts) mutant
suppressor (suppressor mutation)
a mutation at a second site that suppresses the phenotypic effects of another mutation. also referred to as a suppressor mutation
intragenic suppressor
a suppressor mutation that is within the same gene as the first mutation that it suppresses
intergenic suppressor
a suppressor mutation that occurs in a different gene than the gene that contains the first mutation
breakpoint
a region where two chromosome pieces break apart and rejoin with other chromosome pieces
position effect
a change in phenotype that occurs when the location of a gene changes from one chromosomal site to a different one
germ line
refers to cells that give rise to gametes
germ-line mutation
a mutation in a cell of the germ line
somatic cell
any cell of the body except for gametes and germ-line cells that give rise to gametes
somatic mutation
a mutation in a somatic cell
genetic mosaic
an individual that has somatic regions that differ genotypically from each other
replica plating
a technique in which replicas of bacterial colonies are transferred to new growth plates
random mutation theory
according to this theory, mutations are random events - they can occur in any gene and do not require exposure of an organism to an environmental condition that causes specific types of mutations
hot spots
regions within a gene that are way more likely to mutate than others
spontaneous mutation
a change in DNA structure that results from natural biological or chemical processes
induced mutation
a change in DNA structure caused by an environmental agent
depurination
the removal of a purine base from DNA
apurinic site
a site in the DNA that is missing a purine base
deamination
the removal of an amino group from a molecule. for example, the removal of an amino group from cytosine produces uracil
tautomeric shift
a temporary change in chemical structure, such as an alternation between the keto and enol forms of the bases that are found in DNA
tautomers
chemically similar forms of certain small molecules, such as bases, which can spontaneously interconvert
reactive oxygen species (ROS)
products of oxygen metabolism that are produced in all aerobic organisms and that can, if they accumulate, damage cellular molecules, including DNA, proteins, and lipids
oxidative stress
an imbalance between the production of reactive oxygen species (ROS) and an organism’s ability to break them down
oxidative DNA damage
changes in DNA structure that are caused by reactive oxygen species (ROS)
trinucleotide repeat expansion (TNRE)
a type of mutation that involves an increase in the number of tandemly repeated trinucleotide sequences
anticipation
the phenomenon in which the severity of an inherited disease tends to get worse in subsequent generations
mutagen
an agent that can alter the structure of DNA, causing a mutation
nitrous acid
a type of chemical mutagen that deaminates bases, replacing amino groups with keto groups
nitrogen mustard
an alkylating agent that can cause mutations in DNA
ethyl methanesulfonate (EMS)
a type of chemical mutagen that alkylates bases (i.e., attaches methyl or ethyl groups)
acridine dye
a type of chemical mutagen that intercalates between adjacent base pairs in DNA and causes frameshift mutations
proflavin
an acridine dye, which is a chemical mutagen that causes frameshift mutations
5-bromouracil (5BU)
a base analog that acts as a chemical mutagen
2-aminopurine
a base analog that acts as a chemical mutagen
thymine dimer
two adjacent thymine bases in a DNA strand that have become covalently linked
mutation rate
the likelihood that a gene will be altered by a new mutation
mutation frequency
the number of mutant genes divided by the total number of copies of that gene within a population
Ames test
a test using strains of bacterium, Salmonells typhimurium, to determine if a substance is a mutagen
photolyase
an enzyme found in bacteria, fungi, most plants, and some animals that can recognize and split thymine dimers, which returns the DNA to its original condition
photoreactivation
a type of DNA repair mechanism in which photolyase recognizes and splits thymine dimers; the mechanism requires light
alkyltransferase
an enzyme that can remove methyl or ethyl groups from guanine bases
base excision repair (BER)
a type of DNA repair in which a modified base is removed from a DNA strand. following base removal, a short region of the DNA strand is removed and then resynthesized using the complementary strand as a template
DNA N-glycosylase
an enzyme that can recognize an abnormal base and cleave the bond between it and the sugar in the DNA backbone
AP endonuclease
a DNA repair enzyme that recognizes a DNA region that is missing a base and makes a cut in the DNA backbone near that site
nucleotide excision repair (NER)
a DNA repair mechanism in which several nucleotides in the damaged strand are removed from the DNA and the undamaged strand is used as a template to resynthesize a normal strand
base pair mismatch
a DNA abnormality in which two bases opposite each other in a double helix do not conform to the AT/GC rule. for example, if A were opposite C, that would be a base-pair mismatch
mismatch repair system
a DNA repair system that recognizes base pair mismatches and repairs the newly made daughter strand that contains the incorrect base
homologous recombination repair (HRR) (homology-directed repair)
a mechanism for repairing double-strand breaks in which the DNA strands from a sister chromatid are used to repair a lesion in the other sister chromatid. also called homology-directed repair
nonhomologous end joining (NHEJ)
a repair mechanism for double-strand breaks in which the ends of the DNA are pieced back together
translesion synthesis (TLS)
the synthesis of DNA over a template strand that harbors some type of DNA damage. this occurs via translesion-replicating polymerases
error-prone replication
a form of DNA replication carried out by translesion-replicating polymerases that results in a high rate of mutation
sister chromatid exchange (SCE)
the phenomenon in which crossing over occurs between sister chromatids, which thereby exchange identical genetic material
genetic recombination
the process in which chromosomes are broken and then rejoined to form a novel genetic combination
the process in which alleles are assorted and passed to offspring in combinations that are different from those found in the parents
gene conversion
the phenomenon in which one allele is converted to the allele on the homologous chromosome due to recombination or DNA repair
Holliday model
a model to explain the molecular mechanism of homologous recombination
Holliday junction
a site where an unresolved crossover has occurred between two homologous chromosomes
branch migration
the lateral movement of a Holliday junction
heteroduplex
a region of double-stranded DNA that contains one or more base-pair mismatches
resolution
the last two steps of homologous recombination, in which the entangled DNA strands become resolved into two separate chromosomes
in microscopy, the minimum distance between two objects that enables them to be seen as being separate from each other
double-strand break model
a model for homologous recombination in which the event that initiates recombination is a double-strand break in one of two homologous chromatids
DNA gap repair synthesis
the synthesis of DNA in a region where part of a DNA strand has been previously removed, usually by a DNA repair enzyme or by an enzyme involved in homologous recombination
chapter 19 summary
a mutation is a heritable change in the genetic material
homologous recombination is the process whereby identical or similar DNA segments are exchanged between homologous chromosomes
19.1 effects of mutations on gene structure and function
a point mutation is a change in a single base pair in the DNA. such a mutation can be a transition or a transversion
silent, missense, nonsense, and frameshift mutations may occur within the coding sequence of a gene
mutations may also occur within a non-coding sequence of a gene and affect gene expression
gene mutations are given names that describe how they affect the wild-type genotype and phenotype
suppressor mutations reverse the phenotypic effects of another mutation. they can be intragenic or intergenic
changes in chromosome structure can have a position effect that alters gene expression
if a mutation occurs early in development, a larger region of the individual is likely to carry the mutation. also, mutations can occur in germ-line cells, which can be passed to offspring, or they can occur in somatic cells, which cannot be passed to offspring
19.2 random nature of mutations
results of experiments performed by Lederberg and Lederberg were consistent with the random mutation theory
19.3 spontaneous mutations
spontaneous mutations result from natural biological or chemical processes, whereas induced mutations are caused by environmental agents
three common ways that mutations can arise spontaneously is by depurination, deamination, and tautomeric shifts
reactive oxygen species (ROS) can cause spontaneous mutations by oxidizing bases in DNA
in individuals with a trinucleotide repeat expansion (TNRE), the number of repeats of a trinucleotide sequence has increased above a critical level and become prone to frequent expansion. this type of mutation is responsible for certain types of human diseases. the repeats can expand due to hairpin formation during DNA replication
19.4 induced mutations
a mutagen is an agent that can cause a mutation. researchers have identified many different mutagens that change DNA structure in a variety of ways
mutation rate is the likelihood that a new mutation will occur. mutation frequency is the number of mutant genes divided by the total number of copies of a gene in a population
the Ames test is used to determine if an agent is a mutagen
19.5 DNA repair
all species have a variety of DNA repair systems to avoid the harmful effects of mutations
photolyase and alkyltransferase can directly repair certain types of damaged bases
base excision repair (BER) recognizes and removes a damaged base
nucleotide excision repair (NER) removes damaged bases and damaged segments of DNA. several inherited human diseases involve defects in nucleotide excision repair
the mismatch repair system recognizes a base-pair mismatch and removes the segment of the DNA strand containing the incorrect base
double-strand breaks can be repaired by homologous recombination repair (HRR) or by nonhomologous end joining (NHEJ)
damaged DNA may be replicated by translesion-replicating polymerases but the replication process is error-prone
19.6 homologous recombination
homologous recombination can occur between sister chromatids or between homologous chromosomes
the Holliday model describes the molecular steps that occur during homologous recombination
the initiation of homologous recombination usually occurs with a double-strand break
several different proteins are involved in homologous recombination
two different mechanisms, DNA mismatch repair and gap repair synthesis, can result in gene conversion during homologous recombination
