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33 Cards in this Set
- Front
- Back
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spontaneous mutation
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changes in the nucleotide sequence of genes that appear to have no known cause; no specific agents are associated with their occurence, and they are generally assumed to be accidental; many arise as the result of normal biological or chemical processes in the organism that alter the structure of nitrogenous bases; often occur during the enzymatic process of DNA replication
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induced mutation
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result from the influence of extraneous factors; may be the result of either natural or artificial agents (ex: radiation
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adaptive (non-random, not spontaneous) mutation
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a concept revolved around the controversial idea that organisms may in some way "select" or "direct" the mutation of their genes in order to adapt to a particular environmental pressure
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fluctuation test
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a statistical test (measures variance) developed by Salvadore Luria and Max Delbruck that demonstrated that bacterial mutations arise spontaneously, in contrast to being induced by selective agents;
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somatic mutations
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mutations occurring in any cell in the body except germ cells; not transmitted to future generations; will have a greater impact if they are dominant or, in males, if they are X-linked; similarily, the impact of dominant or X=linked somatic mutations will be more noticeable if they ocur early in development, when a small number of undifferentiated cells replicate to give rise to several differentiated tissues or organs.
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germ-line mutations
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those occurring in gametes; are of far greater significance than somatic mutations because they are transmitted to offspring as part of the germ line; they have the potential of being expressed in all cells of an offspring
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point mutation (base substitution)
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a change of one base pair to another in a DNA molecule
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missense mutation
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a change of one nucleotide (a point mutation) of a triplet within a protein-coding portion of a gene may result in the creation of a new triplet that codes for a different amino acid in the protein product
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nonsense mutation (nonsense codon; termination codon)
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if the point mutation within a triplet results in a stop codon, it will result in the termination of translation of the protein
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silent mutation
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if the point mutation alters a codon but doesn't result in a change in the amino acid at that position in the protein (due to degeneracy of the genetic code)
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transition
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if a pyrimidine replaces a pyrimidine or a purine replaces a purine
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transversion
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if a purine replaces a pyrimidine or a pyrimidine replaces a purine
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frameshift mutation
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when the insertion or deletion of a nucleotide causes all of the subsequent three-letter codons to be changed; will occur when any nmber of bases are added or deleted, except multiples of three, which would reestablish the initial frame of reading; results can be very severe if they occur early in the coding sequence
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loss-of-function mutation
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reduces or eliminates the function of the gene product; can result from any type of mutation, from a point mutation to deletion of the entire gene
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null mutation
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result in complete loss of function; a loss of function mutation can be either dominant or recessive. a dominant loss-of-function mutation may result from the presence of a defective protein produce that binds to, or inhibits the action of, the normal gene product, which is also present in the same organism
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gain-of-function mutation
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results in a gene product with enhance or new functions; may be due to a change in the a.a. sequence of the protein that confers a new activity, or it may result from a mutation in a regulatory region of the gene, leading to expression of the gene at higher levels, or the synthesis of the gene product at abnormal times, or places. Most gain-of-function mutations are dominant
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neutral mutations
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(because eukaryotic genomes consist mainly of noncoding regions, the vast majority of mutations are likely to occur in the large portions of the genome that do not contain genes); mutations that do not affect gene products or gene expression are neutral mutations
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lethal mutation
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a mutation that interrupts a process that is essential to the survival of the organism; ex: Tay-Sachs disease and Huntington disease
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conditional mutation
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the mutation is present in the genome of an organism but can be detected only under certain conditions; among the best examples are temperature-sensitive mutations
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temperature- sensitive mutations
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a type of conditional mutation; at a "permissive" temperature, the mutant gene product functions normally, but it loses its function and becomes apparent at a different, "restrictive" temperature.
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replication slippage
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DNA replication can lead to the introduction of small insertions or deletions; can occur when one strand of DNA template loops out and becomes displaced during replication, or when DNA polymerase slips or stutters during replication; can occur anywhere in the DNA but seems distinctly more common in regions containing repeated sequences
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tautomeric shifts
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purines in pyrimidines can exist in tautomeric forms (alternate chemical forms that differ by only a single proton shift in the molecule) the biologically important tautomers are the keto-enol forms of theymine and guanine and the amino-imino forms of cytosine and adenine. these shifts change the bonding structure of the molecule, allowing H bonding with non-complementary bases. Hence, tautomeric shifts may lead to permanent base-pair changes and mutations. T-G and C-A pairs, among many others, may be formed. A mutation occurs during DNA replication when a transiently formed tautomer in the template strand pairs with a noncomplementary base. In the next round of replication, the "mismatched" members of the base pair are separated, and each becomes the template for its normal complementary base. The end result is a point mutation.
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base analogs (2-AP and BrdU)
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one category of mutagenic chemical; compounds that can substitute for purines or pyrimidines during nucleic acid biosynthesis; for ex: 5-bromouracil (5-BU), a derivative of uracil, behaves as a thymine analog but is halogenated at the number 5 position of the pyrimidine ring. If it is chemically linked to deoxyribose, the nucleoside analog bromodeoxyuridine (BrdU) is formed. The presence of the bromine atom in place of the methyl group increases the probability that a tautomeric shift will occur. If 5-BU is incorporated into DNA in place of thymine and a tautomeric shift to the enol form occurs, 5-BU base-pairs with guanine. After one round of replication, an A=T to G=C transition results. Furthermore, the presence of 5-BU within DNA increases the sensitivity of the molecule to UV light. Another base analog that is mutagenic is 2-amino purine (2-AP) which can act as an analog of adenine. It has a base-pairing affinity with thymine and can also base-pair with cytosine, leading to possible transitions from A=T to G-=C following replication.
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IA, IB, and IO alleles
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alleles of the I gene that encode the glycolslytransferase enzyme. The H substance is then modified to either the A or B antigen as a result of the activity of this enzyme, depending on whether the enzyme was encoded by the IA or IB allele, respectively. Failure to modify the H substance results from the presense of the null, IO, allele
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Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD)
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recessive, X-linked conditions. DMD is the more severe of the two, with a rapid progression of muscle degeneration and involvement of the heart and lungs. BMD doesn't involve the heart or lungs and progresses slowly. The gene responsible for DMD and BMD- the dystrophin gene- is unusually large, consisting of about 2.5 million base pairs. In normal ppl, transcription and subsequent processing of the dystrophin initial transcript results in a messenger RNA containing only about 14,000 bases (14 kb). It is translated into the protein dystrophin, which consists of 3685 a.a. DMD mutations change the reading frame of the dystrophin gene, whereas BMD mutations usually don't. About 2/3 of the mutations in the dystrophin gene that lead to DMD and BMD are deletions and insertions (only 1/3 are point mutations). The majority of DMD mutations lead to premature termination of translation which leads to degradation of the improperly translated dystrophin transcript and the nearly complete absence of dystrophin protein. In contrast, the majority of BMD gene mutations alter the internal sequence of the dystrophin transcript and protein, but don't alter the translation reading frame. As a result, a modified but somewhat functional dystrophin protein is produced, preventing the severe consequences of DMD.
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trinucleotide repeat expansion
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specific trinucleotide DNA sequences repeated many times. Normal ppl usually have fewer than 15 repetitions of these sequences
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fragile X syndrome mutations
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gene responsible= FMR-1; contains several to several thousand compies of the trinucleotide sequence CGG loacted in the 5'0untranslated region of the gene; ppl with 55-230 copies are normal but are considered carriers; The large regions of CGG repeats in the gene's regulatory region result in loss of expression of the FMRP protein. In fragile X syndrome, the repeat sequence is upstream (beyond 5' end) of the gene's coding region, in an area that is most often involved in regulating gene expression.
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huntington disease (HD) mutation
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a fatal neurodegenerative disease, inherited as an autosomal dominant condition; the gene responsible is located on chromosome 4 and contains the trinucleotide CAG sequence, repeated 11-34 times in normal indicividuals. the CAG repeat sequence exists in significantly increased numbers (up to 120) in diseased individuals. In Huntington's disease, the repeats lie within the coding portion of the gene. This causes the mutant huntington protein to contain an excess of glutamine residues.
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Ames test
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a bacterial assay developed by Bruce Ames to detect mutagenic compounds; it assesses reversion to histidine independence in the bacterium Salmonella typhimurium (from back of the book)
.. uses any of a dozen strains of the bacterium Salmonella typhimurium that have been selected for their ability to reveal the presence of specific types of mutation. Ex: some strains are used to detect base-pair subst, and other strains detect various frameshift mutations. Each strain contains a mutation in one of the genes of the histidine operon. The mutatnt strains are unable to synthesize histidine (his- strains) and therefore require histidine for growth. The assay measures the freq of reverse mutation within the mutant gene, which yields wt bacteria (his+ reverants). |
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mismatch repair
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a form of excision repair of DNA in which the repair mechanism is able to distinguish between the strand with the error and the strand that is correct
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somatic cell hybridization
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strengthened the idea of the link between XP and excision repair. Fibroblast cells from any two unrelated XP patients, when grown together in tissue culture, can fuse together, forming a heterokaryon
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heterokaryon
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a single cell with two nuclei from different organisms but a common cytoplasm
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complementation
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a relationship between two different strains of an organism which both have homozygous recessive mutations that produce the same phenotype (for example, a change in wing structure in flies). These strains are true breeding for their mutation. If, when these strains are crossed with each other, some offspring show recovery of the wild-type phenotype, these strains show "genetic complementation". When this occurs, each strain's haploid supplies a wild-type allele to "complement" the mutated allele of the other strain's haploid, causing the offspring to have heterozygous mutations in all related genes. Since the mutations are recessive, the offspring display the wild-type phenotype.
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