The DNA Mutations

DNA mutations 95 Types of mutations Micromutation that involve small regions of the DNA Macromutations that involve the chromosomes as a whole 96 DNA micromutations Single-base mutations can result in different types of mutations such as: missense: a change of an amino acid nonsense: premature termination of protein synthesis frameshift: altering the reading of amino acid sequence silent: does not lead to any change at the protein level, but contributes to genetic variability among individuals 97 98 DNA micromutations (cont.) Translocations, that bring different regions of gene segments together Deletions of a few nucleotides to long stretches of DNA, Insertions and duplications of nucleotides or long stretches of DNA Inversion of DNA segments 99 Causes of DNA mutation DNA mutations can arise spontaneously or induced. Spontaneous mutations are naturally occurring mutations and arise in all cells. They arise from a variety of sources, including errors in DNA replication and spontaneous lesions Induced mutations are produced when an organism is exposed to a mutagenic agent, or mutagen. 100 Errors in DNA replication An error in DNA replication can occur when an inaccurate nucleotide pair (say, A-C instead of G-C) forms in DNA synthesis, leading to a base substitution Each of the bases in DNA can appear in one of several forms, called tautomers. 101 Properties of Pyrimidines & Purines Keto-enol tautomerism Tautomers are constitutional isomers of organic compounds that readily interconvert by a chemical reaction. Following DNA replication, tautomers lead to either transition or transversion mutations. 103 Transitions A purine substitutes for a purine or a pyrimidine for a pyrimidine 104 Transversions In transversion mutations, a pyrimidine substitutes for a purine or vice versa In this case, a replication error would require mispairing of a purine with a purine or a pyrimidine with a pyrimidine 105 Other errors of DNA replication Frameshift mutations These mutations often occurred at repeated sequences Such mutations change the reading "frame" of codons and leads to changes in the amino acid sequence of the produced protein Deletions and duplication They also often occur at sequence repeats 106 Spontaneous lesions Spontaneous lesions are naturally occurring type of DNA damage that can generate mutations Depurination Deamination Oxidatively damaged bases 107 Depurination Cleavage of the glycosidic bond between the base and deoxyribose 108 Deamination The deamination of cytosine yields uracil Uracil residues will pair with adenine during replication, resulting in the conversion of a G-C pair into an A-T pair (a GC→AT transition) Methylated cytosine can also be deaminated resulting in its conversion to thymine and leading to a transition mutation Oxidatively damaged bases Active oxygen species such as hydrogen peroxide (H2O2) are normally produced during metabolism Example: 8-oxo-7-hydrodeoxyguanosine (8-oxodG, or GO) product mispairs with A, resulting in G→T transversions. 110 111 Mechanisms of mutagenesis Mutagens induce mutations by at least three different mechanisms: replace a base in the DNA (base analog) alter a base so that it specifically mispairs with another base (alkylation) damage a base so that it can no longer pair with any base under normal conditions 112 Incorporation of base analogs Some chemical compounds are similar to the normal bases of DNA that they are incorporated into DNA in place of normal bases; such compounds are called base analogs. These analogs cause incorrect nucleotides to be inserted opposite to them during replication 113 5-bromouracil 5-bromouracil (5-BU) is an analog of thymine The normal structure of 5-BU pairs with adenine When ionized, 5-BU pairs with guanine 5-BU causes transition mutations 114 Specific mispairing Some mutagens are not incorporated into the DNA but are altered and cause specific mispairing Example, the addition of an alkyl group to guanine leads to the formation of O-6-alkylguanine and results in direct mispairing with thymine and a GC → AT transition. 115 Base damage A large number of mutagens damage one or more bases, so no specific base pairing is possible. Ionizing radiation results in the formation of ionized and excited molecules that can cause damage to cellular components including DNA Many different types of reactive oxygen species are produced that can damage bases, cause breakage of the N-glycosidic bond (AP sites), or cause strand breaks 116 Intercalating agents The intercalating agents such as proflavin and ethidium bromide are planar molecules that can insert themselves (intercalate) between the stacked nitrogen bases imitating base pairs The intercalated agent can cause singlenucleotide-pair insertions or deletions 117 Mutagens and carcinogens Mutagenicity and carcinogenicity are clearly correlated when most mutagens are also carcinogens (approximately 90 percent). Rapid tests make use of microbes to test for mutagenicity The most widely used test was developed in the 1970s by Bruce Ames, who worked with Salmonella typhimurium. Chemicals detected by this test can be regarded as potential carcinogens. 118 Features of Salmonella typhimurium The Ames test uses a mutant strain of S. typhimurium cannot grow in the absence of the amino acid histidine because a mutation has occurred in a gene that encodes one of the enzymes necessary for histidine biosynthesis They also carry a mutation that inactivates a DNA repair system They also carry a mutation that eliminates the protective lipopolysaccharide coating of wild-type Salmonella to enable the entry of many different chemicals into the cell 119 Reversion mutation In order for these cells to survive in the absence of histidine, they must have a mutation that corrects the original mutation that prevented the production of the missing enzyme This type of mutation is known as reversion, because this second mutation returns the mutant to the wildtype genotype This reversion can happen spontaneously or as the result of a mutagen 120 What is considered a mutagen? A compound must increase the number of colonies grown in the absence of histidine by double compared to cells grown in the absence of the compound Water Motor oil Alcohol Drug X 10 50 43 9 5 200 121 Role of liver enzymes In mammals, chemicals are normally detoxified or broken down by liver enzymes In some cases, the action of liver enzymes can create a toxic or mutagenic compound from a substance that was originally safe Ames incorporated mammalian liver enzymes in his bacterial test system 122 Example Condition Water Motor oil Alcohol Drug X ‫ قروش‬5 ‫شيبس أبو‬ -liver enzymes 10 50 43 9 200 +liver enzymes 12 22 50 35 500 123 Controversial issue Three-parent babies