Gene Mutation and DNA Repair Xeroderma Pigmentosum wo individuals with xeroderma pigmentosum the boy shows marked skin lesions induced by sunlight Mottled redness erythema and irregular pigment changes in response to cellular injury are apparent Two nodular cancers are present on his nose Charles Darwin s concept of descent with modification As many more individuals of each species are born than can possibly survive and as consequently there is a frequently recurring struggle for existence it follows that any being if it vary in any manner profitable to itself under the complex and sometimes varying conditions of life will have a better chance of survival and thus be naturally selected From the strong principle of inheritance any selected variety will tend to propagate its new and modified form Genetic Variation is the diversity of different genomes that give rise to different individuals Genomes can change over time Changes are heritable through germline DNA is the hereditary material changes in the DNA at the nucleotide level or at higher level Mutation are the ultimate source of genetic variation recombination also gives rise to genetic variation Spontaneous Mutation occur in nature and mainly due to errors in DNA replication Induced Mutations due to chemical or physical damage muller 1927 Stadler 1928 use of x rays Categories of Mutation Somatic Mutations not inherited most likely to be visible if dominant Gametic changes are heritable recessive X linked Nutritional biochemical mutations prototrophs auxotrophs Behavioral mutations hard to discern Regulatory mutations over under expressed gene Lethal mutations cannot survive to reproduce Conditional mutations ex temp sensitive Neutral mutations effect on fitness level of the organism is negligible no effect on gene product on expression Molecular Basis of Mutation Genetic information read in triplets codons Transition pyrimidine replaces pyrimidine or purine replaces purine Transversion pyrimidine replaces purine or purine replaces pyrimidine Missense amino acid changed to a different aa Nonsense amino acid changed to a stop codon Spontaneous mutations occur due to replication errors and base modifications Tautomeric shifts Purines and pyrimidines exist in alternative forms differing by a single proton shift less frequent forms base pair with non complementary bases ex T enol G keto or C imino A amino If these forms exist transiently during DNA replication then transition mutations can occur note that pairing is still between a purine and a pyrimidine Oxidative damage damage to bases by oxidative species such as superoxides and hydrogen peroxide modify bases and lead to mispairing Formation of a T A to C G transition mutation as a result of a tautomeric shift in Figure 14 3 adenine Apurinic Sites and Deamination nucleotide can be inserted Spontaneous loss of purine AP site leads to inadequate template another in deamination an amino group is converted to keto group this causes an alteration in base pairing efficiencies Induced mutations arise from DNA damage caused by chemicals and radiation Base analogs Chemicals that substitute for purines or pyrimidines ex similarity of 5 bromouracil 5 BU structure to thymine structure In the common keto form 5 BU base pairs normally with adenine behaving with adenine behaving as an analog In the rare enol form it pairs anomalously with guanine Alkylating Agents One of the first chemical mutagens known WWI Alkylating agents such as EMS donate an alkyl group to amino or keto groups in nucleotides cause transition mutations Acridine Dyes and Frameshift Mutations ex acridine orange can cause frameshifts they wedge between bases causing slippage or improper pairing leading to addition or deletion of a base pair UV and high energy radiation Shorter wavelength UV radiation is mutagenic major effect is to induce pyrimidine dimers between adjacent base pairs along a DNA strand Ionizing radiation X rays gamma rays cosmic rays electrons are ejected from atoms and they become free radicals leading to several types of reaction that can alter DNA and resulting in point mutations Also chromosome aberrations breakage of DNA bckbone Target Theory 1924 that x rays cause damage at single sites mutation also effect is cumulative Dimer formed between adjacent thymidine residues along a DNA strand Induction of a thymine dimer by UV radiation leading to distortion of the DN the covalent crosslinks occur between the atoms of the pyrimidine ring Single Gene Mutations cause a wide range of Human Diseases A single base pair SNP change can result in a serious genetic disorder OMIM database has over 3000 human genetic diseases cataloged About 30 of SNPs cause stop codons nonsense mutations 15 abnormal splicing Case studies of mutation in human genes Trinucleotide repeats Huntington disease Repeat CAG Normal number 6 53 affected number 36 129 Myotonic dystrophy Repeat CTG Normal number 5 35 affected number 200 Fragile X syndrome Repeat CGG Normal number 6 50 affected number 200 Spinolumbar Musc Dys Repeat CAG Normal number 10 30 affected number 35 60 Loss of CAG makes long tracts of glutamine that can cause proteins to aggregate number of repeats increases in each generation genetic anticipation Sometimes mutation is in coding region other times upstream or downstream Single Gene Mutations Cause a wide range of Human diseases ABO blood type alleles 4 common nucleotide substitutions in IA and IB alleles that change amino acid sequence of protein Io allele lack glycosyltransferase deletion of one nucleotide causing a frameshift and an abnormal and truncated shorter than normal protein Muscular Dystrophy Common severe duchenne DMA and milder becker forms BMD gene 2 mill bp 14 kb message 3685 aa most mutation cause premature termination 65 substantial deletions most DMD changed reading frame but BMD did not Organisms use DNA repair systems to counteract mutations necessary for survival of organisms on earth proofreading and mismatch repair during DNA replication Old DNA strand is methylated to distinguish from new strand in bacteria Before the new strand is methylated a repair enzyme recognizes the mismatch binds to the unmethylated new strand endonucleases cuts the backbone exonuclease removes nucleotides and DNA polymerase fills in gap again Postreplication Repair RecA enzymes recombines non mutated strand into gap left by DNA polymerase is a form of homologous recombination repair SOS repaint In E coli error prone