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BIOL 133
Denise Woodward

Chapter 12 Genetics Exam III Gene Mutations Mutation --- a change in a phenotype •change in nucleotide base sequence of a gene (change in genotype – mutant is change in phenotype) Mutations don't always cause a mutant phenotype •Mutations: ultimate source of genetic variation •Most are bad for individual *Germline mutations affect gametes (subsequent generations) *Somatic mutations affect body cells (not passed on)- often result in cancer Sickle Cell Anemia •1 bp substitution •Val substituted for Glu •Causes hemoglobin to be “sticky” - - sickling of blood cells (Reducancy protects us against mutation --- several base pairs result in the same protein) Causes of Mutations •Spontaneous mutations – generated spontaneously (random mistake in DNAreplication) •Induced mutations – chemicals or radiation causes the mutation Spontaneous mutations •result in genetic conditions that are not inherited. •Ex: ~20% of cancers occur spontaneously – mutations in genes that control cell cycle •DNA nitrogenous bases (A, T, C, G) are chemically unstable and mismatched base pairs can result ex: G pairs withA----- G should match with C •rate = 1/100,000 chance of mutation/gene •Short-lived organisms have ~same mutation rate but evolve more rapidly due to short generation time Mutational hot spots •sites of increased chance of mutation •usually found in regions where base pairs repeat (ATATAT) Induced Mutations •Chemicals and radiation can cause mutations •Example: Sunlight can cause thymine dimers – can lead to skin cancer •Site-directed mutagenesis uses PCR to create specific gene mutations for use in research •Accidental and natural exposure to mutagens can cause disease (especially cancer) Radiation Radiation is naturally occurring but accidents release greater amounts of radiation to the environment Fukushima nuclear reactor-- accident Types of Mutations •Point mutation - change in single DNAbase –Silent mutation – no effect on phenotype -- no change due to redundancy –Missense mutation – replaces one amino acid with another –Nonsense mutation – changes a codon for amino acid into “stop” codon – resulting protein is too short -- often non- fuctionable Types of Mutations •Deletions and Insertions – removal or addition of several DNAbases –frameshift mutation – adding or removing bases throws off reading frame of protein synthesis machinery Conditional Mutations •Amutation that affects the phenotype only under certain circumstances •Ex: G6PD deficiency and anti-malarial drugs and fava beans DNARepair:: Enzymes exist that repair mistakes in DNA Excision repair:: enzymes cut out segment of DNAw/ mistake Mismatch Repair:: proofreading enzymes remove mismatched bases •DNArepair enzymes are proteins. •Mutations in genes that code for these enzymes can lead to disease. Ex: polymerase eta – form of DNApolymerase that fixes and copies DNAdamaged by sunlight (removes thymine dimers). •People with xeroderma pigmentosum lack polymerase eta and develop skin cancer easily •Polymerase eta may be responsible for all terrestrial life Chapter 13 Chromosome Aberrations Health depends on inheriting correct amount of genetic material. Chromosome aberrations •Large scale changes (visible w/ light microscope and stains) •Effects on health –extra genetic material tends to be less damaging than losing material. Cytogenetics: Matching of phenotypes to detectable chromosomal abnormalities People with down syndrome have physical characteristics that show there chromosome abnormalities Chromosome Structure •Chromosomes are made of proteins and DNA •3 important regions: –Telomeres –Origins of Replication –Centromeres Telomers •The spot at the Tips of chromosomes •TTAGGG repeats •Get smaller after each division – lose a piece of themselves after each divisions, controls the division process Origins of Replication: sites where DNA replication begins Centromeres Constricted portion of a chromosome - Metacentric - divides chromosome equally Submetacentric - creates one long arm and one short arm Acrocentric - creates one very short arm (P arm- small arm- always facing up) Karyotype: Metaphase chromosomes: squashed on slide and stained w/ DNAdyes. Organized according to size. Banding patterns help define different chromosomes. •Clincial uses - diagnosis of genetic conditions •Public Health uses - toxins can cause chromosome aberrations •Evolutionary studies - similar karyotypes may indicate evolutionary relatedness between species Visualizing Chromosomes •Amniocentesis •Chorionic Villus sampling •Fetal cell sorting •Chromosome microarray analysis Amniocentisis •removal of fetal cells from uterus -14-16 weeks •cells are cultured for ~ 1 week and ~20 cells are karyotyped •causes miscarriage in 1/1,600 cases Chorionic Villus Sampling •removal of fetal cells from chorionic villi - 10th – 12th week •earlier detection of potential problems (but less precise due to mosaicism) •greater risk of miscarriage (1/1000-3000 cases can cause birth defects) Fetal Cell Sorting •separates fetal cells from mother’s blood stream •safe but experimental Chromosome microarray analysis •Allows for detection of smaller scale aberrations (copy number variants) •Uses chemical probes to show aberrant locations on chromosomes Abnormal Chromosome Number •Polyploidy – extra sets of chromosomes •Aneuploidy – one missing chromosome or one extra chromosome Aneuploidy examples •Trisomy – an extra chromosome •Monosomy – a missing chromosome –usually fatal Autosomal Aneuploidy - Trisomy 21 – Down syndrome: delay in developmental milestones, short stature, sparse hair, poor reflexes, hearing and vision loss, characteristic eyelid folds - Trisomy 18 – Edward Syndrome: heart defects, displaced liver, growth retardation, severe mental retardation, characteristically clenched fists Sex Chromosome Aneuploidy Syndromes •Turner: XO females (only 1X). Delays sexual development. •Klinefelter: XXY males. Delays sexual development, causes long limbs. •XYY: Causes tallness and acne. Falsely linked to violent behavior. Uniparental Disomy •Inheriting both copies of a chromosome from one parent (rare) •Arises from 2 nondisjunction events or a trisomy with subsequent chromosome loss •Discovered in a CF patient – only her mother was a carrier Chapter 14 Population Genetics •Population – individuals of same species in same area •Gene pool – all the alleles in a population Population Genetics •Individuals carry 2 alleles per gene •Populations potentially carry many more –Ex:ABO blood groups Allele Frequencies •Calculation of allele frequencies –Risk of heritable disease –Important for study of evolution Microevolution: allele frequency changes in a population Phenotype/Genotype frequencies •Frequency of traits is determined from observation and varies in different populations. Ex. freq. of PKU chineses-1/16,000 Irish-1/5,000 Japs.-1/119,000 Turks-1/2,600 Microevolution •Occurs under these conditions: –Nonrandom mating –Migrations- genes move from 1 population to another population –Mutation- a change in the DNAsequence –Genetic drift- genes from a large pop change and create a separate small pop –Natural selection- genes adapt for the good •Populations change because of these factors Hardy-Weinberg Equation •Amathematical equation used to predict allele and genotype frequencies in a population For a 2 allele situation: ---- allele frequency freq of dominant allele (A) = p freq of recessive allele (a) = q p + q= 1 Hardy-Weinberg Equation •Amathematical equation used to predict allele and genotype frequencies in a population For a 2 allele situation: --- genotype frequency freq of homozygous dominant (AA) = p2 freq of heterozygote (Aa) = 2pq freq of homozygous recessive (aa) = q2 p2 + 2pq + q2 = 1 Forensics Example •DNAfingerprinting is used to calculate the probability that genetic variants occur in 2 places by chance (the 2 places are DNAfrom suspect and DNAfrom crime scene) •Interpreting DNAevidence requires knowledge of population genetics. Short Tandem Repeats (STRs) •inherited regions of our DNAthat can vary from person to person (polymorphisms) •short sequences of DNA, normally of length 2-5 base pairs, that are repeated numerous times in a head-tail manner Ex: gatagatagatagata = 4 head-tail repeats of gata •Individuals vary in number of repeats – different numbers of repeats are different alleles CODIS (Combined DNAIndex System) •a computer software program that operates local, state, and national databases of DNAprofiles from convicted offenders, unsolved crime scene evidence, and missing persons •In 1997, the FBI announced the selection of 13 STR loci to constitute the core of CODIS Chapter 15 Population Genetics II ChangingAllele Frequencies The Hardy-Weinberg principle states that allele and genotype frequencies in a population stay constant unless disturbed (this means a population stays in equilibrium and does not evolve) Evolution = change in allele frequencies over time - can be positive or negative 5 factors that can cause evolution: 1. Nonrandom mating 2. Migration 3. Genetic Drift 4. Mutation 5. Natural Selection - Leads to adaptive change Non-random mating • Mate selection is not random – Disproportionate contribution of genes by single individuals Ex. Genghis Khan – Consanguinity Migration • Gene flow between two previously isolated populations Genetic Drift • Changes in allele frequencies due to chance (in small populations) Ex 1:Amish of Lancaster, PA: maple syrup urine disease, Ellis-van Creveld syndrome Ex 2: Population bottlenecks - in cheetahs Mutations • create new alleles • Most have harmful effects • Occasionally increase biological fitness (ex: antibiotic resistance) Natural Selection • Resource limitations: – prevent exponential population growth – lead to competition • Differential reproductive rates (due to heritable variation) • Individuals with ‘advantageous’traits survive and reproduce What is the ultimate source of genetic variation? A. Mutation B. Migration C. Genetic drift D. Natural selection Another Example: MRSA •Staphylococcus aureus – the organism that causes MRSA(Methicillin resistant Staph aureus) •Became resistant to penicillin in 1940s •Became resistant to methicillin in 2000 •Vancomycin currently used to treat it but many strains are resistant now YetAnother Example: HIV •HIV is RNAvirus •Mutation rate is high because viral genetic material is not repaired Artificial Selection •the intentional breeding for certain traits, or combinations of traits, over others –Cats and Dogs –FarmAnimals –Crops Artificial Selection and Dogs •Some dogs have been bred for traits that are unhealthy Examples: English bulldogs, pugs HeterozygoteAdvantage •maintains recessive deleterious alleles in population •Ex: –malaria and sickle cell –PKU and fungal infection –Cystic fibrosis and diarrheal diseases –Diabetes and famine MOVIE: What Makes us Human? As you watch the video, look for answers to the following questions: 1. How many kinds of hominids existed 30,000 years ago? 4 2. Where did the group that gave rise to modern humans evolve? Africa 3. Genetic evidence suggests that most Neanderthals had fair skin and red hair. What advantage did this provide them? Helped their bodies generate Vitamin D 4. We have mapped
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