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Week 5 Notes 82669 - BIOL 3350 - 001
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This 6 page Class Notes was uploaded by Abigail Towe on Wednesday September 16, 2015. The Class Notes belongs to 82669 - BIOL 3350 - 001 at Clemson University taught by Lisa G Rapaport in Fall 2015. Since its upload, it has received 22 views. For similar materials see Evolutionary Biology in Biological Sciences at Clemson University.
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Date Created: 09/16/15
September 15 2015 Variation among Individuals Producing Genetic Variation 0 Variation among individuals 0 through genetic environmental and genebyenvironment interaction I Genetic Mutations A How do new allele arise 2 general mechanisms whereby new alleles are created Mutations occur through 1 mistakes that occur during the DNA replication process caused by the failure of one of the DNA repair mechanisms 3 ultimate source of genetic variability B Point Mutations 1 Single basepair A C T G substitutions 0 not repaired by proofreading or mismatch repair 0 after synthesis a new allele is created Two general types of point mutations Transitions and transversions O Transitions are interchanges of tworing purines A H G or of onering pyrimidines C H T they therefore involve bases of similar shape 0 purine to purine o pyrimidine to pyrimidine O Transversions are interchanges of purine for pyrimidine bases which therefore involve exchange of onering and tworing structures 0 purine to pyrimidine Which are more frequent Why 9 Although there are twice as many possible transversions because of the molecular mechanisms by which they are generated transition mutations are generated at higher frequency than transversions As well transitions are less likely to result in amino acid substitutions due to Wobblequot and are therefore more likely to persist as quotsilent substitutionsquot in populations as single nucleotide polymorphisms SNPs 2 Insertions amp deletions o Dinucleotide repeats are formed by slippage replication 0 slippage replication a form of mutation that leads to either a tri nucleotide or dinuoleotide expansion or contraction during DNA replication A slippage event normally occurs when a sequence of repetitive nucleotides tandem repeats are found at the site of replication Tandem repeats are unstable regions of the genome where frequent insertions and deletions of nucleotides can take place resulting in genome rearrangements 0 basis of satellite alleles o more common in noncoding regions 0 What are they and how do they occur 0 O rearrangement because tandem repeats are unstable regions Where do they commonly occur during DNA replication occurs within a repetitive nucleotide C Consequences of point mutations 1 Silent substitutions o How do they occur and what is the result 0 single nucleotide changes that do not change the amino acid encoded by the mRNA 0 usually involved changes that will affect the third position of mRNA codons 0 about 70 of point mutations are silent 0 results in NEUTRAL mutations o synonymous same amino acid as before 0 We have this because degeneracy of the code yields many opportunities for silent substitutions 0 when it s the third position that has been changed it s usually the synonymous substitution but sometimes it does change missense substitution 2 Missense substitutions How do they occur and what is the result 0 single nucleotide changes that DO change the amino acid that will be coded o lst or 2nd position alteration 0 create what are called null alleles such mutations are usually recessive and deleterious or lethal o occasionally such mutations create beneficial alleles o which opens new possibility of new adaptation for individual 0 nonsynonymous substitutions because the functioning of the protein is NOT the same 0 example Sicklecell anemia o the hemoglobin protein has a single base pair substitution in the protein that changes the glutamic acid to valine this creates a protein with an impaired function 0 this mutation is lethal in homozygous condition 0 heterozygotes are favored in regions with malaria 3 Nonsense substitutions How do they occur and what is the result 0 changes in DNA that results in insertion of a premature STOP codon into the mRNA ATT UAA ATC UAG UGA early stop signal in mRNA halts translation protein is nonfunction 4 Frameshift mutations o How do they occur and what is the result addition or deletion of one or a few nucleotides causes a shift in reading frame of mRNA that will be transcribed all codons downstream of the indel will be changed 0000 results in production of a nonfunctional protein Chart from class Examples of loss of function mutations Point mutations No mutation Silent Nonsense Missense censervative henconservative DNA level mRNA reves AAG AAA UAG AGG ACG rxotem fever Lys Lys STOP Arg Thr D Mutation Rates 1 Spontaneous mutations How often do they occur 0 mutations occur by change at low frequency 0 about 10quot9 or 1 per billion nucleotides replicated in mammals o spontaneous mutation rate varies among genes and among species 0 many alterations to DNA are repaired and do not become persistent mutations Examples 0 Drosophila fruit fly 0 wild type for eyes red o spontaneous mutation leads to brown eyes I occurs 3X10quot5 times per replication I point mutation on chromosome 2 0 changes from enzyme A scarlet pigment to enzyme B brown 0 but when both enzyme A and B are produced then you have a dark red eye 2 Recurrent mutations 0 mutations that arise repeatedly o albinism in mice is a recurrent mutation 0 wild type to albino 10quot5 gamete generation 3 Induced mutations o Mutagens are substances that increase the mutation rate above the spontaneous rate 0 ultraviolet light 0 ionizing radiation 0 pesticides and industrial chemicals 0 quotcancer causing agents that induce mutations E Mutation is a Random Event 1 Mutation RATES are affected by environment 0 mutations are random events but the rate can be affected by environment 0 the kind of mutation that arises is not directed by environment it s a chance event 0 whether or not the mutation persists depends on the environment 2 Mutation is NOT DIRECTED with respect to the environment 0 exposure to mutagens will increase overall mutational rate but not determine which particular genes will be affected Ill Chromosomal Mutations Large Effects A How do mutations cause changes in genomes 1 Karyotypic mutations o genome duplication polyploidy o Entire sets of chromosomes is duplicated 0 can occur by two different mechanisms sources 0 Allopolyploidy the duplicate sets of chromosomes come from two distinct sources 0 creates hybrid o Autopolyploidy the duplicate sets of chromosomes come from the same source 0 creats triploid 0 Both can result in the nearly instantaneous origin of new species 2 Chromosomal mutations 0 Gene Duplications o unequal crossing over 0 a piece of a chromosome is duplicated during crossing over can involve one or multiple genes 0 caused by replication errors something gets copied twice 0 How do they occur 1 homologous don t pair properly so after crossing over one chromosome has a deletion and the other has a duplication 2 occurs rather easily if DNA contains many tandemly repeated nucleotides or genes microsatellites 3 often the new gene is not functional pseudogene 4 sometimes create new genes that may then diverge in function 0 Why are these errors important 0 thought to be important mechanism in the evolution of multigene families I Example Hemoglobin gene family o paralogous genes and pseudogenes r The major Hemoglobin chains r Different genes for different globins alpha and beta chains r the gene for the alpha chain is derived from duplication of the gene for beta chain recent duplication created the second alpha chain r different hemoglobin proteins are produced at different developmental stages r different globin proteins are produced in different tissues 0 cytoglobin myoglobin etc IV Importance of mutation A Mutation as a evolutionary force 1 mutagenic change in populations a mutation is NOT a STRONG evolutionary force i probability of survival of a rare mutant is low ii survival of a rare mutant is higher if recurrent mutation iii survival probability of a rare mutant is higher if 1 mutant is favor by natural selection and 2 population is rapidly increasing in size 2 importance of mutation as a evolutionary force a mutation creates variation i the only source of unique alleles ii rate of accumulation of mutation variation can be fairly high for polygenic traits b mutation provides the variation needed by other forces of evolution i neutral alleles are acted upon by genetic drift ii advantageous and disadvantageous alleles are acted upon by natural selection 0
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