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Lecture 5

Text Notes BIO130 Lecture 5 Part 1

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Jane Mitchell

Lecture 5 Text Notes pg 263276 281287Introduction to DNA Replicationshort term survival of a cell depends on preventing changes in its DNAlong term survival of a species requires that DNA be changeable through many generationsMutationsDNAmaintenance process failuremutationcan destroy the organism if occurring in vital position in DNA sequencemutation raterate at which observable changes occur in DNAin quickly dividing cells a single cell can generate a large population very quicklypossible to detect the small fraction that have suffered damaging mutations to a particular gene if the gene is not necessary to survivalmany mutations are silentie change a codon but not the amino acid it specifies change an amino acid without affection protein activity coded for by the gene6mutations occur about one in 10 bacterial cell generations1 nucleotide 9change per 10 nucleotide changes per cell generationnematode C elegansgeneration time 4 days selffertilization replicationaverage of two new mutations in haploid genome each generationwith number of cells for spermeggs taken into account mutation rate about 1 9mutation per 10 nucleotides per cell divisiondirect measurement of germline mutation rate in mammals is difficultindirectly can be determined by comparing amino acid sequences of the same protein in several speciesfraction of amino acids that are different between any two species can be compared with estimated number of years since divergence of common ancestor through use of fossil recordcalculate number of years elapsed before inherited change in amino acid sequence of a protein becomes fixedeach change reflects alteration of a single nucleotide in DNA sequence of the gene encoding a proteinuse value to estimate the average number of years required for a single stable mutation in the genefibrinopeptidessequence of one family of protein fragmentsallow genes that encode them to accumulate mutations without being selected against20 amino acid long fragmentsdiscarded when the protein fibrinogen is activatedforms fibrin during blood clottingfunction of fibrinopeptidesdoes not depend on amino acid sequencecan tolerate almost any amino acid changesequence comparisons can be used to estimate mutation rate in germ linetypical protein of 400 amino acidsamino acid alteration about once every 200000 yearsalternate way to estimate mutation rates in humansuse DNA sequencingcompare corresponding nucleotide sequences directly from closely related species in regions of genome that do not carry critical informationproduce similar results as fibrinopeptide studies Necessity of Low Mutation Rates
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