L21 Gene mutation - Gene mutation 1 Last Time ✤ ✤ ✤...

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Unformatted text preview: Gene mutation 1 Last Time ✤ ✤ ✤ from mRNA to proteins quick review: features of proteins Important components of translation machinery ✤ tRNA ✤ ribosomes ✤ Revisiting ✤ Initiation, translation (with some detail) elongation, termination 2 Gene mutation ✤What are mutations and how do they affect phenotype? ✤Common mutations ✤Causes of mutation and Induced ✤Spontaneous 3 Mutations likely result in detrimental effects Beneficial - mutations leading to adaptive change Neutral - have no effect on survival or reproductive success of individual Detrimental - disrupt protein function, affects survival or reproductive success 4 Where and when a mutation occurs is important •Mutations in a gene may affect specific tissue or developmental stage •Germ line mutations •Somatic cell mutation • •The stage of development during which the mutation occurs informs the range of affected cells 5 Mutations are characterized by ability of organism to survive and reproduce Beneficial - mutations leading to adaptive change Neutral - have no effect on survival or reproductive success of individual Detrimental - disrupt protein function, affects survival or reproductive success ..but this is not always a clear cut distinction 6 Many types of mutations chromosomal rearrangements insertions deletions substitutions point mutations http://www.cs4fn.org/biology/images/mutation.jpg 7 Point mutations Point mutations affect one base pair in a gene. wild type mutant ‘Reversion’mutation would convert A back to T p://www.gsc.riken.go.jp/Mouse/AboutUs/mutagen.jpg 8 Point mutations Two types: 1. Base pair substitution (as in previous slide) 2. Base pair insertion or deletion And result in: 1. silent mutations* 2. missense mutations 3. nonsense mutations * these are generally caused by substitutions 9 Silent mutations - no change in amino acid sequence key 10 Missense mutations - code for an incorrect amino acid 11 Nonsense mutations - code for STOP 12 Insertions and deletions cause frameshift errors normal (wild type) reading frame: the bad cat ate the red rat (English example) aug aau gcc uua aua ccg uga (RNA example) deletion of one base pair shifts frame: heb adc ata tet her edr uga aug ccu uaa uac cgu start stop 13 Chromosomal rearrangements 14 Suppressor mutations: wrong + wrong = right Suppressor mutation - a second mutation that suppresses the effects of the first mutation 1st mutation wt mutant 2nd mutation wt 15 Suppressor mutations see Table 16.2 for examples intragenic suppressor - “within gene” gene gene wild type intergenic suppressor - “between genes” gene 1 gene 2 gene 1 gene 2 wild type 16 Mutations in non-coding regions Mutations in Promoters - gene expression is affected up promoter mutation - increases expression of gene down promoter mutation - decreases expression of gene see Table 16.3 for other examples 17 Hotspots: regions of the genome with higher frequency of mutation gene gene gene 18 Trinucleotide repeat expansion (TNRE) diseases ular usc ic M y ton oph Myo ystr D lbar bu hy al & trop spin ar a scul mu ease dis on’s ingt unt H agil Fr ome ndr X sy e CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG 19 What causes mutations? Arise by natural causes (“spontaneous”) or as a result of a physical or chemical agent (“induced”) http://www.ehs.washington.edu/rsononion/uvsign.jpg http://www.reagent.co.uk/images/tpoison.jpg 20 Spontaneous mutations ✤ ✤ ✤ ✤ Polymerase makes mistakes during DNA replication Depurination Deanimation Tautomeric shifts 21 Depurination - loss of purine base (A,G) depurination apurinic site replication A,T,G,C if unrepaired 22 Deanimation - loss of amine group (NH2) 23 Tautomeric shift - migration of a Hydrogen atom A tautomer = interconverting forms of a chemical 24 Tautomeric shift (also G) (also A) 25 Tautomeric shift - consequences T* normal G C* normal A 26 Cells are not defenseless against mutation! DNA is constantly monitored for errors Proofreading and repair mechanisms Correct damage to DNA Over 130 repair enzymes in human cells 27 When errors made during replication are not fixed, mutations result 3,000,000,000 (3 billion) base pairs/human cell and 1 error/1 billion base pairs is not repaired ~6 errors each time DNA is replicated This is the source of genetic variability in nature! 28 Summary • Mutations can be beneficial, neutral or detrimental. They are most often detrimental. • Mutations are changes in DNA sequence arising from natural (spontaneous), chemical or physical (induced) causes • Many types of mutations (point, insertions, deletions, rearrangements) can lead to alteration in gene structure or sequence base changes or modifications, which can lead to mutation if not corrected by repair enzymes • Spontaneous mutations occur within the cell and involved 29 Ungraded problems ✤Go ✤Try over Chapter 16 S1 Chapter 16 C2 - C4, C11, C14, C20 30 Next Time Repair and recombination 31 ...
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