Ch 7 Mutation Heritable change in DNA Random Source of genetic variation may be advantageous, deleterious, neutral

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1 Ch 7 Mutation Heritable change in DNA Random Source of genetic variation may be advantageous, deleterious, neutral Mutation (+ sexual reproduction) + natural selection = evolution

2 FYI: Adaptation vs mutation Adaptation not proven environment induces specific adaptive change Ex. E. coli + ampicillin some cells become resistant in response Mutation mutation is random E. coli DNA mutates randomly statistically, one will result in amp resistance, this cell grows resulting in species adaptation

3 germ-line mutation Types of mutations in sperm or egg (meiosis), will be passed to offspring somatic mutation body cells, not passed on to offspring spontaneous mutation natural aspect of DNA replication induced UV light, chemicals, X-ray etc.

4 Germ line or somatic? cat Drosophila Euphorbia basal cell tumor Spontaneous? Induced?

5 Mutation and natural selection

6 Point mutation single base substitution, deletion, or addition

7 Point mutations Base pair substitutions transition = G to A transversion =? T to C

8 Missense mutation Wrong amino acid encoded Sickle cell Hb Subsititution, addition or deletion? TS or TV? Which codon position?

9 Glu hydrophilic Val hydrophobic (reduces solubility of the molecule) Red blood cells Non-conservative mutation

10 Conservative mutation Example: Val to Leu both hydrophobic

11 Nonsense mutation Premature stop codon Codon AAA UAA lys stop

12 5' - ATG ACT CAC CGA GCG CGA AGC TGA - 3 3' - TAC TGA GTG GCT CGC GCT TCG ACT 5' mrna5' - AUG ACU CAC CGA GCG CGA AGC UGA - 3 Protein: Met Thr His Arg Ala Arg Ser Stop 5' - ATG ACT CAC TGA GCG CGA AGC TGA - 3 3' - TAC TGA GTG ACT CGC GCT TCG ACT - 5' mrna: 5' - AUG ACU CAC UGA GCG CGU AGC UGA - 3' Protein: Met Thr His Stop

13 Neutral mutation - mutation does not change amino acid folding - codon AAA AGA lys arg (both basic aa)

14 Silent mutation No amino acid change, usually 3 rd position TS codon GCA GCG arg arg

15 Frameshift mutation Shifts reading frame --> scrambled message May lead to a premature stop codon The sun was hot but the man did not get his hat.

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17 Transition or transversion and which codon position? TATACCTAT TATACCCTA

18 Other mutations Larger insertions, trinucleotide repeats, deletions, duplications, translocation, inversions

19 Trinucleotide repeat pg. 475 Fragile X Syndrome Xq27.3 FMR-1 gene X-linked dominant 2 nd leading cause of mental retardation (1/4000 males, 1/6000 females)

20 (Reprinted from Medical Genetics, 2nd ed., Jorde LB, et al, 2000, with permission from Elsevier Science.)

21 CGG repeat in promoter region Normal = 6-54 repeats Normal carriers = repeats (premutation expansion of repeat) Fragile X syndrome = repeats

22 triplet repeat amplification FMR-1 FMRP (mrna binding protein) FMRP regulates translation of mrna Affects synaptic plasticity signaling between neurons for learning and memory >200 repeats and FMR1 gene silenced retardation Caused by slipping of DNA polymerase during replication

23 Trinucleotide repeats

24 Mutation rate Human estimate is ~1 x 10-6 spontaneous mutations/gene /generation Mutation rates per gene differ. May be hot spots

25 Causes of mutations 1. Spontaneous mistakes by DNA polymerase = replication errors Most repaired by DNA proofreading and repair systems

26 2. Induced mutations Chemicals Mustard gas, industrial waste, PCBs, DDT Ethidium bromide Base analogues Example: Bromouracil substitutes for thymine Radiation sun (UV), radon, X-ray Breaks, dimers High doses kill cells

27 The Ames test Indirect assay for carcinogens Plate 10 8 auxotrophic bacteria (his-) Add filter disc with chemical Look for revertants to his+ = mutations Add rat liver extract to look at metabolism

28 Spontaneous reversion rate to His+ colonies 1000X higher rate with mutagen

29 Repair of mutations 1. Direct reversal Example: Mismatch repair 3 5 proofreading by DNA polymerase Also, UV repair (bacteria) and alkylation repair (prokaryotes)

30 2. Excision repair Ex. base excision repair - single strand incision at both sides of error; - Excise mistake - DNA synthesis to replace the gap - Ligation - Also UV repair prokaryotes, methyl mismatch repair, SOS response

31 Defective excision repair: Xeroderma pigmentosum (1/250,000) 9q34.1 Autosomal recessive Extreme sun sensitivity High risk of skin cancer (1000X) average by age 8 Metastatic melanoma Corneal damage

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33 Knockout mice for excision repair (XPA gene) XPA gene knocked out UV sensitive and develop tumors Study cancer, drugs

34 Small deletion example: Cystic fibrosis (1989) CFTR gene (chromosome 7) Encodes a transmembrane chloride channel active in cells that line the lungs

35 Cystic fibrosis recessive mutation p508 Will the mrna message be in frame?

36 Effects of p508 CFTR mutation The 3 base deletion results in a channel protein that does not function correctly Thick mucus in the lungs, infections, lung stress

37 Phenotypic effects of mutation: Loss of function Drosophila white eye gene Wildtype = red eyes

38 Gain of function mutation Antennapedia (Drosophila) legs on head (ectopic) in place of antennae Legs are normally expressed in second thoracic segment

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40 Knockout mice for excision repair (XPA gene) Genetically engineered to not express XPA gene Mice are UV sensitive and develop tumors Use mice for DNA repair/cancer study

41 Defective DNA mismatch repair: Hereditary nonpolyposis colon cancer Inherited predisposition to colon cancer ~4 people in family have had colorectal cancer, multiple generations, before age 50

42 Small deletion example: Cystic fibrosis (1989)