Chapter 18 – Gene Mutations and DNA Repair

1. Chapter 18 – Gene Mutations and DNA Repair

2. Mutation • Inheritable change in genetic material • Cells from cell division; offspring from reproduction • Somatic mutations • Mitosis yields genetically identical cells • can lead to mosaicism • Tumor – uncontrolled growth • Germ-line mutations • Arise in cells destined to become gametes • Passed to offspring; present in every cell of organism • Gene mutations • Affect a single gene • Chromosomal mutations • Large-scale changes • May be observable with a microscope

4. Types of mutations • Base substitution/point mutation • One base is replaced by another • Transition • One purine replaced by another purine; one pyrimidine replace by another pyrimidine • Transversion • Purine replaced by a pyrimidine, or vice versa

6. Types of mutation • Insertion or deletion • One or more nucleotides • Frameshift mutation • In mRNA genes, affect all amino acids downstream, unless in groups of three in normal codon place • Expanding trinucleotide repeats • Certain genes contain tandem repeats • Number of repeats can increase in offspring due to strand slippage or uneven crossing over

8. Phenotypic effects • Missense mutation • Causes incorrect amino acid to be placed in polypeptide • Neutral mutation – protein function is not affected due to amino acids having similar properties • Nonsense mutation • Introduces a premature STOP codon • Results in a truncated polypeptide • Silent mutation • Due to codon redundancy, mutation still codes for the same amino acid

10. Phenotypic effects cont • Loss of function • Functional polypeptide is not made • Recessive • Normal gene still makes correct polypeptide • Gain of function • Abnormal polypeptide is produced • dominant

11. Causes of mutations • Spontaneous • Natural changes/errors • Replication errors or chemical changes • Induced • Caused by environmental agents • Chemical, radiation

12. Spontaneous replication errors • Tautomers • Wobble • Strand slippage • Unequal crossing over

13. Tautomers • Various forms of nitrogenous bases • Position change of a proton (hydrogen ion) • Can exhibit unconventional base pairing • Rare form of C can bond with A; rare form of G can bond with T • Originally thought to be major source of mutation – no supporting evidence

15. Wobble • Flexibility in DNA helix • Incorporated error • TA base pair becomes CA • One new molecule will have correct TA, other will have CG • Since all bases are correctly paired, no repair mechanism can fix

16. Strand slippage • Causes small insertions or deletions • One nucleotide loops out • On new strand – results in an insertion • On old strand – results in a deletion

17. Strand slippage in trinucleotide repeats • Slippage of new strand can result in expanded number of repeats in offspring cells • Cause of anticipation

18. Unequal crossing over • Incorrect alignment of homologous chromosomes • Crossing over results in an insertion in one molecule and a deletion in the other molecule • Can also cause expanded trinucleotide repeats

19. Spontaneous chemical changes • Depurination • Nucleotide loses its purine base; apurinic • Can’t act as a template • A is usually the base placed in the new strand

20. Deamination • Removal of an amino group • Deaminated cytosine becomes uracil • Since U is not present in DNA, usually correctly by repair mechanisms • Deaminated methylcytosine becomes thymine • Causes CG to AT – not detected by repair mechanisms

21. Chemically Induced Mutagens • Mutagen – environmental agent with ability to alter DNA sequence • Base analogs • Alkylating agents • Deamination • Oxidative reactions • Intercalating agents

22. Chemically induced mutagens • Base analogs • Have structure similar to normal nucleotides • When ionized, exhibit unconventional base pairing • Transition or transversion mutation shown?

23. Chemically induced mutations • Alkylating agents • Donates alkyl groups to bases • Incorrectly base pair • Deamination • Can occur spontaneously or be induced • Adenine becomes hypoxanthine (pairs with C) • Guanine becomes xanthine (pairs with T)

25. Chemically induced mutagens • Oxidative reactions • Reactive forms of oxygen • Causes transversions • G pairs with A • Intercalating agents • Insert themselves into DNA – distorts molecule • Often causes frameshift mutations

26. Radiation • Ionizing radiation • High energy breaks phosphodiester bonds • Results in double-stranded breaks • UV light • Pyrimidine dimers – usually thymine dimers • Causes TpT to covalently bond • Replication of DNA is blocked and cell dies, or transcription is blocked

27. DNA repair • Mismatch repair • Direct repair • Base excision repair • Nucleotide excision repair

28. Mismatch repair • Corrects replication errors/improper base pairing not fixed by DNA polymerase III • Recognizes structural distortions • New strand section is cut out and replaced • Old strand is methylated – strand distinction

29. Direct Repair • Converts altered nucleotide back to original form • Methylguanine binds with A • Enzymes remove methyl group to return to normal guanine • Photolyase • Found in E. coli and some eukaryotes (not humans) • Break covalent bonds of dimers

30. Base Excision Repair • Repairs abnormal/ modified bases • Nitrogenous base is first removed • Apurinic or apyrimidic site • Followed by removal of rest of nucleotide • DNA polymerase replaces nucleotide; DNA ligase seals nick by forming phosphodiester bond

31. Nucleotide excision repair (NER) • Removes lesions that distort DNA helix • Several enzymes/ genes involved • Recognize distortion • DNA strand is separated; single-strand binding proteins stabilize • Large section is removed • DNA polymerase fills in; DNA ligase seals nicks