1. Chapter 18 – Gene Mutations and DNA Repair

2. Mutation Inheritable change in genetic material Somatic mutations
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 Nonsense mutation Silent 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 Induced 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 UV light
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