Chapter 18 – Gene Mutations and DNA Repair

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

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

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

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

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

Causes of mutations

• Spontaneous– Natural changes/errors– Replication errors or chemical changes

• Induced– Caused by environmental agents

• Chemical, radiation

Spontaneous replication errors

• Tautomers

• Wobble

• Strand slippage

• Unequal crossing over

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

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

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

Strand slippage in trinucleotide repeats

• Slippage of new strand can result in expanded number of repeats in offspring cells

• Cause of anticipation

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

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

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

Chemically Induced Mutagens

• Mutagen – environmental agent with ability to alter DNA sequence

• Base analogs

• Alkylating agents

• Deamination

• Oxidative reactions

• Intercalating agents

Chemically induced mutagens

• Base analogs– Have structure similar

to normal nucleotides– When ionized, exhibit

unconventional base pairing

– Transition or transversion mutation shown?

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)

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

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

DNA repair

• Mismatch repair

• Direct repair

• Base excision repair

• Nucleotide excision repair

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

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

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

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