Mutation

Mutations are heritable changes in base sequences

Wild-type – the most common allele for a gene

Forward mutation – mutation that changes wild-type allele to something different

Reverse mutation (reversion) – changing a mutant allele back to a wild-type allele

Types of mutations classified by their effect on the DNA molecule

-          base substitution – when a base at one position replaced by another

o       can lead to missense or nonsense mutations

-          deletions/insertions – when nucleotides are deleted/added

o       can lead to frameshifts, but always leads to amino acids deleted/added

-          inversions – a piece of DNA is cut out and flipped around

-          translocations – pieces of chromosomes are swapped with each other

Mutation rates

-          always low

-          in mice on order of 10^-6 mutations per gene per gamete

-          different rates in different genes

-          forward mutations usually at higher rates than reversions – many different ways to disrupt a gene, fewer to put it back together once it is mutated

Mutations are random events

-          Fig. 7.4, the classic exp showing this

What causes changes in genome sequence? (Fig. 7.6)

-          Replication errors

o       Rate is very low, but they do happen

-          Depurination/depyrimidination

o       Very frequent, 20,000 or more times per day in human cell

o       Hydrolysis on N-glycosidic bond between base and sugar

o       Leaves an apurinic/apyrimidinc (AP) site

o       When DNA is replicated, what is put opposite that?

o       There is a DNA repair system just for this kind of site

-          Deamination

o       Amino group in cytosine falls off, changing base to uracil

o       So after DNA replication get C:G to T:A transition

-          UV light

o       Creates pyrimidine dimers

o       DNA polymerase makes variety of errors

-          Xrays

o       Strand breaking

-          Oxidation

o       Reactive oxygen species from our metabolism cause a number of different types of damage

-          Unequal crossing over

-          Transposon movement

Mutagens (Fig. 7.10)

-          base analogs

o       chemicals that mimic a base, but pair with different partner

-          chemicals that alter base structure

o       hydroxylating agents

o       alkylating agents

o       deaminating agents

-          intercalators

o       ethidium bromide is an intercalator

Can have mutations that alter DNA sequence outside of coding region of gene that affects the gene

-          what if -10 or -35 boxes were changed in E coli cell?

Fragile X syndrome

-          affected people have some physical symptoms and some degree of mental retardation

-          caused be mutation in the FMR-1 gene (fragile X associated mental retardation) which is on the X chromosome

o       affected have strange thing when chromosomes are looked at by a certain technique, then end has a very narrow spot that sometimes breaks off – this is just a consequence of the way these chromosomes are prepared for microscopy, this doesn’t really happen in the cell

o       section of the coding region of the gene has sequence CGG repeated a number of times (Arg amino acid in protein)

§         unaffected alleles are 5-54 repeats

§         people with fragile X has 200-4000 repeats

§         50-200 is “premutation allele” – a person with this X is not affected but is highly likely to generate gametes with a greatly expanded CGG repeat

·         The more repeats the more likely this is to happen

·         But mostly in eggs – usually don’t see this expansion in sperm

·         There are a few “triplet repeat” diseases and they are all similar in that more repeats is worse, but some differ in how repeat is expanded

§         Makes genetic counseling a challenge

Problems

I, 6, 10, 11