Mutation by mispairing
DNA polymerases have a 3' to 5' exonuclease activity for proof reading but mistakes still occur.
This can stem from the fact that purine and pyrimidine bases exist as tautomers (isomers differing in the location of a hydrogen atom). The common version is the keto form, if this changes to the enol form during replication the wrong nucleotide will be incorporated.
Cytosine can deaminate spontaneously to give uracil. If this is missed by a cell's excision repair sytem then a C-G pair becomes a U-A pair.
Chemicals can be mutagenic. Planar compounds such as proflavin can intercalate between stacked bases causing a frameshift mutation. Chemical mutagens such as EMS can alter existing bases and alkylating agents add alkyl groups to the bases. This generates bulky products that are unrecognisable by DNA polymerases so random nucleotides may be incorporated.
DNA repair and mutation
UV induces adjacent pyrimidines in the same DNA strand to dimerise, giving photoproducts called pyrimidine dimers.
There are four DNA repair mechanisms to deal with UV induced damage:
- Photoreactivation: acts before the damaged DNA replicates, absoprtion of light directly undoes the damage.
- Excision repair: acts before the damaged DNA replicates, removes the ** damaged region and resynthesises the correct sequence.
- Post-replication recombinational repair: fills the gap by physical transfer of DNA strand from an undamaged homologous chromosome.
- SOS proce**ing: outcome is the introduction of random nucleotides opposite the damage in the template strand, explains the link between DNA repair and mutagenesis.
Mutations induced by ionising radiation
Radiations have high energy which create highly reactive free radicals which have multiple effects on DNA including: nucleotide damage (base substitutions) and ** or ds breaks.
Double strand breaks are lethal if released chromosome fragments fail to be transmited during mitosis. One daughter cell will contain a chromosome with a deletion. Multiple breaks followed by abnormal reunion give rise to translocations and inversions.
X-rays cause congenital abnormalities which are present at birth and may have a genetic origin e.g the malformations caused by the use of thalidomide in pregnancy.
Effects of mutations in somatic cells
Muatations in somatic cells are not transmitted to the next generation by sexual reproduction. The effect is influenced by the time of its formation andby dominance relationships.
Potential consequence is carcinogenesis.
Evidence indicating a correlation between mutagenesis and carcinogenesis:
- biological effects of ionising radiations (Hiroshima and Nagasaki bombings)
- phenotype of human genetic disorder xeroderma pigmentosum, on exposure to sunlight individuals develop multiple skin cancers, affected individuals are lacking excision repair
- 90% of human carcinogens are mutagens
Detection of mutagenic chemicals
Potential carcinogens can be detected as mutagens, basis of the Ames test. The system tests for mutagenesis of bacteria which are haploid.
Exploits the fact E.coli can grow on a minimal medium and synthesis all necessary metabloites.A bacteria that is mutant in one of these metabolites will need a substamce not required by the wild-type.
The Ames test involves addition of the test compound to a His requiring mutant and searching for an enhanced frequency of revertants that have suffered a second mutation allowing the cell to grow on minimal medium (lacking His).
The procedure permits selective growth of a minor and rare class of organism is called a selection system.
Spontaneous rate of mutations is: 1 in 10^6 per gene per cell generation which is equal to 1 in 10^6 per gene per round of DNA replication.
The average size of a bacterial gene is roughly 1000 nucleotide pairs. So it is about 1 in 10^9 per deoxyribonucleotide pair polymerised.
Varies according to the gene studied but generally in the range of 1 in 10^5-10^6 per gene per sexual generation.
The approach is generally to rely on dominant mtation such as achondroplasia which give a condition that is significantly pronounced but is not lethal before birth.