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Mutation, Mutagenesis and DNA Repair
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Lecture notes on Mutation, Mutagenesis and DNA Repair
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Mutagenesis and DNA Repair are Covered in Chapter 1 of the unit E-bookDefinitions:
1. Mutations:
a. Change in the nucleotide sequence of DNA
b. Position of mutations within genetic sequence is crucial to function
2. Exonic mutations – ie protein coding regions
3. Synonymous mutations do not change the encoded amino acid
a. redundancy of the genetic code i.e. GGT, GGA, GGC, and GGG all code for glycine
4. Non-Synonymous mutations change the encoded amino acid, mutant protein, potential altered biological/function
5. Splice site mutations – ie at intron/exon boundaries – can exclude exons, include introns in final amino acid, and
potential altered/mutant proteins.
6. Promoter mutations – ie protein coding regions – may affect control of gene expression, hence levels of protein
production.
BAD:
- Mutations can cause significant deleterious effects i.e. uncontrolled cell growth (proliferation) leading to multiple types
of cancer. However, cells have multiple mechanisms to repair nucleotide changes. They’re not 100% effective and may
lead to mutations
GOOD:
- Mutations hold a huge role in evolution. Somatic mutations - can occur in any of the cells of the body except the germ
cells (sperm and egg) and therefore are not passed on to children.
- Germline mutations – occurs in body's reproductive cell (egg or sperm) that becomes incorporated into the DNA of
every cell in the body. These are inherited from parents to offspring. “survival of the fittest”
- Long-term species survival - enhanced by mutation - allows adaptation to changing environments
- Short-term survival - requires accurate maintenance of genetic record
Genetic variation is common (ie eye colour)
- Polymorphism – mutations that are common
- DNA change, with at least one variant (allele) found in at least 1% of the population
- Most common type of variation involves single base pair changes i.e. single nucleotide polymorphism, or SNP
(pronounced snip), but can also be much larger in size and involve long stretches of DNA.
- Non-disadvantageous mutation becoming established in a population
HOWEVER:
- Scientists are studying how SNPs in the human genome correlate with disease, drug response, and other phenotypes
- combinations of multiple SNPS, environmental exposures, = complex genetic diseases ie arthritis, cancer risk
stratification
, Mutation rate - Frequency of DNA sequence change expressed as the number of mutations per biological unit (i.e.,
mutations per cell division, per gamete, or per round of replication).
Estimated indirectly:
1. By comparing the amino acid sequence of the same protein in several different species. Compare with time of
divergence from a common ancestor.
a. Ex. fibrinopeptide (blood clotting) = in an average 400aa protein there will be one aa change per 200,000 years
2. Observe rate at which spontaneous changes arise in the genome of faster growing cells/organisms ie bacteria = get 1bp
change per 109 bp replications = low rates of mutations
a. DNA is exposed to lots of damage i.e. sunlight; evolved DNA repair mechanisms; prevent mutations. Overall:
process of DNA replication is very accurate
- Purines are heterocyclic aromatic organic compounds that consist of two rings
(pyrimidine and imidazole) fused together.
- Pyrimidine, any of a class of organic compounds of the heterocyclic series
characterized by a ring structure composed of four carbon atoms and two nitrogen
atoms. The simplest member of the family is pyrimidine itself, with molecular
formula C4H4N2.
Point mutations: single base change in DNA sequence and have a tendency to back
mutate in rapidly reproducing organisms ie e.coli
Types of mutations:
1. Transitions - changes a pyrimidine to a pyrimidine (T to C or C to T) or a purine to a
purine (A to G or G to A).
2. Transversions - change a pyrimidine to a purine or vice-versa ie (T or C to A or G)
and (A or G to T or C)
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