MBG 2040 Lecture Notes - Lecture 10: Open Reading Frame, Missense Mutation, Nonsense Mutation
2 May 2018
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Mutation and DNA Repair
Lecture Outline:
• Mutation: ultimate source of genetic variability, driving evolution and biodiversity
• Molecular basis of mutation
• Basic features of mutation process
• Phenotypic effects
• Screening chemicals for mutagenicity: The Ames Test
• Molecular mechanisms of DNA repair
Mutation: the ultimate source of genetic variability essential for evolution
• Mutation:
o Permanent change in genetic material of an organism
o Process by which the change occurs
• Mutant: organism that exhibits an altered phenotype
• Types of mutations:
o Changes in the number and structure of chromosomes
o Point mutations - changes at specific sites in a gene (substitution, insertion, deletion)
Mutation and Evolution
• Mutation serves the ultimate source of all genetic variation that is observed in the biosphere
• Recombination mechanisms rearrange genetic information into new combinations
• Natural selection preserves the combinations of genetic information that are best fit to the
environment they are in
Somatic mutation produced delicious apples
• Original mutation occurred in fruit trees that were somatic mosaics
• Vegetative propagation allowed the somatic mutation to be perpetuated
Bacterial and phage mutants are very useful in genetic studies
• Helped establish the discipline of genetics
Molecular basis of mutation:
• Mutations alter the nucleotide sequence of a gene in several ways including…
o Substitution: changes from a single base pair to another
o Indel mutation: deletion or insertion of one or several base pairs
• Tautomeric shifts can result in mutation during DNA replication
o Tautomeric shift: movement of H atoms from one position in a base to another
• Alters base pairing during replication
o Guanine and thymine exist as keto (common) or enol (rare) forms
o Adenine and cytosine exist as amino (common) or imino (rare) forms
o A:C base pairing can form when cytosine in the rare imino form
o G:T base pairing can form when guanine exists in the rare enol form
o *see mechanism by which tautomeric shifts in the bases of DNA cause mutations
• --> transition mutation
o Therefore, transitions in tautomers in nucleotide bases leads to mutation during DNA
replication
• Two types of base substitutions:
o Transition - single base pair mutation in which a pyrimidine (T/C) replaces another
pyrimidine, or a purine replaces another purine (A/G)
find more resources at oneclass.com
find more resources at oneclass.com
o Transversion - single base pair mutation that replaces a pyrimidine with a purine, or visa
versa
o **note: 12 different base substitutions can occur in DNA
• Base substitutions can lead to different changes in amino acids
o Silent mutation - single base pair mutation that leads to no change in the amino acid
o Neutral mutation - change to a similar amino acid does not affect function
o Mis-sense mutation - single base pair mutation that results in a different amino acid
o Non-sense mutation - mutation that results in a stop codon, leading to truncated polypeptide
• *recall: suppressor tRNAs
• Frameshift Mutations
o Open reading frame (ORF): series of consecutive triplets of nucleotides (codons) flanked by a
start codon and one of the stop codons
o Insertion or deletion of 1 or 2 base pairs alters the reading frame of the gene distal to the site
of mutation
• Frequency of mutations
o Spontaneous mutations:
• Bacteria and phages = 10^-8 to 10^-10 per base pair per generation of cell division
• Eukaryotes = 10^-7 to 10^-9 per base pair per generation
▪ Or 10^-4 to 10^-6 per gene per generation (assuming an average gene size is
1,000 bp)
o Treating bacteria with mutagens can increase the mutation rate to >1% per gene (100-
100,000x more likely)
Factors that influence the rate of spontaneous mutations:
• Inherent accuracy of the DNA replication machinery (different polymerase enzymes exhibit
different levels of fidelity)
• Efficiency of the mechanisms and enzymes that repair mutated and damaged DNA
• Degree of exposure to mutagenic agents in the environment
o Radiation: the electromagnetic spectrum
• UV light induces mutations through excitation of nucleotides
▪ Hydrolysis of cytosine hydrate may cause mis-pairing during DNA replication
▪ Cross-linking of adjacent thymine bases forms thymine dimers, which block
DNA replication and may activate error-prone DNA repair mechanisms if
damage is extensive
• X-rays and other high energy radiations induce mutations via ionization
▪ Ionizing radiation causes major changes in chromosome structure
▪ Ionizing radiation breaks chromosomes and causes a variety of major changes,
such as deletions, duplications, inversions, and translocations
▪ *see slide
o Chemical mutagens
• Types:
▪ Chemicals that are mutagenic to both replicating and non-replicating DNA (e.g.
alkylating agents and nitrous acid)
▪ Chemicals that are mutagenic only to replicating DNA (e.g. base analogues,
acridine dyes)
• Alkylating agents add alkyl (methyl or ethyl) group to DNA, leading to structural
changes in nucleotides and mis-pairing during replication
▪ They donate an alkyl group or derivatives to DNA or RNA
▪ They can change base-pairing properties of the alkylated base
▪ They induce all types of mutations (transitions, transversions, frameshifts, and
chromosome aberrations)
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Lecture outline: mutation: ultimate source of genetic variability, driving evolution and biodiversity, molecular basis of mutation, basic features of mutation process, phenotypic effects, screening chemicals for mutagenicity: the ames test, molecular mechanisms of dna repair. Somatic mutation produced delicious apples: original mutation occurred in fruit trees that were somatic mosaics, vegetative propagation allowed the somatic mutation to be perpetuated. Bacterial and phage mutants are very useful in genetic studies: helped establish the discipline of genetics. Molecular basis of mutation: mutations alter the nucleotide sequence of a gene in several ways including , substitution: changes from a single base pair to another. 1,000 bp: treating bacteria with mutagens can increase the mutation rate to >1% per gene (100- Factors that influence the rate of spontaneous mutations: Dna replication and may activate error-prone dna repair mechanisms if damage is extensive: x-rays and other high energy radiations induce mutations via ionization. Ionizing radiation causes major changes in chromosome structure.
