Chapter 14 - Gene Mutation, DNA Repair, and Transposition
Chapter 14 - Gene Mutation, DNA Repair, and Transposition
14.1 - Gene Mutations Are Classified in Various Ways
Types of Mutations
Spontaneous mutations
Happen naturally and randomly
Usually linked to normal biological or chemical processes in the organism
Induced mutations
Result from the influence of an extraneous factor, either natural or artificial
Classification by Type of Inheritance
Somatic mutations
Occur in any cell except germ cells
Not heritable
Germ-line mutations
Occur in gametes
Are inherited
Autosomal mutations
Occur within genes located on the autosomes
X-linked mutations
Occur within genes located on the X chromosome
Classification by Type of Molecular Change
Point mutations
Base substitutions where one base pair is altered
Transition
A pyrimidine replaces a pyrimidine or a purine replaces a purine
Transversion
A purine and a pyrimidine are interchanged
Frameshift mutations
Result from insertions or deletions of a base pair (or any number of bases except multiples of 3)
Classification by Type of Codon Change
Missense mutations
Alter a codon, resulting in an altered amino acid within a protein-coding portion of a gene
Nonsense mutations
Change a codon into a stop codon, resulting in premature termination of translation
Silent mutations
Alter a codon but do not change the amino acid at that position of the protein
Classification by Effect of Mutation
Conditional mutations
Have effects depending on the environment in which the organism finds itself
Lethal mutations
Interrupt essential processes, resulting in death
14.3 - Spontaneous Mutations Arise From Replication Errors and Base Modifications
Spontaneous Mutations
DNA polymerase occasionally inserts incorrect nucleotides due to mispairing
Slippage during replication can lead to small insertions or deletions
DNA may suffer oxidative damage from by-products of normal cellular processes
Integrations of transposons can act as naturally occurring mutagens
Tautomeric Shifts
Result in mutations due to anomalous base pairing
Depurination and Deamination
Most common cause of spontaneous mutation
Depurination
Loss of a purine base from the DNA
Deamination
Removal of an amine group from a base, which can result in a base change
14.4 - Induced Mutations Arise From DNA Damage Caused by Chemicals and Radiation
Mutagens
Natural or artificial agents that induce mutations
Base analogs
Can substitute for purines or pyrimidines during nucleic acid replication
Acridine dyes
Cause frameshift mutations by intercalating between purines and pyrimidines
Ethidium Bromide
An example of an acridine dye
UV Radiation
Creates pyrimidine dimers that distort DNA conformation, resulting in errors during replication
Ionizing Radiation
Takes the form of X rays, gamma rays, and cosmic rays
Induces point mutations and chromosomal aberrations
14.6 - Organisms Use DNA Repair Systems to Counteract Mutations
Proofreading
During DNA synthesis, bacterial DNA polymerase III inserts the wrong base approximately 1 time per 100,000 insertions, with an error rate of approximately 10^{-5}
DNA polymerase III can recognize and correct errors during replication, a process known as proofreading
It corrects about 99% of its insertion errors
Postreplication Repair
Occurs when DNA replication skips over a lesion and requires homologous recombination
Nucleotide Excision Repair (NER)
Repairs bulky lesions in DNA
Involves:
Removal of mutation by a nuclease
Gap filling by DNA polymerase
Sealing of the nick by DNA ligase
14.7 - The Ames Test Is Used to Assess the Mutagenicity of Compounds
Ames Test
Utilizes strains of Salmonella typhimurium selected for increased sensitivity to mutagens and the ability to reveal specific types of mutations
Some chemicals are not mutagenic or carcinogenic in their original form but become converted into mutagens or carcinogens upon metabolic processing
The test often includes a mixture of liver enzymes reflecting this metabolic conversion