Molecular Basis of Mutations Flashcards
How do mutations alter nucleotide sequences?
Mutations alter the nucleotide sequences of genes in several ways:
- Substitution of one base pair for another
- Deletion or addition or one or a few base pairs
What is the mechanism used for mutation?
Watson & Crick pointed out that the structure of bases in DNA are not static:
- H atoms can move from one position in the purine or pyrimidine ring to another position (called Tautomeric shifts)
- Stable keto forms of T & G -> enol form
- Stable amino forms of C & A -> imino form
Study the diagrams of the Keto and Enol forms of bases.
https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing
What is a Tautomeric Shift?
- The spontaneous isomerization of a nitrogen base to an alternative hydrogen-bonding form, possibly resulting in a mutation
How does a Tautomeric shift link to mutations?
A Tautomeric base form, present at the moment of replication or incorporation into a nascent DNA chain can lead to a mutation
- Rare imino -> A:C base pairing
- Rare enol -> G:T base pairing
What is the effect of a mutation caused by a Tautomeric Shift?
Effect of mismatched base pairs following replication is:
- Rare imino -> A:C base pairing -> C:G base-pair substitution
- Rare enol -> G:T base pairing -> T:A base-pair substitution
What are the types of Base Substitutions?
- A “Transition” replaces a pyrimidine with another pyrimidine or a purine for another purine
- A “Transversion” replaces a pyrimidine with a purine or a purine with a pyrimidine
- 3 substitutions/base pair (1 transition & 2 transversions), 12 in total -> Point mutations
What is a Frameshift Mutation?
- A mutation caused by the addition or deletion of a base pair or base pairs in the DNA of a gene resulting in the translation of the genetic code in an unnatural reading frame from the position of the mutation to the end of the gene.
- Worse to have the frameshift mutation at the beginning of a gene
What is a point mutation?
- A mutation affecting only one or very few nucleotides in a gene sequence.
What are Induced mutations?
- Induced mutations occur upon exposure to physical or chemical mutagens
What experiment was done for Induced Mutations?
- Muller and Alternburg measured the frequency of X linked recessive lethal mutations in Drosophila
- Muller demonstrated that exposing Drosophila sperm to X-rays increased the mutation frequency
- However, better understanding of mutations at the molecular level was provided by the discovery of chemical mutagens that have specific effects on DNA
What are the different types of chemical mutagens?
- Chemicals that are mutagenic only to replicating DNA (e.g., base analogs and acridine dyes)
- Chemicals that are mutagenic to both replicating and non-replicating DNA (e.g., alkylating agents and nitrous acid)
What does Intercalation mean?
- The insertion of molecules between the planar bases of deoxyribonucleic acid (DNA). This process is used as a method for analyzing DNA and it is also the basis of certain kinds of poisoning
What does the Intercalation of an Acridine Dye cause?
- Causes Frameshift
Mutations - Intercalate or sandwich between stacked base pairs in DNA
- Increases rigidity and alters conformation of double helix
- Subsequent DNA replication-> additions & deletions of one to a few base pairs results
What are Alkylating Agents?
- Chemicals that donate alkyl (CnH2n+1) groups to other molecules
- Induce transitions, transversions, frameshifts, and chromosome aberrations
What do Alkylating Agents do?
- Alkylating of bases can change base-pairing properties
- EMS -> ethylation (+ C2H5) of bases at 7-N & 6-O.
- Eg. 7-ethylguanine base pairs with thymine: G:C -> A:T transition
- Can also activate error prone DNA repair processes
- Less specific mutagenic effects
What mutagenic affects does smoking have?
- 60 different mutagenic chemicals identified
- All bind DNA, many intercalate or chemically modify DNA
- Most common cancer in humans with the highest morbidity (150 people died in the past hour)
- 23,000 mutations induced by smoking (15 cigarettes = 1 mutation)
- Genome sequencing of small-cell lung cancer
- 15% of all lung cancer cases, 2 year survival <15%
- Several anti-cancer genes mutated
What is the Ames test?
- The Ames test provides a simple and inexpensive method for detecting the mutagenicity of chemicals with histidine auxotrophic mutants of Salmonella
What types of radiation cause mutation?
- X-rays induce mutations through ionization: breaks chromosomes and can cause deletions, duplications, inversions, and translocations
- Ultraviolet light induces mutations through excitation (nonionizing radiation)
- The increased reactivity of atoms present in DNA molecules is responsible for most of the mutagenicity
What is significant about Mutagenesis by Ultraviolet Irradiation
- UV is potent mutagen in unicellular organisms
- Two main products of UV absorption
1. Pyrimidine hydrates
2. Pyrimidine dimers
What results form UV absorbtion?
- Hydrolysis of cytosine to a hydrate may cause mispairing during replication
- Cross-linking of adjacent thymine forms thymidine dimers, which block DNA replication and activate error prone DNA repair mechanisms.
What does it mean when something is Transposable?
- A transposable element is a DNA sequence that can change its position within a genome, sometimes creating or reversing mutations and altering the cell’s genetic identity and genome size. Transposition often results in duplication of the same genetic material.
How is a mutation induced by a Transposon?
- The insertion of a transposon into a gene will often render the gene nonfunctional
- Mendel’s wrinkled allele in the pea and the first mutation causing white eyes in Drosophila both resulted from the insertion of transposable elements
What are Trinucleotide Repeats?
- Simple tandem repeats are repeated sequence of one to six nucleotide pairs
- Trinucleotide repeats (repeats of 3 nucleotide pairs) can increase in copy number and cause inherited diseases
